

VrVITAllOUS ANIMALS. 391 



'when they occur externally, are contained in a scrotum 

 formed by the common integuments. They are enveloped 

 by two coats, the first of which, termed the tunica vagina- 

 lis, is derived from the peritoneum, and the second, called, 

 from its white colour, tunica albuginea, is merely a reflect- 

 ed duplicature of the former. This second coat adheres 

 closely to the surface of the testicle, and is only in union 

 with the first in a line on the dorsal edge *. 



Each testicle is usually of an oval form, and of a firm 

 compact substance, consisting of an infinite number of 

 ramifications of arteries and veins, termed spermatic, unit- 

 ed together by a cellular substance. The spermatic tubes 

 take their rise in different parts of the testicle, and gradu- 

 ally uniting, at last emerge from its body, in the form of a 

 single canal, termed the spermatic duct. This duct, which, 

 at its commencement, where it is exceedingly convolu- 

 ted, adheres to the surface of the testicle by cellular sub- 

 stance, and is in part covered by the tunica alhugmosa, is 

 termed Epididymis. The remaining part of the spermatic 

 duct, termed Vas deferens, usually preserves a straight 

 course towards the base of the urethra, where it terminates. 

 In many species, however, it is slightly tortuous, and its 

 walls, in some cases, assume a considerable degree of thick- 

 ness, and become obviously glandular. In a few instances, 

 it is enlarged in its diameter towards its termination. The 

 spermatic ducts usually terminate at the base of the urethra, 

 but, in some cases, they are continued to the extremity of 

 the penis. 



* In the human foetus, the testes are at first lodged in the belly, and only 

 descend into the scrotum a little before birth. In some instances, the de- 

 scent never takes place. This, however, is unnatural, but in many animate 

 termed tcsticondi^ the testes are always internal, and in a few the descent is 

 periodical. 



