EEPRODUCTIVE FUNCTIONS. 919 



process is similar. In the case of the horse the process is more pro- 

 longed. Here the glans, instead of being pointed, as in the case of the 

 bull, possesses a considerably larger diameter than other portions of the 

 organ, and it is found that complete erection of this portion of the penis 

 does not take place until after the introduction of the organ within the 

 vagina of the female. The penis completely fills the vagina of the mare, 

 and more or less prolonged friction between its surface and that of the 

 penis is necessary before the act of ejaculation is accomplished. 



In the dog, on the other hand, the process differs from what lias 

 been described in either of the other groups of mammals. After the 

 introduction of the penis within the vagina of the female spasmodic con- 

 traction of the sphincter eunni muscle sets in, and, as a consequence, the 

 withdrawal of the male organ from the body of the female is, on account 

 of its peculiar shape, rendered impossible until almost complete relaxa- 

 tion has taken place. In the dog the process of relaxation is slower than 

 in other animals, and may not be completed until within half an hour or 

 even so long as two hours after the act of ejaculation. In these animals, 

 therefore, after the act of coition is accomplished, the penis becomes in- 

 verted on itself, until, instead of projecting anteriorly, it is drawn back- 

 ward between the hind legs of the animals through the attempts of the 

 male to leave the body of the female, and the two posterior surfaces are 

 kept in contact by the imprisonment of the male organ. 



Ejaculation is accompanied by the contraction of various other 

 muscles. The cremaster muscle elevates the testicle within the scrotum, 

 and the walls of the urethra contract in its various portions, serving to 

 compress the prostate and Cowper's glands and so cause the extrusion 

 of their contents. In the stallion the act of ejaculation is accompanied 

 by a rhythmical contraction of the muscles inserted into the tail. By 

 every contraction of the ejaculator urinas muscles the muscles passing 

 from around the anus to the tail are compressed, and, as a consequence, 

 the rhythmical motion of the ejaculator seminae is communicated to the 

 tail, and, therefore, by its movement affords an index to the completion 

 of the act of ejaculation. After the act of copulation is completed the 

 male organ is withdrawn and erection gradually disappears, while there 

 may be an escape of seminal fluid from the genital organs of the female. 

 This is especially observable in the case of the mare, where it has become 

 the custom immediately after the completion of the act of copulation to 

 either load the back of the mare or to make her move rapidly, so as to 

 prevent to a certain extent this loss. 



A single spermatozoon is, in all probability, sufficient to fertilize an 

 ovum, and this is accomplished by the entrance of the spermatozoon into 

 the ovum by a boring movement through the vitelline membrane, either 

 through the porous canals or the micropyle. 



