Part 61

B. =Muscles on the Inner Surface of the Cartilages of the Larynx.=

=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.

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.

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.

IV. THE UROGENITAL SYSTEM. APPARATUS UROGENITALIS.

1. The Excretory Organs.

KIDNEY. REN (Figs. 108 and 109).

FIG. 109.--MEDIAN LONGITUDINAL SECTION OF KIDNEY.

Fig. 108.--a, renal artery; b, renal vein; c, ureter.

Fig. 109.--a, medullary portion; b, cortical portion; c, papilla; d, pelvis; e, renal artery; f, renal vein; g, ureter.]

The kidneys of the cat are compact (i.e., not lobulated) and have the usual kidney or bean form. They lie in the abdominal cavity, one on either side of the vertebral column, against the dorsal body wall, in the region between the third and fifth lumbar vertebræ. The right kidney is one or two centimeters farther craniad than the left, and the long axes of the two converge craniad a little. Each is covered by peritoneum on its ventral surface only (i.e., it is retroperitoneal). At the border of the kidney, where the peritoneum passes from it to the body wall, there is an accumulation of fat, which is most abundant at the cranial end of the kidney. Within the peritoneal investment the kidney is enclosed in a special loose fibrous covering, the =capsule= or =tunica fibrosa=, which is continuous with the fibrous coat of the ureter and pelvis. In the middle of the median border of each kidney is a notch, the =hilus=. It gives exit to the ureter (Fig. 108, c) and renal veins (b), and entrance to the renal artery (a). On the ventral surface of the kidney within the capsule are seen grooves radiating from the hilus. They contain blood-vessels. If the substance of the kidney is sliced away parallel to the ventral surface for some distance (Fig. 109), there is exposed a cavity, the =sinus=, which lies near the medial border and the opening of which is the hilus. It contains the pelvis (d) (the expanded beginning of the duct of the kidney), and also renal vessels (e and f) with their branches. These structures are enclosed in fat, which fills the remainder of the sinus. Upon opening the pelvis the kidney substance is seen to project into it in the form of a cone, the =papilla= (c), the apex of which is directed mediad. On the apex of the papilla are the numerous openings of the uriniferous collecting-tubes, some of them opening at the bottom of an apical depression of the papilla.

In a section made parallel to the ventral surface and in the median plane, the substance of the kidney is seen to consist of a peripheral darker and more granular =cortical portion= (Fig. 109, b), and of a central, lighter, less granular =medullary portion= (a). Both portions are marked by lines which converge to the apex of the papilla (c).

THE URETER (Fig. 108, c; Figs. 111 and 112, b).--The duct of the kidney begins as the =pelvis= (Fig. 109, d), a conical sac the base of which encloses the base of the papilla. From the apex of the papilla the urine passes into the pelvis. The outer wall of the pelvis is continuous with the capsule of the kidney. At the hilus the pelvis narrows to form the =ureter= (Fig. 109, g). The ureter passes caudad in a fold of peritoneum which contains fat. Near its caudal end it passes dorsad of the vas deferens (Fig. 111, c), turns ventrocraniad, and pierces the dorsal wall of the bladder (Fig. 111, a) obliquely near the neck. On the inside of the bladder the openings of the ureters appear as pores about five millimeters apart, and each is surrounded by a white, ring-like elevation of the surface.

THE BLADDER. VESICA URINARIA.--The bladder (Fig. 111, a) is pear-shaped. It lies in the abdominal cavity between its ventral wall and the rectum and a short distance craniad of the pubic symphysis. Caudad it is continued into a rather long, narrow =neck= (f) which passes dorsad of the symphysis to the pelvic cavity.