THE URO-GENITAL SYSTEM 395 



integument of either the inguinal or perinaeal regions. The 

 scrotum is originally double, furnishing a separate sac for each 

 testis, but usually the two are fused into single median sac in 

 which the suture of union is usually apparent. In relation to 

 the penis the scrotum is originally anterior to it, prepenial, as 

 in all marsupials that possess one, but in placental mammals 

 these relations are reversed and the scrotum becomes postfcnial 

 through the migration of the penis in an anterior direction. 

 No definite cause for the descensus is known, either phylo- 

 genetic or physiological, and the phenomenon has gained rather 

 than lost in complexity through recent researches which show 

 the cooperation of several distinct elements previously not 

 taken into consideration. Formerly a mechanical explanation 

 was found in the gradual contraction of the band of perito- 

 neum which extends from the testis to the inguinal region 

 (Plate III, b), and termed the gubernaculum in reference to 

 its supposed function, but the matter is not as simple as that, 

 since this band itself is composed of several originally dis- 

 tinct elements, and, furthermore, can hardly be considered to 

 exert the tension ascribed to it. The initiative in the process 

 seems to be a slight invagination of the abdominal wall at the 

 point of insertion of the inguinal ligament. Through a sub- 

 sequent evagination followed by a second invagination a coni- 

 cal body is formed, the conus inguinalis (Fig. in, A), which 

 involves the muscular layers, and by a final outpushing of 

 this and the surrounding structures a subcutaneous muscular 

 pouch is formed, the bursa inguinalis, in the bottom of which 

 lies the conus, which serves as a point of insertion of the in- 

 guinal ligament. The bursa is lined by a pocket of perito- 

 neum, the processus vaginalis, which is reflected up over the 

 conus. The inward development of the conus absorbs and 

 shortens the inguinal ligament, and eventually the testis comes 

 to lie in the bursa, covered internally by the reflected peri- 

 toneum. As shown above, the bursa may or may not become 

 placed in a scrotal sac, but when it does, a scrotal ligament 

 (chorda gubernaculi) extends from the bottom of this sac to 

 the base of the conus. 



