Part 25

_Urogenital Sinus_ (_n_).--The =urogenital sinus= extends from the caudal end of the vagina (_m_) to the external opening, which is situated ventrad of the anal opening. It is about a centimeter long and nearly as wide, and is marked off from the vagina by a circular fold of mucosa, while its inner surface presents longitudinal folds. On its ventral wall at its cranial end is the opening of the neck of the bladder, which is enclosed by a ring-like elevation of the mucosa, most prominent at the sides. The external entrance to the urogenital sinus forms the =vulva=.

_The Clitoris._--The =clitoris= is a minute organ homologous with the penis and lying on the ventral floor of the urogenital sinus. Its distal end lies at the entrance of the urogenital sinus on its ventral border. In adult specimens the =prepuce= of the clitoris appears as a slight elevation of the integument surrounding a central vascular structure which appears red in the fresh organ. From the clitoris there are two small =corpora cavernosa clitoridis= (_o_) passing craniad and then diverging to be attached to the ischiatic rami. The ischiatic portion of each is covered by a muscle (ischiocavernosus). The clitoris is said to contain a bone.

=Mammary Glands.=--The mammary glands secrete the milk, and lie on the ventral surface of the body beneath the integument. The separate glands are closely gathered into two chief masses, one on each side the ventral middle line. Each of these extends from about the region of the fourth rib to the caudal end of the abdomen, ending over the pubic symphysis. On each side the glands are gathered into five groups, each of which is furnished with a =nipple=. The nipple is a projection of the integument, having near its distal end numerous fine openings for the ducts of the glands. The first two nipples are on the thorax, the other three on the abdomen, the most caudal ones being about two or three centimeters in front of the cranial edge of the pubis.

Rudimentary mammary glands and nipples are present in the male.

MUSCLES OF THE UROGENITAL ORGANS, RECTUM, AND ANUS (Figs. 113 and 114).--The muscles connected with the caudal openings of the alimentary canal and of the urogenital organs are closely interrelated, a single muscle sometimes acting on parts of both systems. For this reason all these muscles are described together.

The region lying between the anus and the external opening of the urogenital organs is known as the =perineum=. The perineum is formed chiefly by muscles and fascia.

_a. Muscles common to the Male and Female._--=M. sphincter ani externus= (Fig. 113, _i_; Fig. 114, _a_).--This muscle is confounded with the levator scroti (Fig. 113, _j_) or the levator vulvæ (Fig. 114, _b_). The two take origin in common from the integument on the dorsum of the root of the tail dorsad of the fifth caudal vertebra. There the fibres from the opposite sides are intermingled. The common muscle passes ventrad about the tail as a flat band close to the integument. Dorsad of the anus the inner fibres of the muscles of the opposite sides are united. They then separate and surround the anus as a band five millimeters wide situated beneath the integument. Ventrad of the anus the fibres are again intermingled. Some fibres on each side then continue to the scrotum as the levator scroti (Fig. 113, _j_) or to the vulva as the levator vulvæ (Fig. 114, _b_). Fibres also pass onto the anal pouch and unite with the sphincter ani internus, forming the =constrictors of the anal pouch= (Strauss-Durckheim).

=M. sphincter ani internus= (Fig. 113, _h_; Fig. 114, _c_).--The sphincter ani internus is a broad and thick band of striated muscle-fibres which surrounds the rectum at the anus. Dorsad the band is about two centimeters broad, while ventrad it is less than one. In the ventral median line some of the fibres pass craniad to help in forming the bulbocavernosus muscle. The muscle surrounds the anal sac.

(The muscle here described under this name is that described under the same name in the cat by Strauss-Durckheim and Mivart; it corresponds, however, to a part of the sphincter ani externus of the dog, as described by Ellenberger and Baum.)

=M. levator ani= (or =pubiocaudalis=) (Fig. 162, 11).--This muscle lies in the pelvic cavity. Each muscle forms a nearly vertical sheet, and between the two are the rectum and the urethra.

