grg MECHANISM OF ERECTION. 



Keener de Graaf obtained complete erection of the penis by forcibly injecting its blood- 

 vessels (1668). 



The preliminary phenomena consist in a considerable increase of the, arterial 

 blood-supply, the arteries being dilated and pulsating strongly. The arteries are 

 controlled by the nervi erigentes. The nervi erigentes [called by Gaskell the 

 pelvic splanchnics (fig. 439] arise chiefly from the second (more rarely the third) 

 sacral nerves (dog), and have ganglionic cells in their course (Loven, Nikolsky). 

 These nerves contain vaso-dilator fibres, which can be excited in part reflexly 

 from the sensory nerves of the penis, the transference centre being in the centre 

 for erection in the spinal cord ( 372, 4). Sensory impressions produced by 

 voluntary movements of the genital apparatus (by the ischio- and bulbo-cavernosi 



and crem aster muscles) can also dis- 

 charge this reflex ; while the thought 

 of sexual impulses, referable to the 

 penis, tends to induce erection. The 

 nervi erigentes also supply the longi- 

 tudinal fibres of the rectum (Fellner). 

 The centre for erection in the 

 spinal cord ( 362, 2) is, however, 

 controlled by the dominating vaso- 

 dilator centre in the medulla oblon- 

 gata ( 372), and the two centres are 

 connected by fibres within the cord ; 

 hence stimulation of the upper part 

 of the cord, as by asphyxiated blood 

 ( 362, 5) or muscarin, may also be 

 followed by erection (Nikolsky). [The 

 seminal fluid is frequently found dis- 

 charged in persons who have been 

 hanged.] 



The psychical activity of the cere- 

 brum has a decided influence on the 

 genital vaso-dilator nerves. Just as 

 the psychical disturbance which ac- 

 companies anger or shame is followed 

 Anterior wall of the pelvis with the urogenital sep- by dilatation of the blood-vessels of 

 turn seen from the front. The corpus cavernosum the head, owing to stimulation of the 



vaso-dilator fibres, so when the atten- 

 tion is directed to the sexual centres 

 there is an action upon the nervi eri- 

 fascia (J) ; 6, vena profunda penis"; 7, artery and gentes. This action of the brain is 

 vein of the bulbo-cavernosus. more comprehensible, since we know 



that the diameter of the blood-vessels is affected by the cortex cerebri ( 377). The 

 fibres probably pass from the cerebrum through the peduncles of the cerebrum and 

 the pons ; as a matter of fact, if these parts be stimulated erection may take place 

 ( 362, 4) (Eckhard). 



When the 'impulse to erection is obtained by the increased supply of arterial 

 blood, thzfull completion of the act is brought about by the activity of the follow- 

 ing transversely striped muscles : (1) The ischio-cavernosus arises from the coccyx, 

 and by its tendinous union surrounds the root of the penis (fig. 172). When it 

 contracts, it compresses the root of the penis from above and laterally, so that the 

 outflow of blood from the penis is hindered. It has no action on the dorsal vein 

 of the penis, as this vessel lies in a groove on the dorsum of the penis, and is 

 therefore protected from compression by the tendon. (2) The deep transversus 



(4) with the urethra (3) is cut across below its 

 exit from the pelvis. 1, symphysis pubis ; 2, 

 dorsal vein of the penis ; 5, part of the bulbo- 

 cavernosus ; t, deep transversus perinei with its 



