o .-" <

wags—iv:—
.
-I

CCMCARAWE msmosv as m: 'TMCHEA
or m Cm MC CAT ‘

ma far if» mam affM. s.
mamam sure umvsksm
Robert J. {N‘iowanhuis
T961

'0 o 9‘00."”.0"‘94.0"”Q‘flfly'0"”””fgnn’."w
. ; ' ‘ I ~, .

~ .
. n
o
. . 1
~ I '
o o

 

mssza

IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII

\lHIWllHHIMIMllUHHLI‘l I‘llmlnllhlfllml 2m $157“)

31293 006129

LIBRARY

Michigan Scam
University

 

COMPARATIVE HISTOLOGY OF THE TRACHEA

OF THE DOG AND CAT

BY

Robert J. Niewenhuis

A THESIS

Submitted to the College of Veterinary Medicine of Michigan
State University of Agriculture and Applied Science
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Anatomy

1961

ACKNOWLEDGEMENTS

The writer is sincerely appreciative to Dr. M. Lois
Calhoun, Professor and Head of the Department of Anatomy,
for her untiring help and encouragement in the guidance of
this investigation and for her patient assistance in the pre-
paration of this manuscript. He is also deeply grateful
to Dr. Madan B. Bharadwaj and Dr. Esther M. Smith, Associate
Professors of the Department of Anatomy, who so generously
assisted with their time, knowledge and talents.

Thanks are due Mr. James Tucker and Mr. Andrew Moore
for their aid in preparation of the photomicrographs and
Mrs. Esther M. Colby for her technical assistance and interest.

The writer wishes to thank Mr. Richard Witter who began
the work on this problem as well as Betty Mason, Pauline Ho
and Effie Lou Ball who have in one way or another contri-
buted to this investigation.

Many thanks are extended to my wife who unselfishly con-
tributed many hours in the preparation of this manuscript,
and also to the members of the faculty and staff of the
Anatomy Department who offered helpful suggestions and

assistance.

ii

TABLE OF CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . 1
REVIEW OF LITERATURE . . . . . . . . . . . . . . . . 2
Anatomy of the trachea in general 2
Mucosa 3
Submucosa 9
Fibroelastic cartilagenous layer 10
Trachealis muscle 13
Lymphatics 14

Blood vessels l4

Nerves 15
Tracheal movements and structural exceptions l6

Anatomy of the trachea of the dog and cat 16
MATERIALS AND METHODS . . . . . . . . . . . . . . . 21
RESULTS AND DISCUSSION . . . . . . . . . . . . . . 23
The trachea of the cat 23
Epithelium 23

Lamina propria and elastic layer 27
Submucosa 28
Cartilage rings and annular ligaments 3O
Trachealis muscle 32
Adventitia, nerves and vessels 33

iii

Sexual differences

The trachea of the dog
Epithelium
Lamina propria, elastic layer and submucosa
Cartilage rings and annular ligaments
Trachealis muscle
Adventitia, Nerves and Vessels
Sexual differences

SUMMARY AND CONCLUSIONS. . . . . . . . . . . . . .

LITERATURE CITED 0 O O O O O O O 0 O O O O O O O 0

iv

34

39

39

42

45

47

48

49

50

54

TABLE

II.

III.

LIST OF TABLES

PAGE

Tracheal measurements in the cat . . . . . . 35—36

Tracheal measurements in the dog . . . . . . 37—38
Measurements of epithelial thickness in

longitudinal section . . . . . . . . . . 41

LIST OF PLATES

PLATE PAGE
I. Comparative sketch of dog and cat trachea. . . 60
II. Sketch showing shape and relationship of
cartilage rings. . . . . . . . . . . . . . 62
III. Oblique section through an epithelial fold-
cat. . . . . . . . . . . . . . . . . . . . 64
IV. Transitional type of epithelium in the dorsal
region - cat . . . . . . . . . . . . . . . 66
V. Junction between pseudostratified and transi-
tional type of epithelia - cat . . . . . . 68
VI. Variations in epithelial thickness and gland
distribution - cat . . . . . . . . . . . . 70
VII. a. Longitudinal section through ventral area

of cat trachea

b. Longitudinal section through dorso-lateral

area of cat trachea. . . . . . . . . . 72

VIII. Leucocytes in the epithelium - cat . . . . . . 74
IX. Lymphoid invasion of the mucosa - cat. . . . . 76
X. Mucosa and submucosa in the cat. . . . . . . . 78
XI. Mucosa and submucosa in the dog. . . . . . . . 80
XII. Acini external to cartilage ring - cat . . . . 82

vi

XIII.

XIV.

XVII.

XVIII.

XIX.

Acinus in the lamina propria - cat . . . . . .
Acini in the annular ligaments - cat . . . . .
Muscle attachment to cartilage ring - cat. . .
Relationship of ganglion, muscle and cartil-
age - dog. . . . . . . . . . . . . . . . .
Transitional like epithelium in the dog. . . .
The tracheal glands - cat. . . . . . . . . . .
Longitudinal section through the ventral area
of dog trachea . . . . . . . . . . . . . .
A characteristic large gland duct - dog. . . .
Nerve trunks in the dorsal submucosa - dog . .
Longitudinal section through membranous part

Of traChea - dog 0 o o o o o o o o o o o 0

vii

84

86

88

90

92

94

96

98

100

102

INTRODUCTION

Although the microscopic anatomy of the trachea of man,
or at least certain features of it, has been thoroughly in-
vestigated and recorded in medical literature, relatively
little can be found concerning the detailed histology of the
trachea of domestic animals. The few English sources which
are available treat the subject very superficially. Several
German works (Oppel 1905, Sussdorf 1911, Loginoff 1911, and
Paul 1913) are excellent concerning the microscopic anatomy
of the trachea of domestic animals, however, they are not
complete in all details.

This study is an attempt to supplement existing informa—
tion by comparing the histology of the dog and cat trachea
with the hope that it may be useful in the fields of Anatomy,

Cytology, and Pathology.

REVIEW OF LITERATURE

Present day information regarding the histology of the
trachea stems from the collective work of many investigators
who have studied the trachea as part of a general histolog-
ical study, have concentrated on some particular feature of
it, or have considered it in a study of the respiratory
system. These studies included a variety of animals, the
majority of them have been done on man. Therefore, in re-
viewing the literature it is advantageous to consider in
detail, the work done on the trachea in general before
concentrating specifically on the limited amount of compara-

tive work done on the dog and cat.
ANATOMY OF THE TRACHEA IN GENERAL

Many authors (Dawson 1948. Nonidez and Windle 1949,
Cowdry 1950, Maximow and Bloom 1957, Copenhaver and Johnson
1958, and Ham 1957) recognized the following layers in the
tracheal wall: (a) mucosa, (b) submucosa, (c) fibroelastic
membrane which contains the cartilagenous rings, (d) adven-
titia, and (e) muscularis found only dorsally. Bremer and
Weatherford (1946) and Jordan (1952) combined the fibro-
elastic membrane, adventitia and muscularis into one layer.

Trautmann and Fiebiger (1952) did not recognize the submucosa

as a separate layer. Von Hayek (1960) mentioned that such a
layer is justifiable because the mucosa is displacable and
subject to folding especially in the dorsal region; a feature

permitted by the loose submucosa underneath.

Mucosa

The trachea in man and domestic animals is lined with
pseudostratified ciliated columnar epithelium and contains
goblet cells which are often numerous. The epithelium rests
on a basement membrane which varies greatly in thickness
among different species. Lucas (1932) and Miller (1932)
divided the epithelium into four layers of cells: (a) basal
cells, (b) intermediate or non-cilated, (c) ciliated, and
(d) goblet. Garvin (1957) described the basal cells as
rounded or polygonal cells with large nuclei which do not
reach the surface but lie between the feet of the taller cells.
These cells, according to Miller (1932), develop into interme-
diate cells which become elongated and often reach the sur-
face. Intermediate cells do not bear cilia and usually
taper to a point at their outer surface. They in turn give
rise to the ciliated columnar cells, which are normally the
most numerous and goblet cells, which may represent a mucous

degeneration of ciliated cells. Miller also mentioned that

regeneration of tracheal epithelium is very slow under normal
conditions. Mitotic figures are rarely seen.
Chambers and Renyi (1925) found ciliated cells structur—
ally related and held together at their ciliated surface by
a cuticular border which bears the cilia. This is not an
organic relationship as they believed was true of the cells
in stratified squamous epithelium. Rhodin (1959) by means
of electron microscope studies showed that the surface of the
epithelium consists of microvilli and is striated in nature
similar to the striated border in the intestinal tract.
Florey _t_§l. (1932) studying mucous secretion in the
trachea of cats, noticed that goblet cells are not affected
in any way by nerve stimulation but instead are discharged
by contact or direct stimulation. Chronic irritations caused
by constant injections of formalin or dust inhalation resul—
ted in a great increase in the number of goblet cells. Lucas
and Douglas (1935) studying ciliary activity noticed that
the normal mucous layer covering the tracheal epithelium
consists of two layers of fluid; an inner one of thin watery
fluid and an outer layer composed of more viscous mucoid
material. They concluded the rate of mucus removal is more
likely due to changes in the composition and proportion of

these layers than to changes in the rate of ciliary beat.

Rhodin (1959) referred to a second type of secretory cell
which he identified in the tracheal epithelium of man and be-
lieved it to be responsible for a serous secretion. The cell
is columnar but does not assume a goblet appearance.

Barclay g; a1, (1938) noticed that insufflated dusts
are excreted rapidly from the lower respiratory tract of the
cat due to ciliary activity and rapid propulsion of mucus.
Fluids, however, are not arrested by the cilia but pass into
the lungs.

