E strum 



47 



When the egg has fully ripened and the ovisac has com- 

 pleted its growth and is fully distended with fluid, estrum, 

 or "heat", occurs. 



2. Estrum 



Preliminary to reproduction, an irresistible sexual desire 

 occurs, leading to coition. 



Estrum finds expression in various ways in different 

 species of animals. In general there is a nervous excitabil- 

 ity, the external genitals are swollen and vascular, and there 

 is an increased secretion of mucus from the vulva and the 



GermiTial Epitkeliurn 



rrimaTy Follicle 



'^'i'i 



Fig. 22 — Section of ovary of cow showing a follicle, 

 0.6 mm., with ovum. 



vagina. In the mare, when other horses — especially stal- 

 lions — are present, there are frequent emissions of urine in 

 small quantities, and the vulvar lips are frequently opened 

 and the erected clitoris protruded. lii the cow and the sow, 

 the female imitates the copulatory act of the male by mount- 



