_The Vagina_ (_m_).--The =vagina= extends from the os uteri (at _l_) dorsad of the symphysis of the pelvis to a point a short distance craniad of the caudal border of the ischiatic symphysis. At this point it joins the neck of the bladder (_k_) to form the =vestibulum= or =urogenital sinus= (_n_), which is homologous with the urethra of the male. _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.