THE ACCESSORY REPRODUCTIVE ORGANS 263 



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 unquestionably 

 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 ligaturing 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. Kolliker 1 

 suggested that their action is temporarily inhibited, and that the 

 relaxation of the trabeculae, which consequently follows, permits 

 the vascular spaces to distend. According to Valentin, 2 these muscles 

 contract, and in so doing cause a dilatation of the walls of the vessels, 

 which thereby increase in volume. Langley and Anderson's observa- 

 tions, which support Kolliker's suggestion, are described 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 longitudinally arranged, 

 unstriated fibres, inserted at the attachment of the prepuce, and 

 continued backwards in the middle line 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. When it contracts it causes a marked dorsal 

 curvature of the penis. 3 



1 Kolliker, Verhandl. der Wiirzburger Phyx. Mvd. G'esell., vol. ii., 1851. 



2 Valentin, Lchrbuch der Physiologic, vol. ii., 1844. 



3 Langley and Anderson, "The Innervation of the Pelvic and Adjoining 

 Viscera : Part III. The External Generative Organs," Jour, of Physiol., vol. xix., 

 1895. The retractor muscle is remarkable for its sensitiveness to changes of 

 temperature, and at the same time for being unusually tenacious of life. It 

 can be cut out of the body and preserved in blood-serum, in a cool place, for 

 days at a time, and afterwards, on wanning, will relax and undergo spontaneous 

 contractions. At a temperature of 40 C. it is quite placid ; but, on cooling 



