3<so A Manual of Veterinary Physiology, 



At the moment of intercourse, the uterus becomes erect, 

 and the introduction of the male element into it is assisted 

 by the aspiration following its subsidence. It would appear 

 necessary in the lower animals for the fluid to pass into the 

 neck of the uterus, and there is probably a great deal to 

 say in favour of the common practice of preventing the 

 mare from straining immediately after copulation, and 

 thus rejecting the spermatic fluid. 



The actual mechanism of ejaculation is produced by the 

 contraction of the vesiculse seminales, and probably of the 

 vasa deferentia, through the reflex action of the ejaculation 

 centre in the lumbar and sacral portions of the cord ; by 

 this means the seminal fluid is forced out of the vesiculse 

 into the urethra, and by means of the muscles of the 

 perinaeum is forcibly ejected from the urethra. 



The amount of seminal fluid ejaculated by the stallion 

 and bull is estimated by Colin at 770 grains to 930 grains. 



The Ovum. — The ovaries prepare the female element, the 

 ovum. The ovum is exceedingly small ; it is contained in a 

 thick envelope, the zona pellucida, within which is the yelk, 

 and at one part of the yelk is the germinal vesicle ; within 

 the vesicle is the germinal spot. When the ovum is about 

 to ripen, it passes from the periphery of the gland towards 

 the centre and undergoes certain changes, becoming large 

 through the accumulation of thud within the Graafian 

 follicle; it now passes to the surface of the gland once 

 more, preparatory to bursting. 



As the ova ripen, changes take place in the system of the 

 animal, known as ' heat ' or ' rutting.' It manifests itself by 

 uncontrollable sexual desire, which comes on at fairly 

 definite periods, and lasts for a certain number of days. In 

 the lower animals it is not accompanied by anything 

 approaching the menstruation in the human female 

 Although slightly blood-coloured discharges have been 

 observed in animals, wo may fairly say that menstruation 

 proper is not a normal occurrence, due probably to the tact 

 that neither horses, cattle, nor pigs have anything of the 

 nature of a uterine decidua. 



