THE MALE REPRODUCTIVE ORGANS. 897 



sacral spinal nerves, first to third (dog), on each side and help to 

 form the pelvic plexus. They contain vasodilator fibers to the penis, 

 as well as to the rectum and anus, and also visceromotor fibers to the 

 descending colon, rectum, and anus. Eckhard, Loven, and others * 

 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 organ. 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 or rigidity of 

 the congested organ is completed by a partial occlusion of the 

 venous outflow, which is effected by a compression of the efferent 

 vein by means of the extrinsic muscles (ischio and bulbocavernosus) 

 and possibly by the intrinsic musculature as well. This compres- 

 sion does not occlude the blood-flow completely, but serves to in- 

 crease greatly the venous pressure. This explanation of the act of 

 erection, while no doubt correct, so far as it goes, leaves undeter- 

 mined the means by which the dilatation of the small arteries is 

 produced. Vasodilator nerve fibers in general are assumed to pro- 

 duce 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 hypo- 

 gastric nerve and plexus and the pudic nerve.' No such result of 

 their section is reported and it seems that in the matter of erec- 

 tion the actual mechanism of the great dilatation caused by the 

 nervi erigentes still contains some points that need investigation. 



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 



* See especially Francois-Franck, " Archives de Physiol. norm, etpathol.," 

 1895, 122 and 138. 

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