268 THE VISCERA. 



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 open- 

 ing of the urogenital organs is known as the perineum. The 

 perineum is formed chiefly by muscles and fascia. 



a. Muscles common to tlie Male and Female. M. sphincter 

 ani externus (Fig. 113, z; Fig. 114, a}. This muscle is con- 

 founded with the levator scroti (Fig. 113,7) or tne levator 

 vulvae (Fig. 1 14, 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. Ven- 

 trad of the anus the fibres are again intermingled. Some fibres 