Replacement of tracheal epithelium by stratified squamous
epithelium (metaplasia) has been reported. Miller (1932) re-
ferred to the work of Griffini in 1875 and Baraban in 1890 who
first described the presence of stratified squamous epithelium
in the trachea of man. Baraban believed that this transition
resulted from a low grade irritation which led to loss of
cilia and appearance of mucus in the cells. Miller himself
often found stratified squamous epithelium in the bronchi
of patients suffering from tuberculosis and assummed that
constant coughing played the key role in the transition.
Florey _£__l, (1932) found that constant and intense irrita-
tion of the tracheal epithelium of cats resulted in a change
Of pseudostratified into transitional epithelium similar

t<> that found in the urinary bladder. Sussdorf (1911)

reported spots of stratified squamous epithelium in the upper
trachea of old cats.

Leucocytes are frequently seen in tracheal epithelium.
Andrew and Burns (1947) studied this feature in detail in
mouse tracheae. They stated that lymphocytes are the most
numerous but polymorphonuclear leucocytes are found frequently.
Monocytes and eosinophils are rarely seen. Occasionally
leucocytes are found intracellularly. Age has no affect
upon leucocyte infiltration and migration. Most of the
leucocytes are found at or below the level of the epithelial
nuclei; those found near the surface of the epithelium show
marked pycnosis. They also mentioned that Flemming found
leucocytes in nearly every mammalian trachea as early as
1885. Von Hayek (1960) referred to their presence in human
tracheal epithelium.

Ropes (1930), using rabbits, found ciliated tracheal
epithelial cells capable of exhibiting phagocytic activity.
He divided these phagocytic cells into two major groups;

a columnar slightly phagocytic group and a cuboidal or low
columnar, highly phagocytic group with many transitional
stages between the two. These cells increase in number after

exposure to carbon.

According to Ham (1957) the basement membrane is a
condensation of the lamina propria. Trautmann and Fiebiger
(1952) stated that many animals have only a rudimentary base-
ment membrane. The basement membrane in man is about 10 u
thick and is not a homogeneous membrane but a network of
argyrophilic fibers (Von Hayek 1960). All the cell types
in the epithelium are firmly fastened to it. Goblet and
ciliated cells may be attached to it by a foot, a single
pointed prolongation or a number of fine prolongations
(Miller, 1947). Rhodin (1959) recently found, with electron
microscopy, that some ciliated and goblet cells in the
tracheal epithelium of man do not reach the basement membrane.
This contradicts the commonly held idea that all cells in
pseudostratified epithelium reach or rest on the basememt
membrane. He also noted that the basement membrane in the
rat trachea is a homogeneous continuous structure, while
in man there is a layer of densly packed collagen fibers
in random orientation on the subepithelial surface of the
membrane. Stirling (1890) mentioned that the basement
membrane is not as distinct in the cat as it is in man.

The lamina propria was described by Trautmann and Fie-
biger (1952) as the area between the basement membrane and

the elastic membrane. It consists of fine fibers and often

many lymphocytes and lymph nodules. Von Hayek (1960)
called this area the membrana propria and included the
elastic layer or membrane as a deeper layer of the same area.
The cells in the superficial layer of the membrana propria
consist mostly of lymphocytes which are clustered around
the gland ducts. Blood capillaries, lymphatic vascular
networks and a loose network of collagenous and elastic
fibers are found in this area.

The elastic membrane, generally considered to take the
place of the muscularis mucosa found in the gastrointestinal
tract, was described by Macklin (1922). He dissected out
this membrane from a pig and demonstrated that it is the
most important recoil mechanism of the respiratory tract.
The elastic fibers vary in thickness, the smaller ones are
scattered through the lamina propria near the basement mem-
brane. The fibers form a dense thick meshwork which is
thicker in the trachea than other parts of the respiratory
tract. Lateral branches are given off which connect to
the perichondrium of the cartilages, the muscle, and some
even mingle with fibers of the adventitia.

Von Hayek (1960) reported that this elastic layer in
man is wider and the fibers thicker in the region of the
muscle than they are in the area of the cartilage rings.

He also mentioned that Schaffer described bundles of

smooth muscle in this elastic membrane. Miller (1947) ob-
served that the elastic fibers appear in bundles in the dorsal
part of the tube but this is solely the result of the folds

of the mucosa. Rhodin and Dalhamn (1955) mentioned that

the elastic fibers are l - 2 u in diameter in the rat and

are in turn composed of longitudinally arranged fibrils of

the magnitude of about 70 2, In contrast to the report of
Macklin they did not notice any branching of elastic fibers

in their studies on the rat.

Submucosa

This layer lies between the elastic membrane and cartilage
rings. It is composed of loose connective tissue, is rich in
elastic fibers and fat and blends with the perichondrium of
the cartilage rings. The glands of the trachea are found
mainly in this area.

According to Krélling and Grau (1960) these glands are
mostly mixed tubular and are most abundant ventrally and la-
terally.. Smooth muscle fibers are usually associated with
the ducts. Miller (1947) and Von Hayek (1960) reported mixed
glands in man (both mucous acini capped with serous demilunes
and pure serous acini). They are embedded in the submucosa
between tracts of elastic fibers which connect the elastic

membrane with the perichondrium and annular ligaments. In

10

longitudinal section the glands are seen grouped between the
cartilages, except in the dorsal region where they lie largely
outside of the trachealis muscle. The ducts often open on
the surface of the mucosa by a funnel-shaped enlargement.
The larger ducts are frequently lined for some distance with
ciliated epithelium.

Florey §§_al, (1932) found that glands of the trachea
are activated by stimulation of the recurrent laryngeal nerves
and administration of pilocarpine. Ham (1957) noted that in
man some glands may occasionally be found in the lamina pro-
pria, in between bundles of smooth muscle or in the adven-
titia. Garvin (1957) mentioned that the glands are more num-
erous in some animals, such as the cat, than they are in the

human species.

Fibroelastic cartilagenous layer

Von Hayek (1960) referred to this layer of the tracheal
wall as the tunica fibrosa or tunica cartilagineo-fibrosa
because it includes the hyaline cartilage rings plus the
fibroelastic membrane covering and connecting them. In a
longitudinal section of the trachea from the ventral area,
he observed that the rings are flattened on their external
surface but convex internally. The perichondrium is thicker

externally than internally. The periphery of the cartilage

11

itself is also copiously supplied with collagenous fibers
which extend inward from the perichondrium. Stirling (1883)
noticed that the cartilage rings in the dog, cat and man
become thinner in the dorsal region and overlap adjacent
rings. Chauveau (1873) also mentioned this as a characteris-
tic of domestic animal tracheae. The rings are sometimes
bifurcated and fused with each other although all are in-
complete dorsally. Straus (1931) found differences in the
structure and shape of the cartilage rings in many primates.

Miller (1920) described the marked irregularities in the
shape of the cartilage rings in man. He also studied the
carina tracheae in the cat (1904) and described it as a
"...prominant ridge running anteroposteriorly across the
bottom of the trachea between the origin of the two bronchi."
The carina tracheae may be derived from a tracheal or bron-
chial cartilage ring or a combination of these; occasionally
it is absent altogether. He stated later (1947) that the
same is true in man.

The cross-sectional shape of the cartilage rings in mink
was described by Stowe and Calhoun (1961) as oval ventrally,
elliptical laterally and spatulate in the dorsal region

where they overlap.

12

Sussdorf (1911) and Trautmann and Fiebiger (1952) stated
that the hyaline cartilage rings often calcify and ossify in
old animals. According to Maximow and Bloom (1957) and Bremer
and Weatherford (1946) the cartilages in man become fibrous
or partly calcified in old age. Stowe and Calhoun (1961)
found that calcification in the tracheal cartilages of mink
has already started in the 49-day-old and is complete in about
68 days.

Wolf-Heidegger (1947) stated that the annular ligaments,
the sheets of fibrous tissue between the cartilagenous rings,
consist of three layers in man; (a) a strong outer layer at-
tached to the outer perichondrium, (b) a middle layer which
connects the edges of the rings, and (c) a weaker inner layer
attached to the inner perichondrium. Fat cells and glands are
frequently found between these layers of collagenous and elas-
tic fibers which are oriented somewhat obliquely so as to
form a crisscrossing, distensible latticework. A strong
longitudinal band consisting predominantly of elastic fibers
and continous with the fibroelastic membrane spans the dorsal
ends of the tracheal rings. These layers of fibroelastic
tissue blend with the perichondrium but can be distinguished
from it because they contain a greater percentage of elastic

fibers. Edwards (1956) described it as one continuous sheet

13

or membrane of fibroelastic tissue from top to bottom. Nonidez
and Windle (1949) also made reference to a fairly dense fibro-
elastic membrane which lies outside or dorsal to the trachealis
muscle and spans the area between the ends of the cartilage
rings. This forms a unified structure with the rest of the
fibroelastic layer. According to Copenhaver and Johnson (1958)

the trachealis muscle is embedded in this fibroelastic membrane.

Trachealis Muscle

Miller (1947) mentioned that Cuvier was the first to des-
cribe the different trachealis muscle attachments in man and
various animals. The general rule is that in carnivores the
attachment is external to the cartilage rings, in herbivores
the attachment is internal. Man who lives on a mixed diet has
a muscle attachment which is internal but relatively close
to the ends of the rings. There are various exceptions to
these rules and Miller admitted that he has found no good
reason for this variation.

Von Hayek (1960) described the trachealis muscle as a
unified layer of thick bundles of smooth muscle transversly
oriented which insert by means of elastic tendons into the
perichondrium and annular ligaments. Occasionally isolated
bundles of longitudinal muscle are found external to the

main body of muscle. These insert into the tunica fibrosa

l4

and infrequently into the ventral wall of the esophagus.
Borelli (1949) observed that the muscle in man is heavier in
proportion to the rings at the top of the trachea than at

the bifurcation. Towers (1953) described the transition of
the trachealis muscle into the bronchial muscle. Greep (1954)
mentioned that there are many elastic fibers scattered among
the muscle cells. Krolling and Grau (1960) observed that
contraction of the trachealis muscle causes longitudinal

folds in the mucosa of the dorsal part of the tube.

