Mammalia: Visceral Systems 641 



posterior position on the colon. In some cases (e.g., gianl anteater and 

 some whales) there is no definite ileocolic valve. 



In degree of development the cecum exhibits a wide range of 

 diversity, even among mammals of the same Order or Family. In 

 general, it is more capacious and more highly differentiated in herbi- 

 vores than in carnivores. In the horse it is about 2}^ feet long and 8 

 inches in diameter. In the rabbit it is 15 to 18 inches long and an inch 

 or more in diameter, except in the distal fourth or third of its length. 

 This region is much narrower and its thickened wall contains much 

 lymphoid tissue. The narrow region is known as the vermiform 

 appendix. In the wider part of the rabbit's cecum an inward-project- 

 ing fold of its lining winds spirally from end to end of the lumen, 

 greatly increasing the functional surface of the cecum. It strikingly 

 resembles the spiral valve of the elasmobranch intestine. In the cat 

 the cecum is little more than a half-inch long, and there is no appendix 

 unless the somewhat pointed and thicker-walled distal tip of the 

 cecum can be regarded as a vestige of an appendix. In man the cecum 

 is very short but it bears an appendix which is usually 3 or 4 inches 

 long (Fig. 485). The human appendix is highly variable and sometimes 

 has no lumen. Its wall contains much lymphoid tissue, but physio- 

 logically the organ seems to be more of a liability than an asset. 



IV Urinogenital System 



Excretory Organs 



The mammalian kidneys are metanephric, each developing at 

 the anterior end of a duct, the ureter, which grows forward from the 

 mesonephric (Wolffian) duct, arising near the junction of the latter 

 with the embryonic cloaca. In the adult the kidneys are on the dorsal 

 body-wall and near the middle of the length of the abdominal cavity, 

 but often not exactly opposite one another. In the several groups of 

 mammals there is considerable variation as to the external form and 

 general anatomy of the kidneys. 



In development of the metanephros, branches bud out 

 from the anterior tip of the embryonic ureter (Fig. 487). Each branch 

 becomes the outlet of an elaborate system of numerous renal tubules, 

 whose excretory parts are derived from the kidney-forming (nephro- 

 genic) tissue of the adjacent mesoderm of the body-wall (p. 82), the 

 whole complex constituting what is called a renal lobule. The number 

 of lobules varies greatly, ranging from only a few in some mammals up 

 to scores or hundreds in others. In general, they are more numerous 

 in the larger mammals, whose kidneys are correspondingly larger. In 

 the adult the lobules may remain distinct from one another so thai I he 



