As we proceed to study the anatomy of the human sexual apparatus we shall be constantly struck with the remarkable correspondence between animals and vegetables in the structure and functions of the sexual apparatus. Sexual Organs of Animals.--The male reproductive element is called a _spermatozoon_ or _zoosperm_. The female element is called an _ovum_, literally, an egg. The Spermatozoon.--The male reproductive element of animals is formed by an organ called the _testis_, or _testicle_, of which each male possesses two. They are elastic, glandular bodies, and are formed within the cavity of the abdomen, near the kidneys, but usually pass out of the abdominal cavity and descend to their permanent position before birth. The opening in the abdominal wall is usually completely closed in a short time; but occasionally it remains open, giving rise to congenital hernia, an accident in which a loop of intestine follows the testicle down into the scrotum, either completely or partially. In a few animals, as in the porcupine, the opening is never fully closed, and the testis remains in the cavity of the body most of the time, passing out only at certain periods. We also occasionally meet cases of human beings in which the testes have never descended from their place in the abdominal cavity, giving the individuals the appearance of eunuchs. This condition, however, though an abnormal one, does not in any way interfere with the function of the organs, as those who happen to possess it often imagine. We have also met with cases in which the organs were movable, and could readily be pressed up into the abdominal cavity, through the unclosed inguinal cavity, which afforded them a passage downward in the process of development. As before remarked, these peculiarities do not affect the functions of the organs in any appreciable degree, although they not infrequently give rise to some apprehension on the part of those subject to them. The left testicle is sometimes a little smaller than the right, another fact which is seized upon by quacks as a means of exciting the fears of young men who have been addicted to bad habits, although the peculiarity is generally without important significance. The testicles are connected with the urinary passage by means of two ducts which terminate near the base of the bladder, at which point they connect with the urethra. We need not dwell at further length upon the structure of the testicles, as this subject receives fuller attention elsewhere.