Lymphatics

Sbernini and Bozzana (1953) reported two highly developed
anastamosing lymphatic networks in the trachea, one in the
mucosa and the other in the submucosa. The one in the mucosa
may be reduced with increasing age while the one in the sub-
mucosa shows no reduction during lifetime. Miller (1947)
stated that in the area of the cartilage rings the princi-
ple lymph vessels run parallel to the rings while in the mem-
branous portion of the trachea the vessels run primarily in

a longitudinal direction.

Blood vessels
Schaeffer (1953) and Edwards (1956) stated that branches

of the inferior and superior thyroid, bronchial and mammary

15

arteries furnish the arterial blood to the human trachea. Ven-
ous radicles come together in the annular ligaments and join
lateral veins on each side of the trachea which in turn empty
into a plexus of neighboring thyroid veins. Trautmann and
Fiebiger (1952) referred to a submucous, periglandular and
subepithelial plexus of vessels which supply blood to all

parts of the trachea in the domestic animals.

Nerves

One of the most thorough studies of the innervation of
the trachea was done by Honjin (1954) in the mouse. He found
all the ganglia concentrated in the adventitia of the dorsal
wall of the trachea. They are connected by nerves to form
a "...gangliated nerve meshwork..." called the tracheal plexus.
The trachea is supplied from the recurrent laryngeal nerves
or directly from the vagus. Branches from the right and left
cervical sympathetic cords join with the vagal branches and
innervate the trachea in company with the parasympathetics.
A few small sympathetic twigs innervate the trachea directly.
Elftman (1943) described various types of nerve endings in
the muscle, lamina propria, epithelium and perichondrium in
the dog. Larsell and Burget (1924) found that double vago-
tomy of rabbits and dogs abolishes the cough reflex. Sampaolo

(1950) observed Ruffini's corpuscles among the smooth muscle

16

fibers of the trachea. Trautmann and Fiebiger (1952) mentioned
the presence of both myelinated and non-myelinated nerve fi-

bers in the trachea.

Tracheal movements and structural exceptions

Macklin (1925), Oowdry (1950), Bruckner (1952), and Von
Hayek (1960) discussed passive changes in the trachea during
inspiration, expiration, swallowing, head movements, coughing
and changes in body positions. They showed that the trachea
is not a rigid stationary tube but continually undergoes
changes in length and diameter. Sauser (1957) correlated
movements in the respiratory system with differences of rig-
idity in various parts of the respiratory tract and noted in—
creased flexibility from top (nasal portion) to bottom (bron-
chioles).

Atwood (1947), Zeek (1951), and Cover (1953) noted inter-
esting exceptions to the general tracheal structure discussed

above in whales, penguins, aquatic and domestic birds.

ANATOMY OF THE TRACHEA OF THE DOG AND CAT

Although the microscopic structure of the dog and cat
trachea are basically the same as that discussed previously,

some differences and special features have been reported.

17

Lucas (1932) referred to the work of Haycroft and Carlier
who, in 1890, observed that pseudostratified epithelium of
the trachea in both the cat and the dog is gradually replaced
by stratified epithelium in the region where the cartilages
overlap. They believed that this results from friction.
Florey g§_§l, (1932) found in their experiments with cats
that there are no goblet cells present in the epithelium over
the posterior tracheal fold. However, they made no mention
of a change in the epithelium in this area. Loginoff (1911)
also mentioned that the cells of the epithelium in the dorsal
or membranous part of the cat trachea have a tendency to lose
their cilia and become flattened. Comparatively he noted
that the epithelial cells of the dog are usually somewhat
higher but smaller than those of the cat. In the dog the
cells in the epithelium over the membranous portion are
shorter than cells elsewhere in the trachea.

Oppel (1905) found goblet cells to be more numerous in the
dog than in man. The tracheal glands are described as less
numerous in the dog than in the pig and all acini lie medial
to the muscle. The basement membrane in the dog is very
thin and indistinct and even more so in the cat. The glands
in the submucosa of the cat are more numerous in the ventral

portion than in the lateral and dorsal areas and are

l8

concentrated in front of the annular ligaments. The gland
ducts are narrowed in the area of the elastic layer and are
enlarged where they open on the epithelial surface. Elastic
fibers penetrate the entire thickness of the hyaline car-
tilage rings in both the dog and cat.

Stirling (1883) described the external muscle attachment
in the dog and cat and the overlapping of cartilage rings
in the dorsal area. He also mentioned that many nerve ganglia
are found in the dorsal area and that they seem to be supplied
by the recurrent laryngeal nerves. The mucous glands of the
cat are more numerous and granular than they are in the dog.
The muscle does not seem to be as thick in the dog and in
both animals longitudinal muscle fibers exist outside of the
transverse ones. No gland acini occur outside the tracheal
muscle as they do in man.

Sussdorf (1911) stated that of all the domestic animals,
tracheal glands are most abundant in the cat and least abun-
dant in the dog. Isolated glands are occasionally found in
the adventitia outside of the cartilage rings and muscularis
in the dog and cat. The acini are both mucous and serous
and the submucosa is small and loosely built in both animals.
Patches of stratified squamous epithelium are occasionally

seen in the upper trachea of old cats.

19

According to Paul (1913) the tracheal epithelium in the cat
is thinner than in the horse and pig. He concluded that the
thickness of the epithelium increases with an increase in the
size of the animal. In the cat, goblet cells are more num-
erous ventrally and laterally than they are dorsally. Leu-
cocytes migrate from the submucosa through the lamina pro-
pria into the epithelium. Lymph nodules are more frequently
found in the ventral and lateral regions. He reported the
presence of flattened epithelium in the dorsal wall in the
trachea of the cat and the pig. The basement membrane in
the cat trachea is a fine structureless border frequently
covered with numerous leucocytes. The glands are limited to
the submucosa, are seldom found outside of the cartilage rings
and never found outside of the muscle.

Paul (1913) described the perichondrium in the cat as
rich in elastic fibers which penetrate the cartilages. The
internal perichondrium is thinner than the external one. The
annular ligaments are also composed of elastic fibers and the
cartilage is not entirely hyaline but more or less elastic
in character. The trachealis muscle is heavier at the distal
end than at the proximal end of the tube. No longitudinal
muscle fibers were found and no glands were seen between
the muscle bundles. The muscle in the cat is thinner and

smaller than in the pig and horse.

20

No references to any differences existing in the histology

of the male and female trachea were found.

21

MATE RIALS AND METHODS

The tracheae from twenty-five animals, fourteen dogs
and eleven cats, were used in this study. Eight dogs and
six cats were males, six dogs and five cats were females.
The animals were obtained from the departments of Anatomy
and Physiology at Michigan State University. The tracheae
were removed from the freshly killed animals and immediately
placed in F. A. A. (Lavdowsky's mixture, Guyer 1949) for
fixation. Those of two dogs were obtained from specimens
embalmed upon death and appeared in good condition.

Sections of each trachea, four or more cartilage rings
in length, were taken from three areas: an upper one immedi-
ately below the larynx, a middle area, and a lower one di-
rectly above the bifurcation. Both ventral and dorsal
longitudinal sections were taken from an upper and lower
area in four dogs and four cats.

After fixation the dehydrating and infiltrating method
of Johnson §§_al, (1943) was followed using "Tissuemat"* as
the embedding medium. Sections were cut at 7 microns and
stained with Harris' hematoxylin and eosin, Weigert's and

Van Gieson's Connective tissue stain to identify elastic

 

*Fisher Scientific Company, Pittsburgh, Pennsylvania

22

and collagenous tissue, toluidine blue (Lillie, 1954) for
determination of mucous acini and goblet cells, and von
Kossa's stain (Lillie, 1954) to aid in the identification
of calcium salts in the cartilage.
Using slides of both longitudinal and cross sections,

the following observations and measurements were made:

Epithelial thickness

Length of cilia

Goblet cell distribution

Elastic layer thickness

Elastic fiber diameter

Submucosal thickness

Distribution of gland acini

Cartilage thickness

Muscle thickness - measurements were taken across

the widest area of the muscle bundle.
Presence and location of nerve trunks and ganglia.
The measurements were made in microns with the aid of an

ocular micrometer and are recorded in Tables I and II. Gob-
let cells and gland acini were recorded as (l), (2), or (3)

depending on the quantity present.

23

RESULTS AND DISCUSSION

I. THE TRACHEA OF THE CAT

The measurements of the various structural elements of

the cat trachea referred to below, are found in Table I.

Epithelium

The trachea of the cat is lined with pseudostratified
ciliated columnar epithelium containing goblet cells. Basal
cells and intermediate or non—ciliated cells are also present,
thus the four types of cells described in man, by Lucas (1932)
and Miller (1932) can be identified here. Ciliated cells are
much more numerous than goblet cells. The distal surface of
the cells is covered by microvilli which appear as a thin
dark membrane under the microscope. The cilia average
4.0 u in length and appear to arise from the striated border.
As a rule they are longer and appear more numerous over
the ventral part of the trachea than the dorsal area.

Goblet cells are more numerous in the ventral and lateral

than the dorsal epithelium (this was reported by Paul 1913),
however, no trend appears in the number of goblet cells be-
tween the proximal and distal areas. Intermediate or devel-

oping columnar cells are infrequently seen and are difficult

24

to identify while basal cells are more numerous rounded cells
whose nuclei lie nearest the basement membrane.

In agreement with Oppel (1905) and Paul (1913) the base-
ment membrane in the cat trachea is indistinct and difficult
to see with ordinary stains. In many areas, however, it can
be distinguished as a very thin, light seam or border between
the epithelium and the lamina propria. It is frequently
covered by leucocytes.

