770 A TEXTBOOK OF PHYSIOLOGY 



passes off during menstruation. The emotional nature of women 

 may change at this time. 



The period of active sexual life in the female ceases about forty- 

 five to fifty years of age the climacteric. The menopause is often 

 attended with nervous symptoms, and is the period of life at which 

 certain ailments are more prone to develop. 



The Process of Insemination. The process of insemination varies 

 in different species of animals. In fishes, the sperm is shed into the 

 water in the neighbourhood of the eggs. With frogs, the male clasps 

 the female, and pours the male secretion over the eggs as they are shed. 

 In birds, the semen is introduced into the cloaca by the male organ, 

 and incorporated in the egg before it is laid. In mammals, the egg 

 is not shed, and the male secretion is deposited in the female tract 

 by introduction of the penis the act of coitus. This act is accom- 

 panied by erection of the organ. This is brought about by an engorge- 

 ment with blood of the vessels of the penis, so that the organ 

 becomes greatly increased in volume, its blood-pressure raised, and 

 its temperature increased. The accepted explanation is that, under 

 the influence of the nervi erigentes, the arterioles of the organ become 

 much dilated, while the efferent veins become compressed by the 

 action of such muscles as the ischio-cavernosus, the transversus 

 perinaei profundus, the bulbo-cavernosus. 



The erection is controlled by the presence of a centre in the spinal 

 cord. If the centre be destroyed, erection is no longer possible. 

 The afferent stimuli to this centre may come locally from the filling 

 of the testes with semen; from the excitation of the sensory nerves 

 of the penis; from the filling of the bladder (especially noticeable in 

 young children); from the rectum, as when haemorrhoids (piles) are 

 present. Stimuli may also come from the great brain the sexual 

 emotions. In animals, the sense of smell plays a considerable part 

 in the process. 



Ejaculation of the semen is caused by strong peristaltic contrac- 

 tions of the muscles of the vesiculse seminales forcing the semen into 

 the urethra. The incoming semen distends the urethra, bringing 

 about a rhythmic contraction of the bulbo-cavernosus muscle, which 

 ejaculates the semen from the urethra. A contraction of the ischio- 

 cavernosus and transversus perinaei muscles also occurs at this time, 

 but these probably play but little part in the actual ejaculation of 

 the semen. 



Ejaculation is brought about reflexly through a centre in the cord. 

 The impulses are brought to the centre by mechanical stimulation 

 of the sensory nerves to the penis. It may, however, be caused in 

 sleep as the result of pressure of semen in the vesicles, or by 

 emotional impulses from the great brain (sexual dreams). It is 

 computed that there are about 60,000 spermatozoa in each c.c. of 

 the ejaculation, the volume of which is about 5 c.c. 



In the female, a corresponding series of events takes place during 

 sexual intercourse. Reflexly excited, the clitoris becomes engorged 

 and erected, the Fallopian tubes and the uterus perform peristaltic 



