Part 22

The inner surfaces of the cartilages are covered by the =mucosa=, which forms a number of ridges. The narial opening is almost completely divided by the prominent ridge which is supported by the free edge (_c_) of the cartilaginous wing above described. This free edge is covered by a thick layer of mucosa, and the entire ridge so formed is continuous caudad with the inferior nasal concha. On the medial side of the partial

## partition formed by this ridge and near the ventral side there begins a

slight distance caudad of the outer opening another ridge, supported by the ventrally incurved portion of the cartilaginous ring. This soon becomes a thick swelling; beneath it opens the lachrymal canal, and that part of the passageway that lies ventromediad of it is the beginning of the inferior meatus of the nose. About one or two centimeters caudad of the external opening a third ridge projects from the dorsolateral wall of the cavity toward the large ridge first described; dorsomediad of it is a narrow passage which is the entrance to the =superior meatus= of the nose.

The =olfactory mucosa=, or that part to which the olfactory nerve is distributed, and which therefore acts as the sensory surface, is confined to the dorsocaudal parts of the nasal cavity, in the region occupied by the cells of the ethmoid. The air penetrates to this region probably only by a definite act of snuffing, the inferior meatus serving as the usual passageway of air to the lungs.

The =nasopharynx= and =pharynx= are considered under the alimentary canal (page 231).

2. =The Larynx.=

The larynx is the enlarged upper end of the air-passage which leads from the pharynx to the lungs. It is a box composed of pieces of cartilage connected by ligaments and moved by muscles, and it is lined by mucous membrane. At the root of the tongue about one centimeter caudad of the body of the hyoid bone is the triangular leaf-like =epiglottis= (Fig. 95, _e_, page 227). When food is taken this closes the opening into the larynx, and the food passes over it into the œsophagus. It is so curved that its apex is directed craniad. A fold of mucous membrane, the =plica glossoepiglottica= or =frenulum= of the epiglottis, extends from the middle of its cranial surface to the root of the tongue, and on each side of this fold is a depression.

From each side of the base of the epiglottis a fold, =plica aryepiglottica= (Fig. 95, _f_), extends caudad to the base of the arytenoid cartilage (_g_). Dorsad of this fold and separated from it by a depression is a ridge which marks the position of the caudal hyoid cornu. The plicæ aryepiglotticæ (_f_) and the epiglottis (_e_) form the boundaries of the =aditus laryngis=, or opening into the larynx.

The cavity of the larynx is divided into three portions. The upper one of these is the =vestibule= of the larynx. It is bounded caudad by two folds of mucosa (_i_) that stretch from the caudal surface of the epiglottis near its base to the tips of the arytenoid cartilages. These folds are the =false vocal cords= (_i_). Their vibration is said to produce purring. Caudad of the false vocal cords two folds of the mucosa stretch from the apices of the arytenoid cartilages to the thyroid cartilages, near the base of the epiglottis. These folds are nearer the median plane than the false vocal cords. They are the =true vocal cords= (_j_). The =middle portion= of the laryngeal cavity is that between the true and the false vocal cords. It is produced laterally into a very small pouch or pocket on each side, the =ventriculus=. The narrow slit between the true vocal cords is the =glottis= (_h_). It can be narrowed and widened by the action of muscles. The vocal cords, which bound it, are set vibrating by currents of air transmitted from the lungs, and the voice-sounds are thus produced. The =caudal portion= (inferior portion) of the laryngeal cavity is that between the glottis and the first tracheal cartilage. It is narrowed near the glottis.

CARTILAGES OF THE LARYNX (Fig. 104).--There are three unpaired cartilages, the =thyroid= (1), =cricoid= (3), and =epiglottic= (2), and two paired cartilages, the =arytenoids= (4).

[Illustration: FIG. 104.--CARTILAGES OF LARYNX, WITH SIDE VIEW OF HYOID BONE.

_b_, ceratohyal; _c_, epihyal; _d_, stylohyal; _e_, tympanohyal; _f_, thyrohyal. 1, thyroid cartilage; 2, epiglottis; 3, cricoid cartilage; 4, arytenoid cartilage; 5, cricothyroid ligament; 6, thyrohyoid ligament; 7, trachea.]