The epithelium is often folded especially in the dorsal
region. For this reason an oblique section through a fold
can result in what appears to be an intraepithelial pocket
lined with cilia and containing cellular debris as illustrated
in Plate III.

In the dorsal area of the trachea the epithelium differs
from the usual pseudostratified type to one similar in appear-
ance to transitional epithelium found in the urinary tract
(Plate IV). This epithelial transition in the cat has been
described by Haycroft and Carlier (Lucas 1932), Loginoff (1911)
and Paul (1913) who refer to the epithelium in the dorsal area
as a flattened stratified type. This epithelium was found to
a greater or lesser extent in all the cats used in the study
and varied in height from 2 - 8 layers of cells. At times it

spanned the entire area between the ends of the cartilage rings

25

while in other cases patches were found medial to both ends
of the cartilage rings and pseudostratified epithelium was
present between them. Occasionally it was located dorso—
laterally rather than directly above or between the ends of
the cartilage rings. In places where the transitional epithe-
lium was stretched the cells became flattened and appeared
similar to the cells in stratified squamous epithelium. The
amount of transitional epithelium varied at different levels,
but no definite trend could be established. The transition
between the two types of epithelia is often abrupt (Plate V)
but in many instances it is more gradual. In agreement with
the findings of Florey §£_§l, 1932, cilia and goblet cells
are absent in the transitional-type epithelium thus accounting
for the decreased amount of these elements in the dorsal
region. Adjacent to the junction of the epithelial tran-
sition, the pseudostratified epithelium as well as the cilia
it bears are usually lower than elsewhere in the trachea.
The average thickness of the epithelium is greater in the
ventral area than in the lateral and dorsal regions.

By examining a longitudinal section through the ventral
part of the trachea another interesting variation in the
epithelium becomes apparent. The epithelium is thicker and

the goblet cells more numerous over the glandular areas than

26
over the cartilage rings (Plate VI). Table III shows that
there is an average difference in thickness of about 4 u
between the epithelium covering the two areas. The dif-
ference between the epithelial thickness over the two areas
was found to be analytically significant at a 1% level in
the cats. The epithelium above the concentrated glandular
areas bulges into the lumen (Plates VI and VIIa). A change
in the height of the epithelium and number of goblet cells is
not recognizable in the dorsal region where the cartilage
rings overlap and the glands are more evenly dispersed.

Leucocytes are frequently found among the cells of both
types of epithelium and can be easily mistaken for epithelial
cell nuclei. Lymphocytes are the most numerous but poly-
morphonuclear leucocytes were seen frequently (Plate VIII).
Lymphocytes are occasionally concentrated in nodules in the
submucosa and invade the epithelium in large numbers. Many
times they appear to erode it away (Plate IX). Lymphoid in-
filtration of the epithelium does not occur to the same ex-
tent in each animal. Lymphocytes are usually more numerous
in and below the epithelium in the ventral and lateral re-
gions than in the dorsal area which is in agreement with the
findings of Paul (1913). Lymphoid infiltration occurs to a
greater extent in the epithelium that lies over concentrated
gland areas. Andrew and Burns (1947) reported that in

the rat most of the leucocytes were found at or below the

27

level of the epithelial nuclei; the ones near the surface of
the epithelium are usually pycnotic. This is also true in

the tracheal epithelium of the cat.

Lamina Prgpria and Elastic Membrane

The lamina propria in the trachea of the domestic animals
has been defined by Trautmann and Fiebiger (1952) as the area
between the basement membrane and the elastic layer. It is
composed of elastic fibers, a few fine collagenous fibers
and usually many lymphocytes. It varies in thickness from
15 to 40 u and does not blend with the submucosa. The
elastic layer is often considered as a deeper or external
part of the lamina propria.

The elastic layer or band consists of many longitudinally
oriented elastic fibers closely associated into a compact
layer (Plate X) which varies in average thickness from 28 to
33 u. The band is thickest in the dorsal region, however,
the fibers in this area are less closely packed; a fact which
appears to be related to the number of glands in the under-
lying submucosa as well as the thickness of the submucosa.
The elastic layer is thinnest in the lateral areas. In
longitudinal section through the ventral region it is ob-
served that the fibers over the bulging gland areas are more

tightly packed than they are over the areas in between.

28

The individual elastic fibers average 2.7 u in diameter
and no large amount of variation in average fiber size is dis-
tinguishable in the various areas around the lumen. The fibers
are not all the same size however; the smaller ones are
found in greatest numbers on the medial surface of the mem-
brane and are scattered through the lamina propria. Fibers
extend from the membrane on both surfaces, many extend up to
the basement membrane and a few mingle with fibers in the
submucosa and annular ligaments. A few fine collagenous
fibers could be distinguished scattered among the heavier
elastic fibers of the membrane. Lymphocytes are frequently
found migrating through the elastic membrane often in large
numbers (Plate IX). The fibers bend around gland ducts

which traverse the membrane.

Submucosa

This area is bounded internally by the elastic membrane
and externally by the cartilage rings and annular ligaments.
It is composed of a network of collagenous and elastic
fibers, blood vessels, fat cells, glands and leucocytes.
In longitudinal section through the ventral region the sub-
mucosa is much thicker in the area of the glands between the

cartilage rings, than directly medial to the rings (Plate VIIa).

29

Stowe and Calhoun (1961) described this characteristic in the
mink trachea. Averaged measurements derived from both longi-
tudinal and cross sections show the submucosa to be thickest
ventrally, thinnest laterally and of medium thickness
dorsally.

The glands in the cat compose a large percentage of the
submucosa and, as stated by Oppel (1905), increase in abun-
dance dorsally to laterally to ventrally and are concentrated
in areas medial to the annular ligaments (Plates VI & VIIa).
They are so abundant in the ventral area that the mucosa pro-
trudes into the lumen (Plate VI). In the dorsal region the
glands are not concentrated in specific areas but are scat—
tered along the entire length of the tube (Plate VIIb).

Both mucous and serous acini are present independently
as well as mucous acini with serous demilunes (Plate XVIII).
By counting the number of acini with and without mucus, the
percentage of mucus secreting acini was roughly estimated to
be about 25 percent.

The gland secretions reach the surface by means of nu-
merous ducts. The ducts vary in size and the walls of the
larger ones are often composed of a stratified layer of
cells while the smaller ones are usually lined by a single

layer of pyramidal cells. As Oppel (1905) explained, the

30

ducts are narrowed as they pass through the elastic membrane
but often enlarge as they open on the epithelial surface.
Cilia are found lining the duct openings and in the larger
ducts they often extend as far as the acini. Collagenous
fibers could be seen surrounding and scattered among the
gland acini. Elastic fibers are less frequently seen among
the glands.

Lymph nodules often occur in the submucosa especially
in the ventral and lateral regions and are most frequently
situated in the glandular areas.

Sussdorf (1911) reported that gland acini are occasionally
found in the adventitia outside the cartilage rings. An ex-
ample of this is shown in Plate XII. Acini are also rarely
found in the lamina propria (Plate XIII) and in the area of
the annular ligaments (Plate XIV). Acini were lacking out-
side the trachealis muscle which agrees with the reports

of Stirling (1883) and Paul (1913).

The Cartilage Rings and Annular Liqaments

Plate II is a drawing illustrating the change in shape
and relationship of the cartilage rings as they surround the
tracheal lumen. Examination of the sections and measurements
reveal that the rings decrease in width ventrally to laterally

to dorsally, and that they are heavier in the proximal portion

31

of the trachea than in the distal part. In longitudinal sec-
tion the rings appear more or less oval shaped in the ventral
area having a flattened external surface and a rounded inner
surface. Von Hayek (1960) described this characteristic in
man and observed that the convexity of the inner surface of
cartilage rings permits supportive action similar to that of
the perpendicular ridge of a T-beam. The rings appear ellip-
tical in the lateral area and spatulate dorsally (Plate II).
This is similar to the description given by Stowe and Calhoun
(1961) for the mink. In the dorsal area where the rings be-
come greatly flattened they overlap each other in random
fashion (Plates II & VIIb). Because of this it is common,

in cross section, to see two p1ates.of cartilage adjacent to
one another in the dorsal lateral region (Plate XV).

The cartilage rings are hyaline in nature and surrounded
by a perichondrium which is composed primarily of collagenous
fibers. This contradicts the statements of Paul (1913) and
Oppel (1905) who described the perichondrium as composed
primarily of elastic fibers and the cartilages as somewhat
elastic in composition. In agreement with Paul (1913) the
external perichondrium is thicker than the internal one.
Elastic fibers are found in the outer perichondrium and are

present in small amounts in the inner one. The collagenous

32

fibers penetrate the periphery of the rings in large numbers.
Spanning the area between the ends of the cartilage rings is

a rather heavy collagenous sheet which appears continuous with
the perichondrium and annular ligaments. Bifurcation of the
rings does occur but not frequently as it does in man (Miller
1947).

The annular ligaments are observed best in longitudinal
section through the ventral area as they link the cartilage
rings to one another. They are composed primarily of col-
lagneous fibers but many elastic fibers are present and
appear more numerous in the external part of the ligaments.
Between the rings the ligaments are thicker in the ventral
area than in the dorsal region. They appear to be contin-
uous with the perichondrium. Dorsally the ligaments can
still be distinguished as they attach to the edges of the
flattened, overlapping rings but they consist of relatively
few bundles of fibers. Gland acini occasionally invade the

area of the annular ligaments (Plate XIV).

