=M. thyreoarytenoideus= (Fig. 105, _d_).--The thyroarytenoid is a
triangular, flat muscle of considerable size. It lies within the wing of
the thyroid cartilage (2), and its fibres are nearly dorsoventral in
direction.
_Origin._--The median longitudinal crest on the dorsal surface of the
thyroid cartilage (2).
_Insertion._--The fibres converge to the insertion into the cranial lip
of the laterocaudal angle of the arytenoid cartilage (4).
_Action._--Turns the arytenoid on its oblique articulation so as to
close the glottis.
=M. cricoarytenoideus lateralis= (Fig. 105, _c_).--Triangular, a little
smaller than the preceding, caudad of which it is situated, so that it
also is covered by the wing of the thyroid.
_Origin._--The lateral part of the cranial border of the cricoid
cartilage (3). The fibres converge, passing dorsad.
_Insertion._--The caudal lip of the laterocaudal angle of the arytenoid
cartilage (4).
_Action._--Similar to the preceding, so that it closes the glottis.
3. =The Trachea= (Fig. 105, 5; Fig. 106, _a_).--The trachea is that part
of the air-passage which extends from the larynx to the bronchi (Fig.
106). It is a straight tube composed of a lining mucosa with ciliated
epithelium, and a connective-tissue covering which encloses supporting
cartilages. Each tracheal cartilage is incomplete dorsally where it lies
against the œsophagus, the gap between the two free ends of each ring
being filled with muscular and connective tissue. As a result of this
the diameter of the trachea is not fixed, but can be increased and
diminished. The first ring is broader than the others. Where the
œsophagus leaves the median line, the dorsal surface of the trachea lies
against the longus colli muscles (Fig. 72, _g″_). Its ventral surface is
against the sternohyoid (Fig. 65, _e_) and sternothyroid (Fig. 65, _g′_)
muscles. Its lateral surfaces are partly covered by the thyroid gland
(Fig. 96, 6), and are in close relation with the carotid artery (Fig.
119, _a_, page 284), the vagus and sympathetic nerves (Fig. 156, _i_),
and the internal jugular vein (Fig. 119, _b_). In the thoracic cavity
the great vessels coming from the heart lie against the ventral surface
of the trachea (Fig. 129, 2). At about the level of the sixth rib the
trachea divides into the two main =bronchi= (Fig. 106). Each bronchus is
supported by incomplete rings of cartilage like those of the trachea and
has otherwise in general the structure of the trachea. In the lungs the
bronchi become divided into many branches (Fig. 106), in the manner
described in the account of the lungs.
4. =The Lungs. Pulmones.=--Immediately after division of the trachea the
two bronchi enter the lungs (Fig. 106). These are two large, much-lobed
organs, which fill the greater part of the thoracic cavity. The main
lobes of the lungs are completely separated from each other, except in
so far as they are connected by the bronchi and connective tissue; the
main lobes may also be partly subdivided into secondary lobes that are
not thus completely separated. The two lungs are completely separated
from each other, except at the =radix=, where they are united by the
bronchi; they lie in the right and left halves of the thoracic cavity,
with the mediastinal septum between them. The bronchi on entering the
lungs divide in the following manner. Each divides at first into two
main branches. The cranial branch on the right side is known as the
=eparterial= bronchus (_b_), because it lies craniad of the pulmonary
artery. All the others are =hyparterial=; i.e., they lie caudad of the
pulmonary artery. The right cranial bronchus does not further subdivide
into large bronchi, but gives off numerous small branches. The right
caudal bronchus divides into three main branches. There are thus four
main branches of the right bronchus. The left cranial bronchus divides
into two main branches; the left caudal bronchus continues caudad as a
main trunk giving off small branchlets. Of the left bronchus there are
thus but three main subdivisions. The main subdivisions of the bronchi
on the two sides correspond, as will be seen, with the lobulation of the
lungs.
The =right= lung (Fig. 106, 1-4) is slightly larger than the left
(1′-3′). It divides into three smaller proximal lobes (1-3), and one
large distal one (4). The most cranial one of the proximal lobes (1) is
sometimes partly subdivided. The third one of the proximal lobes (3)
lies mediad of the others; it is partly subdivided and one-half projects
into a pocket in the mediastinum, so that it comes to lie across the
middle line, extending a short distance onto the left side. This lobe is
frequently called the =mediastinal= lobe (3). The caudal lobe (4) of the
right lung is large and flat, containing about half the substance of the
lung.
[Illustration: FIG. 106.--RAMIFICATIONS OF THE BRONCHI, WITH OUTLINES
OF THE LOBES OF THE LUNGS, VENTRAL VIEW.
1-4, lobes of the right lung; 1′-3′, lobes of the left lung. _a_,
trachea; _b_, eparterial bronchus.]
The =left= lung is divided into three main lobes (1′-3′); the two
cranial ones (1′-2′) are, however, partly united at the base, so that
they may be considered subdivisions of but a single lobe; thus the left
lung has but two distinctly separated lobes.
Each lung is attached to the aorta, vertebral column, and diaphragm by a
fold of pleura, the =pulmonary ligament=. This is broadest at the caudal
lobe of each lung. Each pulmonary ligament is double, being formed of
two sheets of the pleura.
[Illustration: FIG. 107.--POSITION OF THYMUS GLAND, FROM LEFT SIDE.
_a_, heart; _b_, aorta; _c_, œsophagus; _d_, thymus gland; _e_,
lymphatic gland; _f_, left subclavian artery; _g_, internal mammary
artery. _I_, cut ends of first rib; _XI_, eleventh rib.]
=The Thyroid Gland. Glandula thyreoidea= (Fig. 96, 6).--The thyroid
gland consists of two lateral lobes (6) and a median lobe or =isthmus=
(7). Each lateral lobe (6) is an elongated, flattened, lobulated mass
with round ends. It is about two centimeters long and about one-fourth
as broad. It lies at the side of the trachea (4), dorsad of the lateral
margin of the sternohyoid muscle. Its cranial end is at the level of the
caudal border of the cricoid cartilage. The =isthmus= (7) is a delicate
band two millimeters wide which connects the caudal ends of the two
lateral lobes. It passes ventrad of the trachea and in close contact
with it. The thyroid has no duct.
=The Thymus Gland. Glandula thymus= (Fig. 107, _d_).--The thymus gland
is best developed in young kittens; in the adult cat it has partly or
almost completely degenerated. It is an elongated, flattened organ, of a
pinkish-gray color, which lies in the mediastinal cavity, between the
two lungs and against the sternum. It extends caudad as far as the heart
(_a_), overlying the pericardium at its posterior end. At its cranial
end it projects, when well developed, a short distance (about one
centimeter) outside of the thoracic cavity into the neck region. The
caudal end is forked, and the left lobe thus formed is usually larger
than the right. The cranial end may also show indication of a division
into two lobes, but this is frequently not the case.