_Origin_ from the symphysis of the pelvis.

_Insertion_ into the midventral line of the centra of the third, fourth, and fifth caudal vertebræ, close to the muscle of the opposite side. This muscle is frequently continuous with the iliocaudalis (Fig. 162, 11′).

_Action._--Bends the tail and compresses the rectum.

=M. ischiocavernosus= (Fig. 113, _m_; Fig. 114, _e_).--A small, flat, spindle-shaped muscle which lies upon the crus of the penis or clitoris. Each has

_Origin_ from the caudal border of the ramus of the ischium, about one centimeter from the median line.

_Insertion_, in the male, into the whole outer surface of the crus penis, or bulb of the corpus cavernosum penis. In the female the muscle is smaller than in the male, and the insertion is into the ventral surface of the urogenital sinus, at the base of the clitoris.

=M. transversus perinei= (Fig. 114, _i_).--A small bundle of fibres which arises from the medial surface of the ischium, just dorsad of the origin of the ischiocavernosus, and passes mediad to join the sphincter ani internus (_c_).

=M. caudoanalis= (S.-D.) (Fig. 113, _f_; Fig. 114, _g_).--A slender, flat bundle of fibres having origin on the middle line of the ventral surface of the second and third caudal vertebræ. It passes caudoventrad, lying between the levator ani and the caudorectal (Fig. 113, _e_; Fig. 114, _j_), and unites with the ventral portion of the sphincter ani internus (Fig. 113, _h_).

_Action._--Draws the anus craniodorsad.

=M. caudorectalis= (Fig. 113, _e_; Fig. 114, _j_).

_Origin_ from the ventral surface of the sixth and seventh caudal vertebræ. A small, at first unpaired band two or three millimeters wide is formed, which passes cranioventrad, soon dividing into two lateral halves. These spread out over the sides of the rectum, forming a broad sheet of fibres which pass into the walls of the rectum, among the transverse fibres of the latter. This muscle is covered by the caudoanalis (Fig. 113, _f_), caudocavernosus (_g_) (or caudovaginalis, Fig. 114, _h_), iliocaudalis, and levator ani.

_b. Muscles Peculiar to the Male_ (Fig. 113).--=M. levator scroti= (_j_).--This is a band of fibres which passes ventrad in the median line from the sphincter ani externus (_i_) onto the scrotum. Here it spreads out beneath the skin, forming especially a well-marked bundle in the median furrow between the two testes.

=M. rectocavernosus=, or =retractor penis= (_k_).--A small bundle of fibres which arises in two parts from the ventral surface of the sphincter ani internus (_h_). The two parts unite into a single bundle which passes caudad on the middle line of the ventral surface of the penis; it is inserted into the corpus cavernosum just proximad of the glans. The muscle is covered only by integument, and overlies the ischiocavernosus (_m_).

=M. caudocavernosus= (S.-D.) (_g_).--A slender bundle just craniad of the caudoanalis (_f_).

_Origin_ on the median ventral line of the first two caudal vertebræ. The muscle passes caudoventrad, lying between the levator ani and the caudorectal (_e_). It divides into two bands, one of which is inserted into the base of the corpus cavernosum, while the other extends farther caudad and is inserted at the distal extremity of the corpus cavernosum.

_Action._--Flexes the penis (bends it backward).

=M. bulbocavernosus= (=accelerator urinæ=) (_l_).--The two muscles cover the ventral surface of the penis.

_Origin_ of each from a median raphe, which passes from the bulbous portion of the urethra toward the anus. The fibres pass toward the distal end of the penis and have their

_Insertion_ into the distal half of the lateral surface of the corpus cavernosum penis.

=M. compressor urethræ membranaceæ= (_n_).--A thick layer of striated muscle-fibres which surrounds the urethra between Cowper’s gland (3) and the prostate (4). The fibres have a circular course, and the cranial ones are attached to the crura of the penis. The other fibres have no fixed attachment.

