THE UROGENITAL SYSTEM 4I 5 



monotremes for the passage of sperm only, while urine passes from the 

 bladder to the cloaca by way of a urinary canal. In addition to the 

 erectile tissue of the corpus cavemosum urethrae, paired masses of 

 erectile tissue, the corpora cavernosa penis, make their appearance in 

 the phallus of monotremes. The posterior free portion of the phallus 

 of monotremes becomes more elongated than in reptiles, and is surrounded 

 by an integumentary fold, the preputial sac, which disappears during 

 erection. (Fig. 340) 



In marsupials and placental mammals, with the disappearance of 

 the cloaca the phallus becomes more independent as an external penis. 

 The urinary canal of monotremes disappears, and the urine is conveyed 

 to the outside through the urogenital canal or urethra. The glans penis 

 with an enlarged corpus cavernosum urethrae persists. In the female, it 

 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 difficult to describe them separately, 

 their wide divergence in function makes this desirable. 



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. 

 Each is approximately four inches long, two inches wide, and an inch 

 and a half thick. The upper end of each kidney overlaps the twelfth 

 rib, the left kidney being usually somewhat higher than the right. The 

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



