372 ACCESSORY MALE ORGANS. [BOOK iv. 



the nervi erigentes will not produce complete erection ; the factors 

 just mentioned are absent, and the blood, though it more or less 

 fills the venous sinuses, Hows freely away by the veins. 



The dilating action of the nervi erigentes and the nervous 

 impulses leading to the subsidiary acts in erection may be set 

 going as part of a reflex action, by stimulation of the glans penis. 

 Of such a reflex act the centre lies in the lumbar spinal cord and 

 erection, with emission of semen, has been witnessed in a dog 

 after division of the spinal cord in the thoracic region. But 

 erection also takes place as the result of emotions, in which case 

 we may suppose that impulses descending from the brain affect the 

 lumbar centre in a direct manner ; and indeed erection has been 

 experimentally brought about by stimulation of certain parts of 

 the brain. 



The antagonistic act, namely, constriction of the blood vessels 

 and retraction of the penis may, in the cat, be brought about by 

 stimulation of fibres coming from the upper lumbar (and possibly 

 the lower thoracic) region, and reaching their destination by way 

 of the sympathetic. 



948. The emission of semen, for which act erection is 

 preparatory, is carried out by a succession of agencies. The 

 epididymis with its coni vasculosi may be regarded as a re- 

 servoir filled by the secretory activity of the seminal tubes ; 

 hence its relatively enormous length. It is possible that the act 

 may begin with an increase of secretory activity on the part of 

 the seminal tubes, bearing perhaps especially on the fluid parts of 

 the semen, by which the epididymis becomes overfilled ; we have no 

 positive evidence of this. Nor have we evidence of any pressure, 

 either intrinsic by means of the plain muscular fibres which are 

 said to occur scantily in the septa of the testis, or extrinsic through 

 the cremaster or other muscles, being brought to bear on the con- 

 tents of the seminal tubes. Hence we may conclude provision- 

 ally that the act begins with a propulsion of the contents of 

 tha distended epididymis by means of peristaltic contractions of 

 the muscular walls of that tube. In any case the flow of fluid 

 having reached the vas deferens, is carried along that tube by 

 the peristaltic contractions of its much stouter and much more 

 muscular walls. In the monkey stimulation of the anterior roots 

 of the second and third lumbar nerves leads to a powerful con- 

 traction of the vas deferens, sweeping down it in a single wave. 



One effect, possibly a chief effect, of the flow along the vas 

 deferens is to fill and distend the vesiculre seminales ; or we may 

 suppose that preparatory feeble contractions of the epididymis fill 

 and distend both the vas deferens and the vesiculae seminales, 

 and that the act really begins with a more powerful contraction 

 of both these distended organs by which their contents are rapidly 

 ejected into the prostatic urethra ; at the same time contractions 

 of the muscular fibres of the prostate discharge the secretion of 