The =thyroid= cartilage (=cartilago thyreoidea=) (1) has nearly the form of a visor of a cap, but is relatively broader at its ends than a cap visor. It forms about two-thirds the circumference of a circle, and is so situated that it embraces the other cartilages ventrally and laterally. Its caudal and cranial borders are oblique to its caudocranial axis and are directed dorsocaudad. To the middle of the cranial border is attached the epiglottic cartilage (2), and the whole cranial margin is connected by membrane (6) to the body and caudal cornua (_f_) of the hyoid bone. The dorsal border projects craniad into a considerable cornu which is attached to the free end of the caudal hyoid cornu (_f_). The border also projects caudad into a process which articulates with a facet on the lateral surface of the cricoid cartilage (3). In the middle of the dorsal surface is a longitudinal ridge for attachment of the vocal cords and origin of the thyroarytenoid muscles (Fig. 105, _d_), and at the caudal end of this ridge the caudal border presents a considerable rounded notch.

The =cricoid cartilage= (=cartilago cricoidea=) (3) has the form of a seal ring with its broader part dorsad. The broad dorsal part of the ring is partly embraced by the wings of the thyroid cartilage (1). Its caudal border is undulating and nearly at right angles to its caudocranial axis. It is connected by membrane with the first tracheal ring. The cranial border is oblique to the long axis and lies in a plane which passes from the ventral side craniodorsad. Near the median line on each side it articulates by an oblique facet with one of the two arytenoid cartilages (4). Near the midventral line the cricothyroid ligament (5) is attached and it stretches thence to the midcaudal notch of the thyroid cartilage (1).

The outer surface presents at the middle of each side a facet for articulation with the thyroid, and is marked in the median dorsal line by a ridge for the posterior cricoarytenoid muscle (Fig. 105, _b_).

The =arytenoid cartilages= (=cartilago arytenoidea=) (Fig. 104, 4) are triangular pyramids with base and sides nearly equilateral triangles. One side articulates with an oblique facet on the cranial border of the cricoid near the mid-dorsal line. Near the opposite apex is attached the vocal cord. At the lateral angle of the base is attached on its dorsal side the posterior cricoarytenoid muscle (Fig. 105, _b_), and on its ventral side the thyroarytenoid (Fig. 105, _d_) and lateral cricoarytenoid (Fig. 105, _c_) muscles. The movements produced by these muscles carry the arytenoid ends of the vocal cords toward or from the median plane and thus open or close the glottis.

The =epiglottic cartilage= (Fig. 104, 2) is flexible (fibro-cartilage) and of a cordate form. It is so curved that its caudal surface is convex dorsoventrally and concave from side to side. It supports the epiglottis. Its cranial surface presents a slight median ridge for attachment of muscles. By its base it is attached to the midventral part of the cranial border of the thyroid cartilage (1). Its position varies so that it either stands erect with its apex directed craniad to allow the passage of air to the lungs, or, as in the act of swallowing, it is turned caudad over the aditus laryngis so as to allow food to pass over it and into the œsophagus.

The =vocal cords= are two fibrous elastic bands. Each is attached at one end to the apex of the arytenoid cartilage, and at the other end to the median ridge on the dorsal surface of the thyroid. Each supports a projecting fold of mucous membrane, the vibration of which causes the voice.

MUSCLES OF THE LARYNX.--1. Muscles moving the entire larynx.

A. =Elevators. M. thyreohyoideus= (Fig. 96, _p_, page 229).--A flat band on the lateral side of the larynx.

_Origin_ on the lateral part of the caudal border of the thyroid cartilage.

_Insertion_ on the medial two-thirds of the caudal border of the caudal cornu of the hyoid.

_Action._--Raises the larynx.

The stylohyoid (Fig. 65, _d_, page 109) and the median and inferior constrictors (Fig. 96, _j_ and _k_) of the pharynx, already described, have the same action.

B. =Depressors.=--The sternothyroid (Fig. 65, _g′_), already described (p. 141).

2. Muscles which move the parts of the larynx one upon another.

A. =Muscles on the Outer Surface of the Larynx.=

=M. cricothyreoideus= (Fig. 96, _n_).--A broad flat band which with its fellow covers the ventral surface of the cricoid cartilage and the cricothyroid ligament.