Trachealis Muscle
The muscle is composed of bundles of transversly oriented
smooth muscle fibers. In the cat the trachealis muscle lies

external to the cartilage rings to which it attaches in the

33

lateral regions. Measurements indicate that the muscle is
heaviest in the area near the larynx and decreases in size
toward the bifurcation. This trend is contradictory to the
one expressed by Paul (1913) who stated that the muscle
increases in size from larynx to bifurcation. The muscle
is thickest in the area between the ends of the cartilage
rings and tapers at its point of attachment to the outer
perichondrium and annular ligaments. The end of the muscle
blends with transverse elastic fibers which attach to the
outer perichondrium (Plate XV). Medial to the muscle the
outer perichondrium is lacking in elastic fibers. External
to the muscle is a thin sheet composed of collagenous and
elastic fibers. Loose connective tissue is often found
between this sheet and the muscle. This fibroelastic sheet
blends with the outer perichondrium at the point of muscle
attachment. In contradiction to Stirlings' (1883) report
but in agreement with Paul (1913) no examples of longitud-
inally oriented muscle fibers or gland acini were found with-

in or outside the muscle.

AdventitiaL Nerves and Vessels
The adventitia is the outermost layer and is composed

of loose connective tissue containing many blood vessels,

34

nerve trunks, ganglia and occasional lymph nodules. In accor-
dance with the findings of Elftman (1943) and Honjin (1954)

in the dog and rat respectively, the nerve trunks and ganglia
are concentrated in the dorsal adventitia external to the
muscle (Plates XV & XVI). Nerve trunks are quite frequently
found in the lateral adventitia and occasionally in the
ventral area. Ganglia are rarely seen ventrally but are

more numerous in the dorsal and lateral regions. No nerve
trunks or ganglia were observed medial to the cartilage rings.
Blood vessels or varying size are found in abundance in the

adventitia.

Sexual Differences

Comparative measurements of male and female animals show
that differences between them are minimal. However, the
averages of many measurements on both sexes indicate that in
the females the epithelium, elastic layer, and submucosa are
all of slightly larger proportions than in the males. The
cartilage rings, on the other hand, are lighter and thinner
in the females. The other factors remain unchanged between

the sexes.

35

TABLE I#

THE CAT

TRACHEAL MEASUREMENTS IN MICRONS

 

 

 

DLV Total
*a b c averages average
Epithelial
thickness **D 33.9 37.4 42.6 37.9
L 35.6 40.4 37.8 37.9 40.2
V 46.4 47.4 40.9 44.9
Ciliary length
D 3.4 3.9 4.1 3.8
L 3.7 4.0 4.2 4.0 4.0
V 3.8 4.5 4.5 4.3
Goblet cell ***
distribution D 1.9 2 2 2
L 2 2.2 2.3 2.2 2.1
V 2.5 2.2 1.9 2.2
Elastic layer
thickness D 32.2 35.6 32.1 33.6
L 25.5 27.3 21.6 24.8 29.0
V 27.5 26.0 32.5 28.6
Elastic fiber
diameter 2.9 2.8 2.6 2.7 2.7
Submucosal
thickness D 124.8 94.1 150.5 123.1
L 96.5 92.6 117.8 102.3 136.4
V 131.7 188.5 231.8 184.0
Distribution ***
of glands D 1 2 l 1
L 2 2 2 2 2
V 2 3 3 3
Cartilage
thickness D 133.6 131.8 119.5 128.3
L 403.1 302.7 336.4 347.4 319.3
V 567.2 359.3 357.5 482.2
Muscle
thickness D 98.0 87.4 33.7 73.0 73.0

36

TABLE I (Continued)

 

#Every figure in the Table represents an average of four to
six measurements obtained from each of ten animals.

*a--Proximal level **D——Dorsa1 ***1--Scarce
b—-Middle level L—-Lateral 2--Average
c--Distal level V-—Ventral 3-—Abundant

37

 

 

 

TABLE II#
THE DOG
TRACHEAL MEASUREMENTS IN MICRONS
DLV Total
a b c averages average
Epithelial
thickness **D 42.3 35.8 41.7 39.9
L 41.1 38.2 42.8 40.7 40.9
V 48.1 39.1 42.3 43.1
Ciliary length
D 6.0 6.0 5.2 5.7
L 5.4 6.4 5.6 5.8 5.7
V 6.2 5.5 5.9 5.8
Goblet cell
distribution D ***2 2 2 2
L 2.5 2.3 2.3 2.3 2.2
V 2 2.2 2.5 2.2
Elastic layer
thickness D 155.8 84.8 62 100.8
L 68 55 87.8 70.2 94.3
v 83.7 110 142.7 112.1
Elastic fiber
diameter 2.9 3.4 2.9 3.0 3.0
Submucosal
thickness D 426.6 369.4 290.1 362.0
L 309.7 308.8 280.8 299.7 328.9
V 406.9 348.5 220.0 325.1
Distribution
of glands D ***l 1 l 1
L 2 l 1 1 1
V l l 1 l
Cartilage
thickness D 303.7 221.8 216.2 247.2
L 700.0 555.1 492.5 582.5 538.2
V 998.0 722.3 634.6 784.9
Muscle
thickness D 433.2 319.5 273.4 342.0 342.0

38

TABLE II (Continued)

 

#Every figure in the table represents an average of four to
six measurements obtained from each of ten animals.

*a--Proxima1 level **D—~Dorsa1 ***1--Scarce
b—-Midd1e level L--Latera1 2--Average
c--Distal level V-—Ventral 3--Abundant

39
II. THE TRACHEA OF THE DOG

In the following pages the histology of the dog trachea
will be discussed and compared to that of the cat.- The
measurements of the various structural elements of the trachea

referred to below are in Table II.

Epithelium

(Pseudostratified ciliated columnar epithelium containing
goblet cells is also found in the trachea of the dog. The
cells are very similar to those of the cat. The average
thickness of the epithelium is very nearly the same as in the
cat. It decreases in thickness ventrally, laterally to dor-
sally which agrees with Loginoff (1911) who reported that the
epithelial cells of the dog are shorter over the membranous
part of the trachea than elsewhere. The cilia are longer
in the ventral area than in the dorsal regibn but there is
no consistent modification in their length between the proxi-
mal and distal areas. The cells are covered on their distal
surface by microvilli. Goblet cells are found in relatively
the same quantity in the dog trachea and are similarly
less abundant over the dorsal area. Intermediate and
basal cells could also be identified. The basement membrane

appears as an indistinct transparent border along the base

40

of the epithelial cells. It is often obliterated by
longitudinal elastic fibers that are most obvious in a
longitudinal section.

A modified type of epithelium similar to transitional
is also found in the dorsal area of the dog trachea but in
much smaller amounts than in the cat (Plate XVII). When
present it is most often found directly medial to the tips
of the cartilage rings and is lower than the pseudostratified
epithelium. Occasionally it spans the entire distance
between the ends of the rings and at times it is absent al-
together. Cilia and goblet cells are not found in the tran-
sitional-type of epithelium. The pseudostratified epithelium
is usually highly folded in a longitudinal direction over
the membranous part of the tube where it protrudes into the
lumen of the fixed specimen. This folded epithelium displays
the characteristic pattern of alternating ridges and creases
as seen in cross section on Plate XVII. The folds are smal-
ler and more numerous than in the cat.

In the ventral region the epithelial lining is thicker in
the area of the annular ligaments than in the region medial
to the rings (Table III). The difference in epithelial thick-
ness between the two regions is less in the dog (significant

at a 10% level) than it is in the cat (significant at a 1% level).

41

Goblet cells generally do not appear more numerous in
the epithelium over the annular ligaments and the epithelium
in this area does not ordinarily bulge into the lumen as it
does in the cat. In the dorsal region the epithelium is

uniform in thickness.

TABLE III

MEASUREMENTS OF EPITHELIAL THICKNESS IN LONGITUDINAL SECTION*

 

 

Epithelial thickness in microns

 

Over annular Over cartilage

ligaments rings

Cat number 1 45 41
2 44 40

3 62 59

4 40 35

Average 48 44
Dog number 1 40 32
2 57 55

3 42 41

4 35 36

Average 43 41

 

*Each figure represents an average of eight measure-
ments taken from the ventral area of the trachea of each
animal. Using the analysis of variance test on the original
measurements, the difference between the epithelial thick-
ness in the two areas was found to be significant at a 1%
level in the cats but at a 10% level in the dogs.

Leucocytes appear in the epithelium and are usually found
at the level of or below that of the epithelial nuclei; those

near the surface are often pycnotic.

42

Lamina Propriai Elastic Layer,iand Submucosa

It is in these areas that some of the greatest differences
exist between the trachea of the dog and cat. For an illus—
tration of these differences refer to Plates X and XI. The
lamina propria cannot be identified as a distinct and separate
area in the dog as it is in the cat. The area directly below
the epithelium composed of fine collagenous and elastic fibers,
many leucocytes and other connective tissue cells may be con-
sidered as the lamina propria, however, this area blends with
the submucosa (Plate XI).

The elastic layer is not a compact band as in the cat but
consists of numerous, loosely arranged, longitudinal fibers
scattered through the lamina propria and submucosa. The layer
varies greatly in thickness but is usually narrowest in the
lateral region and becomes wider in the ventral and dorsal areas.
No trend in the thickness of the elastic layer is evident among
the various levels. Just beneath the epithelium in the dog
trachea are two single layers of longitudinal elastic fibers
separated from each other by approximately 10 — 20 u
(Plate XI). The medial layer is composed of fibers measuring
about 1.5 u in diameter which lie in close contact with the

basement membrane. The external layer is composed of coarser

43

fibers having an average diameter of 3 u or more. Between
the two layers is a small portion of the lamina propria con-
taining a network of fine collagenous and a few small elastic
fibers. Both layers are not always distinguishable.

External to these layers the elastic and collagenous
fibers gradually increase in size and density toward the
cartilage rings. Occasionally the elastic layer extends
through the entire width of the submucosa. Most often it ex-
tends about one third of the distance from the basement mem-
brane to the perichondrium, however, the relationship between
the submucosa and the elastic layer is subject to great vari-
ation. The heavier elastic fibers are most numerous in the
lateral part of the layer where they appear to branch often
and are frequently oriented in an oblique or transverse
direction. The area external to the elastic layer and inter-
nal to the perichondrium and annular ligaments is relatively
free of large elastic fibers but is very rich in collagenous
tissue. The width of this area varies with the extent of
the elastic layer and the width of the cartilage rings as
seen in longitudinal section.

