Part 163

The =clitoris= is long, flexuous, and pointed; beneath it is a cul-de-sac.

The =urethra= is very long; its vulvar orifice is not concealed by a valvular fold.

The =mammary glands= are usually ten or twelve in number, and are arranged in two rows, as in the bitch. Each teat has commonly two excretory ducts.

GENITAL ORGANS OF THE BITCH

The =ovaries= (Fig. 336) are small, elongated-oval in outline, and flattened. Each ovary is situated close to or in contact with the posterior pole of the corresponding kidney, and thus lies ventral to the third or fourth lumbar vertebræ, or about half-way between the last rib and the crest of the ilium. Each is concealed in a peritoneal pouch, the =bursa ovarii=, which has a slit-like opening ventrally. The two folds which form this pouch contain a quantity of fat and unstriped muscle. They are continued to the cornu of the uterus, constituting the =mesosalpinx= and the =ovarian ligament=. The surface of the ovary presents prominences caused by projecting Graafian follicles. There is no distinct hilus.

The =Fallopian= or =uterine tubes= are small and average two or three inches (ca. 5 to 8 cm.) in length. Each passes at first forward and then turns backward, having a straight or only slightly flexuous course. The fimbriated extremity lies in the bursa ovarii, and has a rather large opening. The uterine orifice is very small.

The =uterus= has a very short body and two long narrow horns. In a bitch of medium size the body is about an inch (ca. 2 to 3 cm.) and the cornua five or six inches (ca. 12 to 15 cm.) long. The horns are of uniform diameter, are nearly straight, and lie entirely within the abdomen. They diverge from the body in the form of a V toward each kidney. Their posterior parts are united by the peritoneum. The neck is very short and is fused dorsally with the vagina. It has a thick muscular coat. The mucous membrane of the uterus has long uterine glands and also short tubular crypts.

The =broad ligaments= contain fat and unstriped muscle. They are much longer in the middle than at either end. The posterior part is attached to the anterior part of the vagina. The =round ligaments= are very long, extending through the inguinal canals, and traceable almost to the vulva.

The horns of the gravid uterus present dilatations or =ampullæ=, which contain the fœtuses, and are separated by constrictions. The gravid uterus lies on the ventral abdominal wall, and toward the end of gestation extends forward to the stomach and liver.

The =vagina= is relatively long. It is narrow anteriorly, and has no distinct fornix. The muscular coat is thick and consists chiefly of circular fibers. The mucous membrane forms longitudinal folds. The canals of Gartner are usually absent.

FIG. 418.—GENITAL ORGANS OF BITCH.

Vulva, vagina, and uterus (in part) are slit open. a, Ovarian bursa; b, same opened to show the right ovary, c; d, d, horns of uterus; e, e′, body of uterus; f, neck of uterus; f′, os uteri; g, vagina; h, hymen; i, vulva; k, external urethral orifice; l, urinary bladder; m, urethra; n, n, labia vulvæ; o, fossa clitoridis; p, clitoris. (After Ellenberger, in Leisering’s Atlas.) ]

The =vulva= has thick labia which form a pointed inferior commissure. The mucous lining is smooth. On either side of the urethral orifice there is a small depression. The glandulæ vestibulares majores are absent, but the smaller glands are present, and their ducts open ventrally on either side of a median ridge. The vestibular bulbs are relatively large and join dorsally. The clitoris has a small pointed glans, beneath which is a diverticulum. The corpus cavernosum is a little more than an inch (ca. 3 to 4 cm.) long in a subject of medium size, and is infiltrated with fat.

The =mammary glands= are usually ten in number, and are arranged in two series extending from the posterior part of the pectoral region to the inguinal region; they are, therefore, designated according to location as =pectoral=, =abdominal=, and =inguinal=. The teats are short, and present on their apices six to twelve small orifices of the excretory ducts.

ANGIOLOGY: THE VASCULAR SYSTEM

=Angiology= is the description of the organs of circulation of the blood and lymph—the heart and vessels. The =heart= is the central hollow muscular organ which functions as a suction and force pump; the differences in pressure caused by its contraction and relaxation determine the circulation of the blood and lymph. It is situated in the middle mediastinal space of the thorax, between the two lungs, and is inclosed in a fibro-serous sac—the pericardium. The vessels are tubular and run through almost all parts of the body. They are designated according to their contents as =blood-= and =lymph vessels=.

