REPRODUCTION. 181 



penis muscle by its contraction compresses the veins of the organ, 

 and the veins become turgid with blood. The arteries enter the 

 structure of the erectile tissue along the pubic bone, and are not 

 pressed upon by the contraction of the muscle. 



What is menstruation? 



It is a periodical change which accompanies ovulation in women. 



Is menstruation dependent upon the ripening and discharge of an 

 ovum ? 



No. The phenomena of menstruation may occur without the 

 rupture of a Graafian follicle, and, on the other hand, ovulation 

 may occur in amenorrhceic women. As a rule, however, the two 

 processes are simultaneous, and the discharge of an ovum occurs 

 during the period. 



What is the character of the menstrual discharge ? 



It is a thin, bloody fluid of a dark color and having a peculiar 

 odor. It consists of blood, epithelium, and mucus from the uterus 

 and vagina, together with the decidua menstrualis. The blood 

 does not clot. 



What is the decidua menstrualis? 



It is a membrane developed by the uterine mucous membrane 

 for the purpose of furnishing a site for the implantation and growth 

 of an impregnated ovum. It occurs entirely within the body of 

 the organ, and does not extend into the cervix. If impregnation 

 does not occur, the decidua menstrualis breaks down and leaves its 

 vessels bleeding. 



Why does not the menstrual blood clot? 



Probably because of its mixture with the mucus of the uterus 

 and vagina. 



What is the duration of menstrual life ? 



Usually in temperate climates from fourteen or fifteen to forty-five 

 or fifty years of age. The periods recur regularly at intervals of 

 about four weeks, but the occurrence of pregnancy causes a cessa- 

 tion which lasts through lactation, with occasional exceptions. 



What is the corpus luteum? 



After the escape of an ovum there is an effusion of blood into 

 the cavity of the Graafian follicle. The clot which follows is dis- 

 posed of by the same retrogressive processes which extravasated 



