440 



' Id, 



27'-' 



^ corpus luVcum ^ 



ORGAN SYSTEMS OF MAN 

 DAY'S —I 



--3 ,^-— 



II 



Fig. 16-32. The events that occur during the menstrual cycle in man are graphically outlined. 



gesterone. Once this hormone is released 

 the menstrual cycle is initiated in man. 



The menstrual cycle in man and other 

 primates has its counterpart in the estrus or 

 "heat" cycle in other mammals. It involves 

 a distinct rhythmic cycle of sexual activity 

 during some part of which the female is 

 receptive to the male. Such a cycle is com- 

 pleted every 14 days in guinea pigs, twice 

 a year in dogs, and once every 5 days in 

 rats. In humans and anthropoid apes the 

 estrus cycle is complicated, involving a pe- 

 riodic sloughing off of the highly vascular 



lining of the uterus every 28 days by men- 

 struation. In other mammals the uterine lin- 

 ing returns to the resting state with no 

 sloughing off or bleeding. The word men- 

 struation comes from the Latin mensis, 

 which means month. The menstrual flow 

 consists mostly of the epithelial lining of 

 the uterus together with the incorporated 

 gorged blood vessels. The course of events 

 that lead up to this dramatic event are 

 rather well known, but just tvhy it must 

 occur, since it is not found in lower mam- 

 mals, lacks an immediate explanation. 



