864 THE PHYSIOLOGY OF REPRODUCTION. 



have shown that when these fibers are stimulated there is a large 

 dilatation of the arterioles in the erectile tissue of the penis and a 

 greatly augmented blood-flow to the organs. If the erectile tissue 

 is cut or the dorsal vein is opened the blood-flow under usual con- 

 ditions is a slow stream ; but when the nervus erigens is stimulated 

 the outflow is very greatly increased; according to Eckhard's 

 measurements, eight to fifteen times more blood flows out of the 

 organ. The act of erection is therefore due essentially to a vas- 

 cular dilatation of the small arteries whereby the cavernous spaces 

 become filled with blood under considerable pressure. The caver- 

 nous tissues are distended to the limits permitted by their tough, 

 fibrous wall. It seems probable that the turgidity of the organ is 

 aided also by a partial occlusion of the venous outflow, which is 

 effected by a compression of the dorsal vein by means of the 

 muscle of Houston. This explanation of the act of erection, while 

 no doubt correct so far as it goes, leaves undetermined the means 

 by which the dilatation of the small arteries is produced. Vaso- 

 dilator nerve fibers in general are assumed to produce a dilatation 

 by inhibiting the peripheral tonicity of the arterial walls. If this 

 explanation is applied to the case under consideration it forces us to 

 believe that throughout life, except for the very occasional acts of 

 erection, the arteries in the penis are kept in a constant condition 

 of active tone. Moreover, on this view we should expect that 

 section of the vasoconstrictor fibers to the penis by abolishing the 

 tone of the arteries would also cause erection. These constrictor 

 fibers arise from the second to fifth lumbar spinal nerves, and reach 

 the organ by way of the pudic nerve or the hypogastric (pelvic) 

 plexus. No such result of their section is reported and it seems that 

 in the matter of erection the actual mechanism of the great dilata- 

 tion caused by the nervi erigentes still remains to be investigated. 

 The Reflex Apparatus of Erection and Ejaculation. The 

 dilatation of the arteries of the penis during erection is normally a 

 reflex act, effected through a center in the lumbar cord. This center 

 may be acted upon by impulses descending from the brain, as 

 in the case of erotic sensations, or by afferent impulses arising in 

 some part of the genital tract, from the testes themselves, from 

 the urethra or prostate gland, and especially from the glans penis. 

 Mechanical stimulation of the glans leads to erection, and Eckhard 

 showed in dogs that section of the pudic nerve prevents this reflex 

 from occurring, proving, therefore, that the sensory fibers concerned 

 run in the pudic nerve. Stimulation of these latter fibers leads also 

 to erotic sensations and eventually to the completion of the sexual 

 orgasm. This latter act brings about the forcible ejection of the 

 sperm through the urethra. It is initiated by contractions of the 

 musculature of the vasa deferentia, ejaculatory duct, the seminal 



