304 CHORDATE ANATOMY 



forms the rudimentary glans clitoridis. In the marsupials, in correlation 

 with the presence of a double vagina, the penis has a forked termination. 



While the preputial sac of the phallus of monotremes, marsupials, 

 and lower placental mammals is directed posteriorly, in most higher 

 mammals the sac shifts its direction so as to point anteriorly. Finally, 

 in primates the penis is released from the skin of the abdominal wall, 

 the organ becomes pendulous, and the preputial sac opens downwards. 



In the female, the external genitals correspond with those of the male 

 but in a rudimentary form. The clitoris is the homolog of the glans penis 

 of the male. The remainder of the male phallus is represented in the 

 female by the labia minora. A corpus cavernosum urethrae is lacking 

 in the female. The labia majora correspond to the scrotum of the male. 



THE UROGENITAL SYSTEM OF MAN 



Although the excretory and reproductive systems of mammals are 

 so closely associated that it is difftcult to describe them separately, their 

 wide divergence in function makes this desirable. 



A. Urinary Organs 



The urinary apparatus of man and all placental mammals consists 

 of four parts: excretory glands, the kidneys; urinary ducts, the ureters; 

 a urinary reservoir, the bladder ; and the external outlet of the bladder, the 

 urethra. 



The Kidneys. The kidneys in man are bean-shaped organs, lying 

 in the lumbar region, closely pressed against the dorsal body-wall, and, on 

 account of the large amount of blood in them, of a deep reddish color. 

 The lateral border of each kidney is convex; the medial border is concave, 

 with a slit-like aperture, the hilum. 



Structure. A cross section of the human kidney in the region of the 

 hilum shows that, under its peritoneal and fatty investments, the kidney 

 is covered with a thin but tough fibrous capsule which in a young kidney 

 may be pulled off like the skin of an orange. 



The substance of a kidney consists of an outer cortex and an inner 

 medulla. A variable number, three to twenty, of renal pyramids form 

 the medulla. Each pyramid has its base upon the cortex, and a cone- 

 shaped apex which projects as a papilla into the renal smus. The cortex, 

 which is about half an inch thick, appears striated from the presence of 

 cortical rays. Between the renal pyramids, the cortex extends to the 

 renal sinus in the form of renal coliunns in which blood is conveyed to 

 and from the cortex. The renal sinus is filled by the expanded termination 

 of the ureter, the pelvis. 



Renal Tubules. The structure of the kidney as just described is 

 determined by the arrangement of the kidney tubules, which are the 



