902 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and are brought about by a distention of the cavernous spaces of the erectile 

 tissues with blood. The vascular phenomena are, however, accompanied by 

 complex nervous and muscular activities. As regards the penis, the arteries 

 supplying the organ relax and allow blood to flow in quantity to the corpora 

 cavernosa and the corpus spongiosum. Simultaneous relaxation of the smooth 

 muscle fibres scattered throughout the trabecular framework of the corpora 

 increases the capacity of the blood-spaces. Furthermore, the ischio-cavernosus 

 (erector penis) and bulbo-cavernosus muscles contract and compress the 

 proximal or bulbous ends of the corpora and the outgoing veins. The result 

 of this combined muscular relaxation and contraction is a free entrance of 

 blood into and a difficult exit from the vascular spaces ; this leads to a swelling 

 and distention which aid further in compressing the venous outlets and, being 

 limited by the tough, fibrous tunics of the corpora, result in making the organ 

 stiff, hard, erect in position, and well adapted to its specific function. During 

 the process of erection the cresta of the urethra or caput gallinaginis, which is 

 an elevation extending from the cavity of the bladder into the prostatic por- 

 tion of the urethra and containing erectile tissue, becomes turgid and, by the 

 aid of the contraction of the sphincter urethrce, effectually closes the passage 

 into the bladder. Erection is a complex reflex act, the centre of which lies 

 in the lumbar spinal cord and may be aroused to activity by nervous impulses 

 coming from different directions. Impulses may originate in the walls of the 

 ducts of the testis from the pressure of the contained semen or in the penis 

 from external stimulation of the nerve-endings in the skin, in both cases 

 passing along the sensory nerves of the organs to the spinal centre ; or they 

 may originate in the brain and pass downward through the cord, the impulses 

 in this case corresponding to sexual emotions. The centrifugal paths for the 

 arteries are along the nervi erigentes, which are true vaso-dilator nerves, and 

 in the mammals, where experiment has proved their existence, pass from the 

 spinal cord along the posterior lumbar (monkey) or anterior sacral (monkey,, 

 dog, cat) nerves to their arterial distribution. The ischio- and bulbo-caverno- 

 sus muscles are under the control of their motor nerve supply, consisting of 

 branches of the perineal nerve. 



In the female, anatomists recognize the homologues of the male erectile 

 parts as follows : the clitoris with its corpora cavernosa and glans as the homo- 

 logue of the penis, the two bulbi vestibuli as that of the bulb of the corpus 

 spongiosum, the pars intermedia perhaps as that of the corpus spongiosum 

 itself, and the erector clitoridis muscle as the homologue of the erector penis 

 (ischio-cavernosus). The mechanism of erection is similar to that in the male, 

 and the result is a considerable degree of firmness in the external genital 

 organs. 



The sexual excitement attendant upon copulation is usually much greater 

 in man than in woman, and culminates in the sexual orgasm, when the emis- 

 sion of semen from the penis into the vagina occurs. It will be remembered 

 that the prepared semen is stored in the ducts of the testes. The discharge 

 of the fluid is a muscular act which begins probably in the vasa efferentia. 



