1278 PHYSIOLOGY 



to speak, the undoing of a preparation for the reception of a fertilised ovum. 

 The ovum has been discharged, the mucous membrane has 'been prepared 

 for its reception, but fertilisation not having taken place, ovum and mucous 

 membrane are cast out together in the menstrual flow. Unfortunately 

 almost equally definite clinical evidence has been adduced for the view that 

 ovulation occurs during or after the menstrual period. Light is thrown 

 upon the question by the study of the phenomena of ' nit ' or ' heat ' in the 

 lower animals. In most mammals impregnation and conception can only 

 occur at certain definite periods of the year. At these seasons the female 

 presents a swelling of the mucous membrane of the external genitals, and 

 often a flow of blood or mucus. As a rule it is only when in this condition 

 that it will permit the approach of the male. Thus the bitch ' comes on 

 heat ' as a rule twice in the year ; the cat three or four times ; most car- 

 nivora only once a year. At these periods the uterus shows well-marked 

 histological changes, which may be divided into the following periods : 



(1) The period of rest. During this time, which extends'over the greater 

 part of the year, the mucous membrane is thin and pale. The period of 

 heat being known as the cestrus, this first period is denoted by Heape the 

 ancestrum. 



(2) The period of growth or congestion. This corresponds to the pre- 

 menstrual thickening of the mucous membrane of the human female. 



(3) Period of destruction, associated with haemorrhages into the mucous 

 membrane, desquamation of the superficial epithelial cells, and occasionally 

 discharge of blood and mucus from the vagina. These two periods are 

 grouped together as the pro-oestrum. 



(4) Period of recuperation corresponding to the post-menstrual regenera- 

 tion of the mucous membrane. It is during the first part of this period or at 

 the very end of the last period that ovulation occurs in those animals where 

 ovulation is independent of coitus. It is at this time too that the animal 

 exhibits sexual desire and permits the approaches of the male. If fertilisa- 

 tion occurs, the mucous membrane undergoes rapid hypertrophy, much 

 more marked than that occurring during the pro-oestrum. In the absence 

 of impregnation the mucous membrane returns to the condition of rest, the 

 stage of return being known as the met oestrum. 



These results have been found by Heape and Marshall to apply to a 

 large number of different mammals. We are therefore justified in con- 

 cluding that menstruation is the physiological homologue of the pro-oestrum 

 in the lower mammals, and that ovulation occurs, or at any rate that the 

 ova attains maturity, after or at the very end of the menstrual flow. If we 

 consider that the ovum may take some days to pass down the Fallopian 

 tube to the uterus, and that the spermatozoa may retain their vitality for 

 ten days or more in the Fallopian tubes or uterus, it is evident that in man 

 impregnation may take place at any time between two menstrual periods. 

 Sexual desire is thus not limited to certain seasons, as is the case with most 

 of the lower animals. 