_c. Muscles Peculiar to the Female_ (Fig. 114).--=M. levator vulvæ= (Strauss-Durckheim), or =constrictor cunni= (_b_).--This is homologous with the levator scroti of the male. It consists of a band of fibres which pass ventrad from the external sphincter ani (_a_) and surround the vulva (3), lying immediately beneath the integument.

=M. constrictor vestibuli=, or =rectovaginalis= (Strauss-Durckheim) (_d_).

_Origin_ from the sides of the sphincter ani internus (_c_). The muscle forms a distinct bundle two or three millimeters wide, which passes ventrocaudad and is _inserted_ into the ventral surface of the urogenital sinus, caudad of the insertion of the ischiocavernosus (_e_).

=M. caudovaginalis= (Strauss-Durckheim) (_h_).--A slender band just craniad of the caudoanalis (_g_), and corresponding to the caudocavernosus of the male.

_Origin_ from the median line of the ventral surface of the first two caudal vertebræ. The muscle passes caudoventrad, lying between the levator ani and the caudorectal (_j_), and is _inserted_ into the ventral side of the urogenital sinus, at the base of the clitoris.

=M. urethralis= (_f_).--This consists of fibres surrounding the cranial part of the urogenital sinus and the caudal parts of the vagina and neck of the bladder.

_Origin_ partly on the caudal part of the symphysis of the ischium, partly from the ventral surface of the urogenital sinus, where the fibres are attached to the corpora cavernosa clitoridis. The fibres pass dorsad over the surface of the sinus, and over the surface of the union of the vagina and neck of the bladder, to be inserted into the sides of the vagina and the dorsal surface of the urogenital sinus.

THE CIRCULATORY SYSTEM.

I. THE HEART. COR.

The heart lies in the mediastinum, enclosed in the pericardial sac, and projects rather more toward the left than toward the right. It is an ovoid or pear-shaped organ, with its long axis directed approximately craniocaudad. Its caudal end or apex is, however, directed slightly ventrad and to the left, while the larger cranial end or base faces slightly dorsad as well as craniad.

Laterally and dorsally the heart is largely covered by the lungs. The ventral side and a considerable portion of the lateral surface are, however, not thus covered, so that they lie against the thoracic wall. The heart extends from about the fourth or the fifth to the eighth rib, and its apex touches the diaphragm.

The cavity of the heart is divided by a longitudinal dorsoventral septum into lateral halves--a right and a left side. Each side is again divided by a transverse dorsoventral septum (=auriculoventricular=) into two chambers, one of which, the =auricle=, lies at the base; the other, the =ventricle=, lies at the apex of the heart. There are thus right and left auricles and right and left ventricles. An external groove, partly filled with fat, separates the auricular portion of the heart from the ventricular part; this groove is known as the =sulcus coronarius=.

Each auricle or ventricle has a single set of blood-vessels either leaving it or entering it. Thus the blood enters the right auricle by the =inferior= and =superior= venæ cavæ (Fig. 116, _d_ and _e_) (præcava and postcava). It passes thence into the right ventricle, and from the right ventricle to the lungs by a single =pulmonary artery= (Fig. 115, _f_). It returns to the left auricle by the numerous =pulmonary veins= (Fig. 116, _g_, _h_, _i_), and passes thence to the left ventricle. From the left ventricle it passes to the body by a single =aorta= (Fig. 115, _g_).

When the heart is viewed from the ventral surface (Fig. 115), a considerable blood-vessel, the =coronary artery= (_q_), running from the base to a point a little to the right of the apex, indicates the position of the septum (ventricular septum), between the right (_a_) and left (_b_) ventricles. The apex thus belongs to the left ventricle. From the cranial end of the right ventricle the pulmonary artery (_f_) is seen passing obliquely craniad and toward the left. Beneath the pulmonary artery the aorta (_g_) appears rising from the middle of the base of the heart and passing directly craniad. At the base appears a part of the auricular appendage of the left auricle (_d_), and at the right a part of the right auricular appendage (_c_). The former (_d_) is larger and bent at right angles, so that its free end is directed toward the apex.

