842 THE PHYSIOLOGY OF REPRODUCTION. 



during sexual life the ovaries are completely removed by surgical 

 operation menstruation is brought to a close, this condition being 

 designated as the artificial menopause. 



Structural Changes in the Uterus During Menstruation. Men- 

 struation is a phenomenon of the uterus. The lining mucous mem- 

 brane, the endometrium, in the period of four or five days preceding 

 the flow, becomes rapidly thicker and its superficial layers are con- 

 gested with blood, and indeed in places small collections of blood 

 may be noticed. Opinions differ very much as to the change under- 

 gone by this thickened membrane during the flow. According to 

 some authors, most of the membrane is thrown off and the blood 

 escapes from the denuded surface mixed with pieces of the mem- 

 brane. According to others, no material destruction of the mem- 

 brane occurs, the blood that escapes being due to small capillary 

 extravasations or perhaps mainly to a process of diapedesis. It 

 would seem that the amount of destruction of the endometrium 

 must be subject to individual variations. After the cessation of 

 the flow the mucous membrane is rapidly repaired by regenerative 

 changes in the tissues; the surface epithelium, if denuded, is re- 

 placed by proliferation of the cells lining the uterine glands and 

 the thickened, edematous condition of the membrane rapidly sub- 

 sides during a period of six or seven days. While the escape of 

 blood takes place only from the surface of the uterus, the other 

 reproductive organs the ovary, the Fallopian tubes, and even the 

 external genital organs share to some extent in the vascular con- 

 gestion exhibited by the uterus during the period preceding' the 

 menstrual flow. The mucous membrane of the uterus may be said 

 to exhibit a constantly recurring menstrual cycle which falls into 

 four periods: (1) Period of growth in the few (5) days preceding 

 menstruation, characterized by a rapid increase in the stroma, 

 blood-vessels, epithelium, etc., of the membrane. (2) The men- 

 struation or period of degeneration (4 days), during which the 

 capillary hemorrhage takes place and the epithelium suffers de- 

 generative changes and is cast off more or less. (3) The period of 

 regeneration (7 days), during which the mucous membrane returns 

 to its normal size. (4) The period of rest (12 days), during which 

 the endometrium remains in a quiescent condition. 



The Phenomenon of Heat in Lower Mammals. The phenom- 

 enon known as heat in lower mammals resembles, in many essential 

 respects, menstruation in human beings, and they may be regarded 

 as homologous functions. Heat is a period of sexual excitement 

 which occurs one or more times during the year and during which 

 the female will take the male. This condition lasts, as a rule, for 

 several days, and in the female is accompanied by changes which 

 recall those of menstruation. The external genital organs become 



