BEPKODUCTIVE FUNCTIONS. 917 



be found in the dilatation of the minute arteries through the action of 

 the nervi erigentes. These nerves are to be regarded as containing vaso- 

 dilator fibres which are in connection with a centre in the spinal cord, 

 controlled to a certain extent by the medulla oblongata, and also capa- 

 ble of being reflexly excited by impressions made upon the terminal 

 filaments of the sensory nerves of the penis. The centre in the lumbar 

 portion of the cord is also capable of being controlled by various other 

 afferent impulses, especially by the psychical activity of the cerebrum. 

 When, through any impression on the centre for erection, the blood-ves- 

 sels dilate through the action of the nervi erigentes, the completion of 

 the act of erection is accomplished by the action of several transversely 

 striated muscles. The ischio-cavernosus muscle arises from the coccyx, 

 and by its tendinous union with its fellow from the opposite side sur- 

 rounds the root of the penis, and by its contraction compresses the penis 

 so as, to a certain extent, to prevent the flow of blood from the organ. 

 This muscular action was at one time supposed to be the main factor in 

 the accomplishment of erection, but that this is not the case is proven 

 by the fact that the dorsal vein, tying in the groove between the two 

 corpora cavernosa, is protected from compression, and the congestion of 

 the organ is by no means due to the complete arrest of the venous circu- 

 lation. Otherwise, in certain pathological conditions where erection is 

 continuous, gangrene would result. The deep transverse muscle of the 

 perineum also serves to facilitate erection by compressing the deep veins 

 of the penis as they come from the corpora cavernosa between its con- 

 tracted horizontal fibres; while, finally, the bulbo-eavernosus muscle com- 

 presses the bulb of the urethra and so assists in the erection of the 

 urethral corpus spongiosum. That erection is dependent upon the nerv- 

 ous system is proven by the fact that section of the nerves of the penis 

 will prevent erection, as has been experimentally proven in the case of 

 the stallion. 



In the bull, in the process of erection, the S-shaped curve disappears 

 and the erected organ, protruding from the sheath, may acquire a length 

 of a meter or more, while the increase in diameter, owing to the great 

 development of the fibrous sheath of the organ, is less than in other 

 mammals. 



The erection of the penis has simply for its object the giving such 

 an increase to the rigidity of the organ as to permit of its introduction 

 within the genital organs of the female. 



The contact of the sensitive glans with the rugse of the vagina 

 inaugurates a reflex process which terminates in the discharge of the 

 seminal fluid through the urethra. 



In this process, which is termed ejaculation, two different factors 

 are concerned. In the first place, the passage of the seminal fluid to the 



