172 



PHYSIOLOGY OF FARM ANIMALS 



[CH. 



The prooestruni wliicli is the period of ' eoining on heat' is 

 marked by increased activity on the part of all the generative 

 organs. The Graafian follicles which at any earlier stage were 

 situated more centrally within the ovary, increase in size so as to 

 become easily visible on the surface from which they may protrude. 

 The prooestrous uterine changes may be divided into two stages: 

 (1) growth and congestion, and (2) destruction. In the first stage 

 the uterine mucous membrane becomes thicker and more ex- 

 tensively vascularised , the capillaries being increased both in size 



Fig. 84. Section through prooestrous uterine mucous membrane of dog shosving 

 congested blood vessels between the glands (from Marshall). 



and number and the glands becoming more active. In the second 

 stage the walls of a certain number of capillaries break down and 

 the blood corjouscles are extravasated in the stroma, and tend to 

 become congregated just below the uterine epithelium; the latter 

 ruptures in certain places and blood is poured out into the cavity 

 of the uterus whence it passes down to the vagina and to the 

 exterior. Bleeding from the external vaginal opening in the 

 prooestrous bitch may last for a week or even longer, and the 

 entire jDrooestrum lasts for from ten to fourteen days. The a\ hole 

 process must be regarded as an act of preparation on the part of 



