252 THE PHYSIOLOGY OF REPRODUCTION 



is, that whereas the blood can freely enter the dilated vascular 

 spaces of the penis, its exit is retarded, while this leads to a 

 further distension of the vessels, the venous outlets of which 

 become still more compressed. 



Although the muscular mechanism of the penis unques- 

 tionably assists in the erection of that organ, it is equally clear 

 that it is incapable by itself of causing that phenomenon, since 

 erection cannot be induced by hgaturing the efferent veins. 

 Moreover, the penis can be made to erect in animals in which the 

 muscular mechanism has been paralysed by the injection of 

 curari, but the erection in such cases is incomplete. 



It is stated also that the smooth or unstriated muscle fibres, 

 which are scattered throughout the trabecular framework of the 

 corpora, participate in the process of erection, but there has 

 been some disagreement as to the precise part they play. 

 KoUiker ^ suggested that their action is temporarily inhibited, 

 and that the relaxation of the trabeculse, which consequently 

 follows, permits the vascular spaces to distend. According to 

 Valentin,^ these muscles contra.ct, and in so doing cause a 

 dilatation of the walls of the vessels, which thereby increase in 

 volume. Langley and Anderson's observations, which support 

 Kolhker's suggestion, are d^escribed below in giving an account 

 of the nervous mechanisms of erection and retraction. 



It is obvious, however, that in those cases in which the penis 

 remains erected for a considerable time a constant circulation 

 must be maintained through both the afferent and the efferent 

 vessels of the organ. 



In some animals (dog, cat, horse, hedgehog), but not in the 

 rabbit or Man, the penis possesses an accessory muscle. This is 

 called the retractor penis. It consists of a thin band of longi- 

 tudinally arranged, unstriated fibres, inserted at the attach- 

 ment of the prepuce, and continued backwards in the middle 

 hue over the ventral surface of the corpus spongiosum to the 

 bulbous part of the urethra, where it divides into two halves 

 which separate on either side of the anus. Some of the fibres are 

 continuous with a portion of the bulbo-cavernosus of the same 

 side, while others are connected with the wall of the urethra. 



1 KoUiker, Verhandl. der Wiirzburger Phys. Med. Gesell., vol. ii., 1851. 

 ^ Valentin, Lehrbtich der Physiologie, vol. ii., 1844. 



