Mammalia : Visceral Systems 651 



walls become strongly muscular. This region of the tube is known as the 

 ductus ejaculatorius. 



In size, the testis, like the ovary, varies with the size of the animal. 

 Ovoid in form, the cat's testis is about 1.5 cm. in long diameter and 

 1.0 cm. in short diameter. The human testis, exclusive of the epidid- 

 ymis, is 4 or 5 cm. long and 2.5 to 3 cm. in short diameter. That of a 

 horse is about 11 by 6 cm. Often the testis of one side of the body is 

 noticeably larger than that of the other side. The horse's left is often 

 the larger. 



In all cases the testis produces vast numbers of spermatozoa. It is 

 probable that the number produced in the larger animal is not pro- 

 portional to the greater size of the organ. 



The position of the testes in adult mammals is variable. In 

 OrnUhorhynchus and Echidna they remain at the region of their em- 

 bryonic origin — therefore near the kidneys. But in most other adult 

 mammals they become shifted to more posterior positions. In some 

 insectivores and in elephants they undergo little, if any, displacement. 

 In many insectivores, in cetaceans and sea cows, in armadillos, sloths, 

 American anteaters, and conies (Hyrax), the testes remain permanently 

 in the abdominal cavity but are finally lodged in the extreme posterior 

 (pelvic) region of it. In most other adult mammals, the testes lie 

 temporarily or permanently in a sac, the scrotum, which protrudes 

 externally from the posterior ventral abdominal wall. 



The scrotum is an integumentary pouch occupied by a pair of 

 coelomic pockets. The cavity within each pocket (bursa inguinalis) 

 communicates, primarily if not permanently, with the abdominal 

 coelom (Figs. 492, 493). The several layers of abdominal muscles 

 continue into the wall of the bursa, being represented by layers of 

 fascia if not by actual muscular tissue. The bursa is lined throughout 

 by a continuation of the abdominal peritoneum forming the tunica 

 vaginalis. It has been pointed out (p. 23) that the visceral organs 

 do not, in strict sense, lie in the abdominal cavity. Each organ is 

 separated from the coelom by a layer of peritoneum. The same rela- 

 tions obtain in the scrotal sac. The testis lies, not actually in the lumen 

 of the bursa, but on the dorsal wall of it (Fig. 493) and separated from 

 the lumen by the tunica vaginalis, which is reflected over the ventral 

 surface of the testis. Thus the tunica vaginalis, like the abdominal 

 peritoneum, has its parietal and its visceral part. 



The connection of the testis with the vas deferens, blood-vessels, 

 and nerves are all established before the organ is moved into the 

 scrotum. In its "descent" into the scrotum, the testis carries these 

 structures with it. Where they pass through the abdominal wall, they 

 become wrapped together by connective tissue, the whole complex 