The =blood-vascular system= consists of: (1) the =arteries=, which convey blood from the heart to the tissues; (2) the =capillaries=, microscopic tubes in the tissues which permit of the necessary interchange between the blood and the tissues; (3) the =veins=, which convey the blood back to the heart.

The blood-vessels are divided into the =pulmonary= and the =systemic=. The =pulmonary artery= conveys the blood from the right ventricle of the heart to the lungs, where it is arterialized, and is returned by the =pulmonary veins= to the left atrium of the heart, and passes into the left ventricle. The =systemic arteries= convey the blood from the left ventricle all over the body, whence it is returned by the =venæ cavæ= to the right atrium, and passes into the right ventricle.

It should be noted, however, that the lungs receive arterial blood through the systemic bronchial arteries. This blood is returned in part by the bronchial veins to the right atrium (indirectly), in part by the pulmonary veins to the left atrium.

The term =portal system= is often applied to the portal vein and its tributaries which come from the stomach, intestine, pancreas, and spleen. The vein enters the liver, where it branches like an artery, so that the blood in this subsidiary system passes through a second set of capillaries before being conveyed to the heart by the hepatic veins and the posterior vena cava.

The =arteries= (Arteriæ), as a rule, divide at an acute angle, giving off finer and finer branches. In some cases branches come off at a right angle, and others are recurrent, i. e., run in a direction opposite to that of the parent stem. The intercommunication of branches of adjacent arteries is termed =anastomosis=. Most commonly the connections are made by a network of numerous fine branches (Plexus vasculosus). Relatively large communicating branches (Rami communicantes) occur in certain places; they may be transverse or in the form of arches. Wide-meshed networks of vessels are termed =Retia vasculosa=. =Terminal= or =end arteries= are such as form isolated networks, i. e., do not anastomose with adjacent arteries. A =rete mirabile= is a network intercalated in the course of an artery. A =collateral vessel= (Vas collaterale) is one which pursues a course near and similar to that of a larger vessel.

The =veins= (Venæ) are in general arranged like the arteries, but are usually of greater caliber. When a vein accompanies an artery, it is termed a =vena comitans= or =satellite vein=; in many places two veins accompany a single artery. The primitive venous trunks do not run with the arteries, and most of the superficial veins (Venæ cutaneæ) pursue independent courses. They anastomose even more freely than the arteries, and large communicating branches are very common. The veins form very rich plexuses (Plexus venosi) in many places. Some veins which are without independent walls and are inclosed by dense membranes and run usually in bony grooves are termed (venous) =sinuses=; examples of this are the sinuses of the dura mater of the brain.

=Structure of Arteries.=—The wall consists of three coats. The =external coat= (Tunica externa) consists chiefly of fibrous connective tissue. In the deeper part are some elastic fibers, and in some arteries there are also longitudinal unstriped muscle-fibers. The =middle coat= (Tunica media) is composed of unstriped muscle and elastic tissue in medium-sized arteries. In small vessels there is only the muscular tissue, and in the largest trunks only elastic tissue. The =internal coat= or =intima= (Tunica intima) consists of a layer of endothelial cells, resting on an elastic membrane. The =sheath= (Vagina vasis) is a condensation of the surrounding connective tissue, and is attached more or less closely to the external coat.

=Structure of Veins.=—The walls of veins are similar in structure to those of the arteries, but are very much thinner, so that veins collapse more or less completely when empty, while arteries do not. The =middle coat= is very thin and consists to a large extent of ordinary connective tissue. The =intima= is also less elastic than in the arteries. In many veins this coat forms semilunar =valves=, the free edges of which are directed toward the heart. They are most numerous in the veins of the skin and the deep veins of the extremities (except the foot), while in most veins of the body cavities and viscera they are absent or occur only where the veins open into larger ones or where two veins join.

The walls of the vessels are supplied with blood by numerous small arteries, called =vasa vasorum=. These arise from branches of the artery which they supply or from adjacent arteries, ramify in the external coat, and enter the middle coat also.

The =nerves= of the vessels consist of both medullated and non-medullated fibers. They form plexuses around the vessels, from which fibers pass mainly to the muscular tissue of the middle coat.

BLOOD-VASCULAR SYSTEM OF THE HORSE

THE PERICARDIUM