_Origin._--The lateral half of the ventral surface of the cricoid cartilage. The muscles diverge so as to leave a part of the cricothyroid ligament between them.

_Insertion._--The ventral part of the caudal border of the thyroid cartilage laterad of the median ventral notch.

=M. cricoarytenoideus posterior= (Fig. 105, _b_).--The two muscles cover the dorsal surface of the larynx.

_Origin._--From the dorsal part of the caudal border of the cricoid cartilage (3) and from its median dorsal crest. The fibres pass craniolaterad, converging. The lateral fibres are nearly longitudinal in direction.

_Insertion._--The dorsal border of the caudal end of the arytenoid cartilage (4).

_Action._--Moves the arytenoid on its oblique articulation with the thyroid. The apex of the arytenoid is thus carried laterad, and the vocal cords are separated so as to widen the glottis.

[Illustration: FIG. 105.--MUSCLES OF THE LARYNX, AS EXPOSED BY REMOVAL OF MOST OF THE LEFT HALF OF THE THYROID CARTILAGE.

1, epiglottis; 2, portion of the thyroid cartilage (cut); 3, cricoid cartilage; 4, arytenoid cartilage; 5, trachea. _a_, M. arytenoideus transversus; _b_, M. cricoarytenoideus posterior; _c_, M. cricoarytenoideus lateralis; _d_, M. thyreoarytenoideus.]

=M. arytenoideus transversus= (Fig. 105, _a_).--A small unpaired muscle running transversely between the caudal ends of the arytenoid cartilages, just beneath the mucous membrane and parallel to the cranial border of the cricoid cartilage.

=M. glossoepiglotticus.=--A small longitudinal muscle, lying by the side of its fellow in the frenulum of the epiglottis.

_Origin._--The median fibrous septum of the tongue.

_Insertion._--The dorsal (cranial) surface of the epiglottic cartilage in the median line near its attached border.

_Action._--Draws the epiglottis craniad.

=M. hyoepiglotticus= is a small bundle of parallel fibres lying in the frenulum of the epiglottis craniad and dorsad of the body of the hyoid bone.

_Origin._--The lateral end of the cranial surface of the body of the hyoid. The two muscles pass craniodorsad, converging, and unite with the preceding.

_Insertion_ with the preceding. (The lateral portion of the muscle may continue craniad to the tongue as a second part of the hyoglossus.)

_Action._--Like the preceding.

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.

[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.

IV. THE UROGENITAL SYSTEM. APPARATUS UROGENITALIS.

1. The Excretory Organs.

KIDNEY. REN (Figs. 108 and 109).

[Illustration: FIG. 108.--LEFT KIDNEY, VENTRAL SURFACE.

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.

The bladder is covered by peritoneum and is held in place by its neck and by three folds of the peritoneum. One of these passes from its ventral wall to the linea alba and is the =suspensory= ligament. Two others pass one from each side of the bladder to the dorsal body wall at the sides of the rectum. They are the =lateral= ligaments of the bladder. They form the walls of a partly isolated peritoneal pocket into which the rectum passes; this pocket opens craniad into the peritoneal cavity. The wall of the bladder is composed of an internal epithelium, a layer of plain muscle-fibre bundles which cross one another in various directions, and the external peritoneal layer.

SUPRARENAL BODIES. GLANDULÆ SUPRARENALES.--The suprarenal bodies are two ovoid bodies about a centimeter in the longest diameter, lying craniomediad of the kidneys, but usually not touching them. In a fresh condition they are of a pinkish or yellow color. They are usually imbedded in fat and are covered by peritoneum on their ventral surface. They have no duct and are of uncertain function.

2. The Genital Organs.

A. THE MALE GENITAL ORGANS. _External Genital Organs._--The external genital organs are the =scrotum= and =penis=.

The =scrotal sac= or =scrotum= is a pouch of integument which lies ventrad of the anus in the median line against the ischiatic symphysis. It is marked by a median groove which indicates the position of an internal septum dividing its cavity into lateral halves, within each of which is one of the =testes=.