REPRODUCTION IN MAN 1227 



Sexual 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 spermatozoa 

 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 rhyth- 

 mic contractions, thus forcing the spermatozoa they contain into the 

 urethra. The spermatozoa, mixed with the secretions of the epididymis, the 

 seminal vesicles, the prostatic glands, and the glands of Cowper, form the 

 semen, which is pressed along the urethra by rhythmical contractions, 

 from behind forwards, of the bulbo- and ischio-cavernosi muscles. It has 

 been stated that movements take place coincidently in the uterus, so that 

 its axis more nearly corresponds to that of the vagina. The movement of 

 the semen along the uterus and Fallopian tubes is ascribed by certain 

 observers to an antiperistaltic contraction of these organs. A more im- 

 portant factor is probably the movement of the spermatozoa themselves. 

 As we have already seen, these are introduced into the female passage in 

 countless numbers. They will be chemiotactically attracted by the alkaline 

 mucus, secreted by and filling the cervix of the uterus. When they have 

 entered this organ they will meet the downward stream of mucus impelled 

 by the action of the cilia lining the uterus and Fallopian tubes. It seems 



