VIVIPAROUS ANIMALS. 39% 
‘when they occur externally, are contained in a scrotum 
formed by the commen 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 *. 
Fach 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 acellular 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 albuginosa, 1s 
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 obvieusly glandular. In a few instances, 
it is enlarged in its.diameter towards its termmation. The 
‘spermatic ducts usually terminate at the base of the urethra, 
but, in some cases, they are continued to the extremity of 
the penis. 

* Inthe human fetus, 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 animals 
termed testicond?, the testes are always internal, and in a few the descent is 
periodical. 