Strands of collagenous fibers are found among and around
the gland acini often fusing with the perichondrium and an-

nular ligaments. Blood vessels, fat cells, occasional nerve

44

trunks. leucocytes and glands are scattered through the sub-
mucosa. Lymph nodules are not as abundant in the submucosa
of the dog as in the cat and consequently lymphoid invasion
of the epithelium, which usually occurs near a nodule, is less
frequently seen. The submucosa is thinner in the lateral
region than it is dorsally and ventrally. It also decreases
in thickness proximally to distally. Ventrally and laterally
it is widest in the area of the annular ligaments.

Stirling (1890) noticed that the tracheal glands in the
cat are more numerous than in the dog. Sussdorf (1911)
rePkbrted that of all the domestic animals tracheal glands
are most numerous in the cat and least numerous in the dog.
Cc’1'1'l£>arison of Plate VII and XIX illustrates that the tracheal
glands are much more numerous in the cat than the dog. The
gland acini are the same type as those found in the cat.
stirling (1883) mentioned that the mucous acini in the cat
are more granular than in the dog. This difference, if it
exists, is very minute. The percentage of mucus secretingacini
in the dog was estimated to be about 42 percent which is
greater than in the cat. The gland acini are scattered
thr<>11gh the submucosa and lamina propria, many are embedded
in the elastic layer (Plate XI) while others are situated

lateral to it. There is no visible difference in the quantity

45

of glands between the ventral and dorsal regions, however,
they are more numerous in the area of the annular ligaments
than elsewhere. The small groups of acini are usually sepa-
rated by expanses of collagenous and elastic tissue. Rarely
are acini found outside the cartilage rings or in the annular
ligaments. No acini were seen outside of the trachealis
muscle which agrees with the report of Oppel (1905).

The gland ducts although less numerous have the same
construction as those of the cat. The walls of the larger
ones are often composed of a stratified layer of cells and
are lined for long distances with cilia (Plate XX). The
»smaller ducts consist of a single layer of pyramidal cells.
Some-ducts become narrow as they open on the epithelial
surface others enlarge at their openings. The ducts do not
become narrowed in the area of the-elastic layer as they

do in the cat.

Cartilage Rings and Annular Liqaments

The cartilage rings in the dog trachea are similar to
those of the cat in shape (Plate II) but differ in size
(Plate I). They are thicker in the ventral region and de-
crease in size dorsally. A similar decrease in size is ob-
served from the proximal part of the trachea to the distal

end. .The rings have the same general shape as those of the

46

cat in cross section: oval ventrally (flattened exterior
surface, rounded interior surface), elliptical laterally and
spatulate dorsally where they overlap each other. There ap-
parently is no definite order or pattern in which they over-
lap.

The rings are composed of hyaline cartilage and are
surrounded by a collagenous perichondrium which is thicker
on the external surface than the internal one. Elastic fi-
bers are present in small amounts in the perichondrium of
both surfaces. A heavy sheet of collagenous tissue which
appears continuous with the perichondrium and annular liga—
ments spans the ends of the incomplete rings.

In some of the dogs and cats the central portion of the
rings in the ventral area appeared degenerated, highly
vacuolated and stained very lightly with hematoxylin and
eosin stain. This was first assumed as indicative of cal-
cification, however, the results of Von Kossa's stain showed
that no significant amount of calcification had taken place
in any part of the cartilage rings of either the dogs or the
cats used. It should be noted here that the exact age of
many of the animals was not known.

The annular ligaments are heaviest between the rings in

the ventral area and are heavier in the dog than the cat.

47

They are composed of numerous collagenous fibers, few elastic
fibers, and seem to blend with the perichondrium. In longi-
tudinal section the ligaments are often seen folded inward
toward the mucosa. In the gross specimen lengthwise tension
on the trachea causes movement in the area of the annular
ligaments. Thus the ligaments not only connect the carti-
lage rings but allow movement between them. In the dorsal
area the ligaments can be discerned connecting the overlapping

edges of the flattened rings.

Trachealis Muscle

The transverse smooth muscle bundles are located outside
the cartilage rings and attach to the external perichondrium
in the lateral areasby means of elastic fibers. The average
width of the muscle is 269 u thicker than in the cat which
does not agree with Stirling (1883) who reported that the
trachealis muscle of the dog did not seem to be as thick as
in the cat. The muscle decreases in thickness from the
larynx to the bifurcation. The area of muscle attachment is
similar to that of the cat. Outside of the muscle lies a
thin sheet of fibroelastic tissue similar to that in the cat.
which blends with the outer perichondrium at the point of
muscle attachment. Loose connective tissue is frequently

found between this sheet and the muscle. The muscle is widest

48

in the region between the ends of the cartilage rings and ta-
pers to its point of attachment. No examples of longitudinal
muscle fibers were found outside the transverse bundles and
no gland acini were found within or outside the trachealis

muscle.

Adventitia, Nerves and Vessels

The adventitia is composed of loose connective tissue
and contains numerous blood vessels, nerve trunks, ganglia
and occasional lymph nodules. Blood vessels of varying sizes
and oriented in both transverse and longitudinal planes are
found here.

Nerve trunks and ganglia are concentrated in the dorsal
area external to the muscle like in the cat. Nerve trunks
are found occasionally in the lateral area but only rarely
in the ventral adventitia. Ganglia are found only in the
lateral and dorsal adventitia. Occasionally large nerve
trunks are found in the submucosa of the dorsal area (Plate
XXI).

The tissues spanning the membranous part of the dog and
cat trachea from the lumen outward are: mucosa, submucosa,
collagenous layer continuous with the perichondrium and an-

nular ligaments, trachealis muscle, thin fibroelastic membrane

49

and loose connective tissue of the adventitia containing

vessels and nerves (Plate XXII).

Sexual Differences

Sex variations in the dog trachea are in general less
apparent than in the cat. The only trend which appears on
the basis of measurements is the presence of thinner
cartilage rings in the females than in the males. These
sex differences in both the dog and cat are not of substan—
tial importance because of the size variations among the
various breeds of animals used in this study.

For an illustrated summary of some of the histological
differences between the trachea of the cat and dog see

Plate I.

50

SUMMARY AND C ONCLUS I ONS

Comparative histological studies of the trachea were
made on fourteen dogs and eleven cats, all adults of both
sexes. Routine techniques were used in preparation of the
sections for microscopic study.

The tracheae of the dogs are larger in every respect
than those of the cat. No substantial differences exist in
the average thickness of the epithelium, the elastic fiber
diameter and goblet cell distribution. Except for those
characteristics mentioned, no changes were found among the
various tracheal levels in either species.

Sex differences in both species are minimal but there is
one trend in common, the cartilage rings are thinner in the
females than the males.

A transitional-type of epithelium is commonly found in
the dorsal tracheal wall of both species. It is present to
a greater extent in the cat. Most frequently it is located
directly medial to the ends of the cartilage rings and in
the membranous region of the tube. The change from pseudo-
stratified ciliated columnar to a transitional-type of epi-
thelium is suggestive of continual movement of the mucosa in
the membranous part of the trachea and especially so at the
junction of the membranous and cartilagenous areas.

In the ventral area the thickness of the pseudostratified

epithelium increases in the region of the annular ligaments

51

and decreases near the central portion of the cartilage rings.
The difference is greater and more significant analytically in
the cats than in the dogs. This variation becomes less
obvious in the dorsal area of both species.

The cilia are shorter over the dorsal than the ventral
and lateral areas in both species.

Goblet cells are found in decreased amounts in the dorsal
epithelium of both the dog and cat. In the cat however, they
are abundant in the epithelium over the concentrated gland
areas in the ventral part of the trachea.

The lamina propria represents a distinct area in the cat and
the elastic layer is a compact band composed of longitudinal
elastic fibers located in the outer part of the lamina propria.
In the dog the lamina propria blends with the submucosa and the
elastic layer is a wide band of loosely associated fibers scat—
tered through both areas. In both species the larger fibers are
most numerous in the lateral part of the elastic band.

The submucosa decreases in thickness from the proximal
to distal region in the dog but from the distal to proximal
to middle area in the cat. In both species it is thinnest
in the lateral region and in longitudinal section it is
wider medial to the annular ligaments than medial to the

cartilage rings. Shrinkage of the cartilage rings due to

52

fixation often modifies the width of the elastic layer and
submucosa in the dorsal area.

Tracheal glands are very numerous in the cat and compose
a large percentage of the submucosa. They increase in
abundance dorsally to laterally to ventrally. Gland acini
are occasionally seen outside of the cartilage rings, in the
lamina propria and among the annular ligaments. In the dog
the glands are scarce and collagenous and elastic tissues
compose the bulk of the submucosa. The acini are more evenly
dispersed around the lumen in the dog than in the cat and
they are rarely seen outside the cartilage rings or among
the annular ligaments in the dog. In both species the glands
consist of mucous, serous and mixed acini. The percentage
of mucus secreting acini is estimated to be less in the cat
than the dog. Gland acini are not found within or outside
the trachealis muscle. In both the dog and cat the glands
are concentrated medial to the annular ligaments in the ven-
tral and lateral areas.

The shape of the cartilage rings is the same in the
dog and the cat. They are composed of hyaline cartilage
and are surrounded by a collagenous perichondrium. In lon-
gitudinal section through the ventral area the rings have a

flattened external surface and a rounded internal one.

53

They are elliptical laterally and flattened in the dorsal
region where they overlap. The outer perichondrium is
thicker than the internal one. The rings decrease in
thickness proximally to distally as well as ventrally to
dorsally. No significant amount of calcification was en-
countered in either species. Bifurcation of the rings does
not occur frequently.