In the dorsal view (Fig. 116) the position of the ventricular septum is not indicated. On the surface of the left ventricle a short distance from the ventricular septum and nearly parallel to it are seen branches of the coronary artery and the coronary vein passing toward the apex. Craniad of the left ventricle is seen the left auricle (_b_) with the pulmonary veins (_g_, _h_, _i_) opening into it. Craniad of the right ventricle (_a_) is the right auricle (_c_) with the venæ cavæ (_d_ and _e_) opening into it near the middle. Coming from beneath the auricles are seen the pulmonary artery (_j_) and the aorta (_f_).

The =chambers of the heart=.

1. The =right auricle= (atrium dextrum) (Figs. 115 and 116, _c_). Externally the right end of the right auricle projects so as to form its =auricular appendage= (Fig. 115, _c_) which lies at the right of the base of the aorta (Fig. 115, _g_). The superior vena cava (Fig. 116, _e_) is seen entering near the cranial end of the auricular septum opposite the base of the aorta. The inferior vena cava (Fig. 116, _d_) enters the auricle near the coronary sinus and close to the auricular septum. The netted appearance which the wall, especially that of the appendage, presents externally is due to the muscular thickenings of the wall (=musculi pectinati=).

The cavity (including that of the auricular appendage) is somewhat egg-shaped, with its long axis transverse. The musculi pectinati, which branch and unite into a network, are most abundant on its dorsal wall at the right. The axes of the two venæ cavæ if produced into the cavity of the auricle would meet one another at its centre and nearly at right angles. Caudad of the opening of the inferior vena cava is seen the slit-like opening of the =coronary sinus=, guarded craniad by the semilunar =valve of the coronary sinus= (or valve of Thebesius). The coronary sinus receives blood from =coronary veins=, which collect it from the walls of the heart.

In the auricular septum ventrad of the opening of the inferior vena cava is seen a faintly marked smooth oval depression, the =fossa ovalis=. When the auricular septum is examined by transmitted light it is seen to be thinner over the fossa ovalis. There is an opening, =foramen ovale=, at this point in fœtal life, so that the blood of the inferior vena cava then passes directly from the right auricle to the left auricle. Caudad the cavity of the right auricle communicates with that of the ventricle by the large oval auriculoventricular opening, which is guarded by the tricuspid valves (Fig. 117).

2. The =right ventricle= (ventriculus dexter) (Figs. 115 and 116, _a_) does not reach quite to the apex of the heart, so that it makes up less than one-half of its ventricular portion. It makes a half-spiral turn about the left ventricle, from its lateral side at the apex toward its ventral side at the base, where it ends in the pulmonary artery (Fig. 115, _e_). Its wall is very thin as compared with that of the left ventricle. Its cavity (Fig. 117) is nearly flat on its medial side, convex on its lateral side. It communicates with the auricle craniodorsad. Cranioventrad the cavity is narrower and turns around toward the ventral side of the heart and opens into the pulmonary artery. That portion of the ventricle between the auriculoventricular opening and the pulmonary artery is the =conus arteriosus= (Fig. 115, _e_; Fig. 117, _f_). Internally the wall presents many muscular =trabeculæ= (Fig. 117, _a_) of various sizes. These are more numerous over the ventricular wall than on the septum. They do not occur on the conus (_f_). Surrounding the auriculoventricular opening is the =tricuspid valve= (Fig. 117, _d_, _d′_, _d″_). It consists of three flaps. One of these (_d′_) is septal (i.e., it lies against the septum), while of the two others which do not lie against the septum, one is dorsal (_d″_) and the other ventral. Each flap is thin, semicircular, and membranous, and is attached to the border of the auriculoventricular opening by the diameter of the semicircle. The free border of each is attached to the wall of the heart by numerous delicate tendinous bands, the =chordæ tendineæ= (_c_), some of which are attached to the lower face of each valve. The chordæ tendineæ of the septal valve (or most of them) are attached at their opposite ends to the septum directly, while those of the dorsal and ventral flaps are attached to the ends of three or more band-like muscles, =columnæ= (or trabeculæ) =carneæ= (_b_), which are fixed by their opposite ends to the ventricular wall.

