Chap, i.] MALE ORGANS. 1117 



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 ves- 

 sels 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. 



§ 692. 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 reser- 

 voir 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 contents of the seminal tubes. Hence we may 

 conclude provisionally that the act begins with a propulsion of 

 the contents of the 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 stimu- 

 lation of the anterior roots of the second and third lumbar 

 nerves leads to a powerful contraction 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 vesiculse seminales ; or we 

 may suppose that preparatory feeble contractions of the epididy- 

 mis fill and distend both the vas deferens and the vesiculse 

 seminales, and that the act really begins with a more powerful con- 

 traction 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 that gland into the urethral canal. So far 

 plain muscular fibres only are brought into play ; but the act 

 is completed by the aid of striated muscles, namely, by forcible 