The collagenous annular ligaments are most dense in the
ventral area in both species. Elastic fibers are present
in small amounts. A collagenous layer apparently continuous
with the perichondrium and annular ligaments spans the area
between the ends of the rings.

The trachealis muscle lies external to the cartilage
rings and attaches in the lateral region in both species.
The muscle decreases in size from the laryngeal region to
the bifurcation and is widest in the area between the ends
of the rings. It becomes thinner where it attaches by means
of elastic fibers to the external perichondrium.

In both the dog and cat the nerve trunks and ganglia
are concentrated in the dorsal adventitia. Occasionally
they are found laterally and ventrally. Only in the dog

are nerve trunks seen in the dorsal submucosa.

54

LITERATURE CITED

Andrew, W. and M. R. Burns. 1947. Leucocytes in the tracheal
epithelium of the mouse. J. Morph. 81:317-141.

Atwood, W. H. 1947. A Concise Comparative Anatomy. C. V.
Mosby Co., St. Louis.

Barclay, A. E., K. J. Franklin and R. G. Macbeth. 1938.
Roentgenographic studies of the excretion of dusts from
the lungs. Am. J. Roentgenol. 39:673-686.

Borelli, C. 1949. Sull'architettura del muscolo tracheale
dell'uomo. Atti della Societa Midico-Chirurgica di
Padova e della Facolta di Medicina e Chirurgia della
Universita di Padova. Padua. 27:122-125. (Excerpta
Med. 4:1950, sec. 1, article 1335.)

Bremer, J. L. and H. L. Weatherford. 1946. A Textbook of
Histology. The Blakiston Co., Philadelphia.

Bruckner, H. 1952. Die Anatomie der Luftrohre beim lebenden
Menschen. (Eine Untersuchung Mittels Stereoskopie des
Rbntgenbildes.) Z. Anat. Entw. Gesch. 116:276-298.
(Excerpta Med. 7:1953, sec. 1, article 386.)

Chambers, R. and G. S. Renyi. 1925. The structure of the
cells in tissues as revealed by microdissection. I.

The physical relationships of the cells in epithelia.
Am. J. Anat. 35:385-402.

Chauveau, A. 1873. Comparative Anatomy of the Domesticated
Animals. W. R. Jenkins, New York.

Copenhaver, W. M. and D. D. Johnson. 1958. Bailey's Text-
book of Histology. Williams and Wilkins Co., Baltimore.

Cover, M. S. 1953. Gross and microscopic anatomy of the
respiratory system of the turkey. II. The larynx,

trachea, syrinx, bronchi and lungs. Am. J. Vet. Res.,
14:230-248.

Cowdry, E. V. 1950. Textbook of Histology. Lea and Febiger,
Philadelphia.

55

Dawson, H. L. 1948. Lambert's Histology. The Blakiston
Co., Philadelphia.

Edwards, L. F. 1956. Concise Anatomy. McGraw-Hill Book
Co. Inc., New York.

Elftman, A. G. 1943. The afferent and parasympathetic
innervation of the lungs and trachea of the dog. Am.
J. Anat. 72:1-27.

Florey, G., H. M. Carleton and A. Q. Wells. 1932. Mucus
secretion in the trachea. Brit. J. Exp. Path. 13:269-284.

Garvin, H. S. D. 1957. A Students Histology. E. & S.
Livingstone Ltd., London.

Greep, R. O. 1954. Histology. The Blakiston Co. Inc.,
New York.

Guyer, M. F. 1949. Animal Micrology. University of Chicago
Press, Chicago, Illinois.

Ham, A. W. 1951. HistolOgy, J. B. Lippincott Co., Phila-
delphia.

Honjin, R. 1954. On the ganglia and nerves of the lower
respiratory tract of the mouse. J. Morph. 95:263-287.

Johnson, E., F. N. Andrews and C. L. Shrewsbury. 1943.
The preparation of muscular tissue for histological
study. J. Anim. Sc. 2:244-250.

Jordan, H. E. 1952. A Textbook of Histology. Appleton-
Century-Crafts, Inc., New York.

Kr6lling, O. and H. Grau. 1960. Lehrbuch Der Histologie und
Vergleichenden Mikroskopischen Anatomie Der Haustiere.
Paul Parey, Berlin.

Larsell, O. and G. E. Burget. 1924. The effects of mech-
anical and chemical stimulation of the tracheo-bronchial
mucous membrane. Am. J. Physiol. 70:311-321.

Lillie, R. D. 1954. Histopathologic Technic and Practical
Histochemistry. The Blakiston Co., New York.

56

Loginoff, W. 1911. Zur Morphologie die Flimmerzellen des
Trachealepithels einiger Haussaugetiere. Anat. Anz.
38:353-361.

Lucas, A. M. 1932. Ciliated epithelium. Cowdry's Special
Cytology. Paul B. Hoeber, New York.

Lucas, A. M. and A. C. Douglas. 1935. Principles under-
lying ciliary activity in the respiratory tract. Arch.
Otolarying. 21:285-296.

Macklin, C. C. 1922. A note on the elastic membrane of the
bronchial tree of mammals, with an interpretation of its
functional significance. Anat. Rec. 24:119-136.

Macklin, C. C. 1925. X-ray studies on bronchial movements.
Am. J. Anat. 35:303-329.

Maximow, A. S. and W. A. Bloom. 1957. Textbook of Histology.
W. B. Saunders Co., Philadelphia.

Miller, W. S. 1904. The carina tracheae of the domestic cat.
Anat. Anz. 25:377-382.

Miller, W. S. 1920. A morphological study of the tracheal
and bronchial cartilages. Cont. to Embryol., Carnegie
Inst. Publ. No. 272. 9:285-298.

Miller, W. S. 1932. The epithelium of the lower respiratory
tract. CowdrytsSpecial Cytology. Paul B. Hoeber, New
York.

Miller, W. S. 1947. The Lung. Charles C. Thomas, Spring-
field, Illinois.

Nonidez, J. F. and W. F. Windle. 1949. Textbook of Histology.
McGraw-Hill Book Co. Inc., New York.

Oppel, A. 1905. Atmungsapparat, Lehrbuch der Vergleichenden
Mikroskpischen Anatomie der Wirbeltiere. Von Gustav
Fischer., Jena.

Paul, O. 1913. Beitrage Zur vergleichenden Histologie der
Trachea von Pferd, Schwein und Katze. Inaug. Diss.
Leipzig.

57

Rhodin, J. 1959. Ultrastructure of the tracheal ciliated
mucosa in rat and man. Annals of Otol., Rhin., and
Laryngol. 68:964-974.

Rhodin, J. and T. Dalhamn. 1955. Electron microscopy of
collagen and elastin in the lamina propria of the tracheal
mucosa of rat. Exptl. Cell Res. 9:371-375.

Ropes, M. W. 1930. Phagocytic activity and morphological
variations of the ciliated epithelial cells of the trachea
and bronchi in rabbits. Cont. to Embryol. Carnegie Inst.
Publ. No. 414. 22:77-90.

Sampaolo, C. L. 1950.. Ricerche istologiche comparative sulle
esponsioni nervose sensitive della trachea e die bronchi.
Boll. Soc. ital. Biol. Sper. 26:541-544. (Excerpta Med.
5:1951, sec. 1, article 1567.)

Sauser, G. 1957. Paries elastico-muscularis tracheae. Acta
Anat. 30:705-712.

Sbernini, C. and E. Bozzana. 1953. Ulteriori indagini ana-
tomomicroscopichi sulle reti linfatiche della trachea.
Arch. ital. Otol. 64:50-81. (Excerpta Med. 8:1954,
sec. 1, article 136.)

Schaeffer, J. P. 1953. Morris' Human Anatomy. The Blakiston
Co., New York.

Stirling, W. 1883. The trachealis muscle of man and animals.
J. Anat. Physiol. 17:204-206.

Stirling, W. 1890. Outlines of Practical Histology. Charles
Griffin & Co., London.

Straus, W. L. 1931. The form of the tracheal cartilages
of primates with remarks on the supposed taxonomic im-
portances. J. Mammal. 12:281-285.

Stowe, H. D. and M. L. Calhoun. 1961. Gross and microscopic
anatomy of the mink trachea. Publication pending, Am.
J. Vet. Res.

58

Sussdorf, M. V. 1911. Der Respirationsapparat. Chapter
in Handbuch der vergleichenden Mikroskopischen Anatomie
der Haustiere. Edited by W. Ellenberger, Paul Parey,

Berlin.

Towers, B. 1953. The muscle-cartilage relationship in the
extrapulmonary bronchi. J. Anat. 87:337-344.

Trautmann, A. and J. Fiebiger. 1952. Fundamentals of the
Histology of Domestic Anaimals. Translated by R. E.
Habel and E. L. Biberstein. Comstock Publishing Assoc.,
Ithaca, New York.

Von Hayek, H. 1960. The Human Lung. Translated by V. E.
Krahl. Hafner Publishing Co. Inc., New York.

Wolf-Heidegger, G. 1947. Die funktionelle Struktur der
Ligamenta anularia der menschlichen Trachea. Acta.
Anat. 4:295-310.

Zeek, P. M. 1951. Double trachea in penguins and sea lions.
Anat. Rec. 111:327-339.

59

PLATE I

A sketch to show some basic differences in various tracheal
elements which exist between the dog and cat. Note varia-
tions in the dorsal epithelium, elastic layers, gland

quantity, perichondrium thickness externally and inter-

nally and size of cartilage rings and muscle.

60

Oh

. ... A... ...1........... .2. . as. E.

     

.. ...“?Q S...“ .Huuu 593T. n a “who... .

.1. %. AAA

. €®9 my. . 99 6‘4?

ll'.\

1
o u 1..
Vein-Ir! -
I... 5

....
\h‘.“ .\\."|\
a O Q 0 I

u 0‘
-
- '
I

.8! .
\ o‘.‘)u0!\ I

I l
I .0” \
’II- "~\‘.