Between the conus arteriosus (_f_) and the pulmonary artery are three pocket-like =semilunar valves= (Fig. 117, _g_), one ventral, one dextral, and one sinistral. Between each valve and the wall of the pulmonary artery there is an enlargement of the cavity of the artery, one of the =pulmonary sinuses= (or sinuses of Valsalva). Beyond the sinuses the pulmonary artery divides into right and left branches (Fig. 116, _j_).

3. The =left auricle= (atrium sinistrum) (Fig. 115, _d_; Fig. 116, _b_). The auricular appendage (Fig. 115, _d_) is bent at right angles and its apex turned caudad. The cavity is thus irregular. Muscular trabeculæ occur only in the auricular appendage. When the auricular septum is examined by transmitted light the position of the fossa ovalis is indicated at about the middle of the septum as seen from the left side. Ventrad of it is a fold of the septal wall.

The =pulmonary veins= enter the dorsal wall of the auricle in three groups (Fig. 116, _g_, _h_, _i_). Each group opens into a more or less pronounced sinus or extension of the auricular cavity. The sinuses may be called, on account of their position, dorsal (_g_), sinistral (_i_), and dextral (_h_).

4. The =left ventricle= (Fig. 115, _b_) occupies rather more than the left half of the base of the heart. Its walls are two or three times as thick as those of the right ventricle. Internally its walls present a few muscular bands, comparable to the trabeculæ of the left ventricle. There are two very large columnæ carneæ, one dorsad and one ventrad. At the cranial end it communicates near the lateral wall with the left auricle by the auriculoventricular opening, and near the septum with the aorta. The =bicuspid valve= (or mitral valve), which guards the auriculoventricular opening, consists of two flaps, one septal and one lateral. Their chordæ tendineæ, which come from their free borders as well as from their outer surfaces, are attached to the columnæ carneæ, chordæ passing from both valves to each columna carnea.

The opening into the aorta is guarded by three =aortic semilunar= valves, each of which partly conceals an =aortic sinus= (or sinus of Valsalva). One valve is dorsal, one sinistral, and one dextral. In the dextral sinus is the opening of one of the =coronary arteries=, which carry blood to the walls of the heart.

=Pericardium.=

The pericardium is a sac enclosing the heart. It lies in the middle mediastinum, and the two halves of the mediastinal septum with their fat may be dissected away from it. Its wall is composed of two layers, an external firm fibrous layer and an internal layer of flattened epithelial cells similar to the peritoneal epithelium (=serous= layer). The fibrous layer forms a sac which repeats roughly the form of the heart. This sac is attached to the aorta at the point of origin of the subclavian artery, to the pulmonary artery at its bifurcation, and to the venæ cavæ and pulmonary veins near their entrance into the heart. At these points it is continuous with the fibrous coats of the vessels named, and from them it is reflected over the heart, forming a complete sac enclosing it but not attached to it anywhere. The heart lies within this sac. The serous layer lines the fibrous sac and gives to the surface of the heart and fibrous layer a smooth glistening appearance. It is reflected over the heart. The relation of the heart to it is much the same as the relation of the intestine to the peritoneal sac. The serous layer consists therefore of two portions, =parietal=, lining the sac, and =visceral=, covering the heart. The parietal and visceral portions are continuous along a line which runs approximately parallel to the auriculoventricular groove and encloses all the great blood-vessels. Within this line the heart lies against the fibrous layer of pericardium and is not covered by the serous layer. The serous layer is easily dissected free from the heart-wall, but its parietal portion is closely adherent to the fibrous layer.

II. THE ARTERIES. ARTERIÆ.

1. =A. pulmonalis, the Pulmonary Artery= (Fig. 115, _f_).