Mammalia: Visceral Systems 



653 



Abd. \ 



Fig. 493. Diagrams illustrating the descent of the testis as seen in parasagittal 

 section, (d. def.) Ductus deferens; (Proc. Vag.) processus vaginalis (the diver- 

 ticulum of the peritoneum pushed into the scrotal sac). (Courtesy, Patten: "Em- 

 hryology of the Pig," Philadelphia, The Blakiston Company.) 



known as cryptorchism — and so must remain permanently in the 

 abdominal cavity. 



In some mammals (e.g., tapir, rhinoceros, civet, otter) each testis 

 lies in a bursa which extends more or less deeply into the inguinal or 

 perineal (Fig. 494E) region, but the two sacs do not meet in a common 

 scrotum and they cause little or no external protrusion of the body- 

 wall. 



In marsupials the scrotum, lacking in some of them, is anterior 

 to the penis. In placental mammals the positions are reversed — the 

 scrotum is posterior. 



"Inguinal canal" is a name which may be applied logically and 

 conveniently to the communicating passage between the abdominal 

 cavity and the bursa inguinalis, and is often so used, but confusion 

 results from the fact that the same name is applied, especially in 

 human anatomy, to the passage occupied by the spermatic cord in 

 its course through the abdominal wall after the former passage has become 

 closed. The former (Fig. 493C) connects two regions of the coelom and 

 is internal to the peritoneum. The vas deferens and other structures 

 which constitute the spermatic cord lie in the dorsal wall of the bursa 

 inguinalis and primarily external to the tunica vaginalis (peritoneum). 

 In strict sense, the two passages are not the same, although intimately 

 related. 



The mechanics of the "descensus testiculorum" is a problem 

 not yet fully solved. Apparently several factors are involved. The 

 initial factor in the backward shifting of the gonads, both ovary and 

 testis, seems to be a polarized growth. Persistent growth at the hind 

 end of the gonad is accompanied by atrophy, at a compensating rate, 

 of the anterior end of the organ. This process may even be regarded as 

 a unique method of locomotion in that it moves the organ from one 

 place to another. Other factors are external to the gonad. The change 

 of position is doubtless due in part to unequal longitudinal growth of 

 the body-wall and the membranes and ligaments connecting gonad 