0

 

 

 

 

      

 

  

 

 

 

     

61

PLATE II

A drawing to show the change in shape of the cartilage rings
as they surround the lumen. The rings become flattened in
Ithe dorsal area and overlap each other in random fashion.
The shape and relationship of the cartilage rings is the

-same in both the dog and the cat.

63

PLATE III

CAT

Oblique section through the bottom of an epithelial fold
in the dorso-lateral area giving the appearance of an
intraepithelial pocket lined with cilia. H & E; x 1215.

1. Bottom of epithelial fold. I

2. Cellular debris.

3. Lamina propria.

4. Basement membrane.

5. Elastic fibers.

64

 

65

PLATE IV

CAT

Transverse section through a fold in the dorsal region show-
ing both the transitional type of epithelium and the common
pseudostratified ciliated columnar epithelium. H & E;
x 183.

l. Pseudostratified ciliated columnar epithelium.

2. Transitional-type epithelium.

3. Tip of incomplete hyaline cartilage ring.

4. Lamina propria.

5. Submucosa.

6. Glands.

7. Elastic fibers in the lamina prOpria.

66

 

67

PLATE V

CAT

High power View of the abrupt transition between the two types
of epithelia seen on Plate IV. H & E; x 1166.

l. Pseudostratified ciliated columnar epithelium.

2. Transitional-type epithelium.

3. Lymphocyte.

4. Elastic fibers.

5. Collagenous fibers.

68

 

69

PLATE VI

CAT

Longitudinal section through the ventral area showing the
difference in the number of goblet cells and height of
the epithelium between the region medial to a concentrated
gland area and the one medial to the center of a cartilage
ring. Note how the epithelium protrudes into the lumen
above the aggregation of glands which lie between the
cartilage rings. H & E; x 157.

1. Thick epithelium with numerous goblet cells.

2. Relatively thin epithelium with few goblet cells.

3. Aggregation of glands between cartilage rings.

4. Elastic layer.

5. Blood vessel.

6. Cross section of a cartilage ring.

‘IO

. m- Finnipftkfl.

. . 4
u . ...;

. wl‘rvlt

 

71

PLATE VII

CAT

a. Longitudinal section through the ventral area showing
the gland-cartilage relationship and the shape of the
cartilage rings. Note how the epithelium protrudes into
the lumen above the gland aggregation. H & E; x 83.
b. Longitudinal section through the dorso-lateral re-
gion to show the difference in gland-cartilage relation-
ship and the shape of the cartilage rings in this area
as compared to a. H & E; x 83.

l. Gland acini.

2. Cartilage rings in cross section.

3. Epithelia.

4. Adventitia.

‘I2

0‘ .
a."

J.’ \‘.. ‘Iv 1‘5: .-ILI-
-- .lv . I. .VHKL5&§1‘.M ..
-.- at: . 1 . .

3“

..l.

\ . I n

 

73

PLATE VIII

CAT

Transverse section through a fold of the mucosa in the
dorsal region showing many leucocytes in the epithelium.
The tracheal epithelium of either species normally does
not contain as many leucocytes (especially neutrophils)
as pictured here. H & E; x 1381.

1. Transitional-type epithelium.

2. Neutrophils.

3. Lymphocytes.

4. Tracheal lumen.

 

75

PLATE IX

CAT

Lymphoid invasion of the mucosa as seen in longitudinal

section.

H& E; x 205.
Aggregation of lymphocytes.
Elastic layer.
Lymphocytes in the lamina propria.

Epitheliunn pseudostratified ciliated columnar.

'(6

 

77

PLATE X

CAT

Transverse section from the lateral area showing the
arrangement of various elements in the mucosa and sub-
mucosa. Compare with Plate XI. Weigert and Van Gieson;
x 290.

l. Lamina propria and elastic layer.

2. Tracheal gland acini in the submucosa.

3. Loose elastic and collagenous tissues.

 

79

PLATE XI

DOG

Transverse section from the lateral area showing the arrange-

ment of mucosal and submucosal elements in the dog trachea.

Compare with that of the cat in Plate X. Weigert and Van

Gieson;

l.

x 300.

Area of the lamina propria which blends with the
submucosa.

Two layers of longitudinally oriented elastic fibers
separated by a small area of lamina propria.

Elastic layer which is a wide band of loosely arranged
elastic fibers scattered through the lamina propria
and submucosa. The larger fibers are concentrated in
the deeper part of the band.

Gland acini.

Blood vessels.

Heavy collagenous fibers in the lateral part of the
submucosa. Few large elastic fibers are found in

this area.

80

$2.4;

(r, in.”

LAW.

[r

 

81

PLATE XII

CAT

Transverse section showing acini external to a cartilage
ring in the ventral region. This section is cut through
the edge of a cartilage ring and therefore the perichon-
drium is seen as it blends with an annular ligament in the

lower part of the plate. H & E; x 229.

1. Fat cells in the adventitia.

2. Gland acini external to the ring.

3. Outer perichondrium blending with an annular
ligament.

4. Acini in the submucosa.

82

 

 

 

83

PLATE XI I I

CAT

Gland acinus situated in the lamina propria as seen in longi-
tudinal section. Note the distinct lamina propria and well
defined elastic layer. H & E; x 640.

1. Gland acinus.

2. Epithelium.

3. Lamina propria.

4. Elastic layer.

5. Submucosa.

84

 

85

PLATE XIV

CAT

Gland acini situated in the area of the annular ligaments;

longitudinal section. H & E; x 159.

1.

Elastic layer in the lamina propria.
Gland duct.

Mucus secreting cells.

Serous acini.

Blood vessel filled with blood.
Remaining part of the annular ligament.

Adventitia.

86

 

87

PLATE XV

CAT

Transverse section from the dorso-lateral region showing
two adjacent cartilage plates and the muscle attachment
to the outer perichondrium by means of elastic fibers.
weigert and Van Gieson; x 63.

l. Adjacent cartilage plates.

2. Muscle fibers.

3. Elastic fibers.

4. Outer perichondrium.

5. Adventitia containing adipose tissue, blood vessels

and nerve trunks.

88

 

89

PLATE XVI

DOG

Transverse section through the dorso-lateral region showing

the relationship of ganglion, muscle and cartilage. H & E;

x 223.

Tracheal lumen.
Cartilage ring.
Trachealis muscle.

Nerve ganglion in the adventitia.

90

. , I A
\ ‘ \P‘ ).‘
.' ‘ .4- .“ '
75": “"\i§. \\
‘ ‘\\ - i

0" '
H - '

|’. ~
‘ o
\
\ - f '
K

« %‘ \

 

91

PLATE XVI I

DOG

Transverse section through the area medial to the cartilage-
neo-membranous junction showing a patch of transitional-like
epithelium. Note the highly folded epithelium. H & E;
x 245.
l. Transitional-like epithelium directly medial to
the cartilageneo-membranous junction.
2. Folded pseudostratified ciliated columnar epithe-

lium lining the membranous portion of the trachea.

92

 

93

PLATE XVI I I

CAT

Transverse section showing mucous, serous, and mixed acini.
H & E; X 950.

l. Mucus secreting cells.

2. Serous acinus.

3. Mucous acini with serous demilunes.

94

 

95

PLATE XIX

DOG

Longitudinal section through the ventral region of the trachea.
Note the difference in size and gland quantity from that of
the cat in Plate VII a. H & E; x 71.

1. Gland acini.

2. Cartilage rings.

3. Annular ligament.

96

1 I: .. ..‘UC‘C'A-

Alvurip. M‘QqJNu nus-..va‘fila

J... gifcmrtrt.-. &

. . o. 0‘6 R 0. .II. "a '4’ H on 4. .
Tug. \ - . g .30.

. 0‘ .
I '.V..‘.\ 0' 1’...

 

fl
I»
0‘.1 . \I‘ I." I

-...“ ..... . ”IVA/I .
Rafibmeér ....\C ......”

“~\ 0. ..au.

‘ O

97

PLATE XX

DOG

Typical large gland duct characteristic of both species,
seen in a transverse section. Note the cilia and stra-
tified duct epithelium. H & E; x 560.
l. Gland duct.
2. Stratified epithelium with cilia lining the duct.
3. Stratified epithelium lining the tracheal
lumen.

4. Submucosa.

98

 

99

PLATE XXI

DOG

Transverse section through the dorsal submucosa showing

nerve trunks. Note the irregular epithelium. H & E; x 162.

Tracheal lumen.

Folded or clumped epithelium.
Nerve trunks in submucosa.
Blood vessels.

Cartilage.

Approximate lateral limit of the lamina propria.

100

. 7.4! 4 41o .
h ...“Wflswixufi..i ‘

4‘ . .. " .
. \f .l/.. orss.

.0 . 6 0)! .v. 0‘...
A x.

.&.‘J 6.

\ ...

 

101

PLATE XXII

DOG

Longitudinal section through the membranous part of the

trachea showing the relationship of tissues in this area.

Weigert and Van Gieson; x 266.

l.

20

Epithelium.

Submucosa with scattered elastic fibers of the
elastic layer.

Collagenous sheet continuous with the perichondrium
and annular ligaments.

Trahcealis muscle.

Thin fibroelastic layer external to the muscle.

Adventitia.

102

VHF» “r. . ’. _. .1 “ /

i ' 'u 5%,)“; " 'PV- "4" |
’»~‘ ,: $.31? 3‘” .. ’

”a
.4

. \ ‘
'7‘???“ ” tuf: i i

"w 4. IAo v.1

' 5 “if,“ .

‘ I'M-’ ' 4‘ ‘1';’
11.. ”if.“ “' «0“ ""°‘ 3.5-

‘2‘"):

‘j

.Lv fir:
. ‘4 .
(y .

 

 

9LT 2 4 '52

HICHIGQN STATE UNIV LIBRARIES

3006129542