622 VETERINARY PHYSIOLOGY 



to the muscular coat. The nature of their secretion requires 

 investigation. The nerve supply is considered on p. 633. 



IV. The (Estrous Cycle. 



The female of all species of mammals, and the male of some, 

 pass through a cycle of sexual activity and sexual rest. This 

 has been called the oestrous cycle. 



In the anoesfrum the ovaries, uterus, and other sexual organs 

 are quiescent, and their blood-vessels more or less contracted. 

 In the procestrum one or more ova ripen, the blood-vessels 

 dilate, hsemorrhages occur into the mucous membrane of the 

 uterus, and blood may escape by the vagina. In the cestriim 

 these conditions reach their maximum, and in the lower animals 

 coitus is allowed. 



In the mare, cow, pig, and sheep the cestrum may recur at 

 short intervals at some special season of the year, while in 

 carnivora, e.g. the dog, it occurs only once at a given sexual 

 season. 



In the mare the usual sexual season is early summer. Each 

 cestrwin may last 4 or 5 days. 



In the cow the season varies, and in it cestrum may recur at 

 intervals of about 20 days. 



In the sheep the season is in autumn, and oestrum may 

 recur repeatedly at intervals of about 12 days, lasting each time 

 for only about a day or less. 



In the sow oestrum may recur at intervals of about 20 

 days, and the whole cycle is generally repeated twice a 

 year. 



In the bitch an oestrum cycle recurs generally twice a year. 

 (Estrum usually lasts for about a week. Even if not impreg- 

 nated, the ovary, uterus, and mammary gland show the same 

 chauo-es as occur in pregnancy and which may be secreted. 



In the postcestrum the organs return to their normal condi- 

 tion, and the corpus luteum or corpora lutea grow in the 

 ovaries. 



In some animals, e.g. the rabbit, rupture of the follicle occurs 

 only if copulation takes place. If this does not occur the follicle 

 atrophies. 



