1372 PHYSIOLOGY 



a large number of different mammals. We are therefore justified in 

 concluding 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. Desire is 

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

 lower animals. 



FERTILISATION 



The act of impregnation consists in the introduction of sperma- 

 tozoa into the female genital tract, where they may come in contact 

 with and fertilise the ovum, which is discharged from the ovary by 

 bursting of a Graafian follicle. This is effected in the act of coitus or 

 sexual congress by the insertion of the penis into the vagina of the 

 female. Before this can occur erection of the male organ must take 

 place. The mechanism of erection is twofold. The most important 

 factor, as was shown by Eckhard and Loven, is an active dilatation of 

 the vessels of the penis, especially of the medium-sized and smaller 

 arteries. If the penis be cut across while in the flaccid condition, 

 venous blood merely trickles away from the cut surface, whereas, if 

 erection be excited, the flow of blood from the cut surface is increased 

 eight to ten times, and the blood becomes bright arterial in colour. It 

 is thus possible to excite erection in an animal, in whom the second 

 factor has been abolished by paralysing the muscles by means of 

 curare. This second factor is the contraction of the ischio-cavernosus or 

 erector penis muscle, certain fibres of which pass over the dorsal vein 

 of the penis and compress this vessel when they contract. Since 

 ligature of the veins coming from the penis does not produce erection, 

 the contraction of this muscle must be regarded as simply aiding 

 the effects of the arterial dilatation. 



Before or at the beginning of coitus analogous changes occur in 

 the female organs, leading to erection of the clitoris and of the erectile 

 structures of the vulva. The glands of the vulva, especially the glands 

 of Bartholini, secrete a mucous fluid, thus lubricating the passage 

 into the vagina. The friction between the glans penis and the wall of 

 the vagina causes a reflex discharge of motor impulses in both male 

 and female. In the former the muscular walls of the vasa deferentia 

 and seminal vesicles enter into rhythmic contractions, thus forcing 

 the spermatozoa they contain into the urethra. The spermatozoa, 

 mixed with the secretions of the epididymis, the seminal vesicles, 



