396 HISTORY OF THE HUMAN BODY 



In a complete, or typical descensus, in which the bursa is 

 contained in a scrotal sac, the parts are related as in Fig. in, 

 B. The processus vaginalis of the peritoneum, continuous 

 beyond the sac with that which lines the abdominal wall, 

 wraps itself partly around testis and epididymis, thus forming 

 a membrane, the tunica vaginalis propria, with a parietal and 

 visceral layer, and a serous cavity included between them. 

 This serous cavity is naturally continuous with the main ab- 

 dominal cavity, the ccelom, and the passage between them re- 

 mains open in those mammals in which the external appearance 

 of the testes is periodic; in those, however, in which the 

 descent is final and definite, it closes up during late, often 

 post-natal, development, and all communication between the 

 two cavities is lost. The vessels and nerves of the testis, to 

 which is added the ductus deferens, become united by con- 

 nective tissue into a single structure, the spermatic cord, which 

 escapes from the testis along the side not invested by perito- 

 neum, becomes recurved and enters the abdominal cavity by 

 running along the wall of the pouch, covered by the parietal 

 layers of peritoneum i.e., the tunica vaginalis propria. 



Outside of this come three layers which represent the ab- 

 dominal muscles and their fascia; in order, beginning from 

 within: i, the tunica vaginalis communis, i.e., common to both 

 testis and spermatic cord, a continuation of the fascia trans- 

 versa ; 2, the cremaster muscle, a continuation of the trans- 

 versalis and internal oblique, and 3, the fascia cremast erica 

 [Cooper's], which represents the external oblique, but is with- 

 out muscular fibers. 



Beyond this comes the integument, although this is often 

 differentiated into two layers through the development of its 

 involuntary muscular fibers into a layer of integumental mus- 

 cles, the tunica dartos, which occasions a wrinkling of the 

 surface in response to slight stimuli. 



The external reproductive organs have arisen as one of the 

 adaptations required by the assumption of a terrestrial exist- 

 ence, the ultimate cause being found in the non-suitability of 

 the air as a medium for the transmission of the spermatozoa. 



