CHAPTER II. 



IMPREGNATION. 



IN coitus the discharge of the semen containing the spermatozoa is 

 most probably effected by means of the peristaltic contractions of 

 the vesiculse seminales and vasa deferentia. assisted by rhythmical 

 contractions of the bulbo-cavernosus muscle, the whole being a 

 reflex act, the centre of which appears to be in the lumbar spinal 

 cord. In the dog, emission of semen can be brought about by 

 stimulation of the glans penis after complete division of the spinal 

 cord in the dorsal region. The emission of semen is preceded by 

 an erection of the penis. This we have already seen, p. 210, is in 

 part at least due to an increased vascular supply brought about by 

 means of the nervi erigentes; it is probable, however, that the 

 condition is further secured by a compression of the efferent veins 

 of the corpora cavernosa by means of smooth muscular fibres 

 present in those bodies. The semen being received into the female 

 organs, which are at the time in a state of turgescence resembling 

 the erection of the penis, but less marked, the spermatozoa find 

 their way into the Fallopian tubes, and here (probably in its upper 

 part) come in contact with the ovum. In the case of some animals 

 impregnation may take place at the ovary itself. The passage of 

 the spermatozoa is most probably effected mainly by their own 

 vibratile activity; but in some animals a retrograde peristaltic 

 movement travelling from the uterus along the Fallopian tubes has 

 been observed ; this might assist in bringing the semen to the 

 ovum, but inasmuch as these movements are probably parts of the 

 act of coitus and impregnation may be deferred till some time after 

 that event, no great stress can be laid upon them. 



