CHAP, i.] MALE ORGANS. 1509 



The erectile tissue in each of these structures consists of an 

 irregular labyrinth formed by trabeculse composed of connective 

 tissue with abundant elastic elements mixed up with a large but 

 variable amount of plain muscular tissue. The spaces of the 

 trabeculse are lined by spindle-shaped epithelioid plates, resting 

 in some cases on a layer of plain muscular fibres, and are venous 

 sinuses, into which blood finds its Way chiefly through the terminal 

 capillaries of the numerous arteries lying in the trabeculas but 

 also in some cases by minute arteries opening directly into the 

 spaces; from the sinus the blood finds its way out into smaller 

 regular veins. In the corpora cavernosa, and to a less extent in 

 the corpus spongiosum, the small arteries in the trabeculse are 

 extremely twisted up and looped, bulging into the venous sinuses 

 as arterial coils, the so-called ' helicine arteries.' 



When the arteries supplying these masses of erectile tissue, 

 namely, the branches of the pudic arteries and dorsal artery of 

 the penis, are constricted, and when the plain muscular fibres of 

 the trabeculas are in a state of contraction, whereby the venous 

 spaces are largely closed, the greater part of the blood flowing 

 through the arteries finds its way by ordinary capillaries into the 

 efferent veins, little blood passes into the venous sinuses, and the 

 whole tissue is relatively small in bulk. When on the other hand 

 the arteries are dilated and in addition the muscular bundles of 

 the trabeculfe are relaxed, a large quantity of blood passes into 

 the venous sinuses, these become greatly distended with blood ; 

 the whole mass of erectile tissue becomes turgid, and in propor- 

 tion to the resisting nature of the outer envelope, as is especially 

 seen in the corpora cavernosa, hard and rigid. 



947. In the dog and cat, fibres from the anterior roots of the 

 second and first, or sometimes from the third, sacral nerves form 

 the nervi erigentes, which passing to the pelvic plexus are distributed 

 to the penis and to other organs ; in the monkey the fibres are sup- 

 plied by the seventh lumbar and first sacral, sometimes also by the 

 second sacral nerves. They receive this name because stimulation 

 of them leads to erection of the penis ; and this results from a 

 vaso-dilator action on the arteries supplying the erectile tissue. 

 Erection of the penis is hence to a large extent a vaso-dilator 

 effect. But not wholly so ; the entrance of the blood from the 

 dilated arteries into the venous sinuses is facilitated by the 

 relaxation of the muscular bundles in the trabecula3, whose 

 contraction would offer an obstacle to the spaces becoming filled. 

 Further the filling of the venous sinuses tends of itself to 

 compress the large longitudinal veins running in the centre of 

 the corpora cavernosa and thus to increase the distension already 

 begun ; moreover contractions of the striated muscles, the trans- 

 versus perinaei, and the bulbo-cavernosus, between the bundles 

 of which the veins pass, also tend to check the outflow and so to 

 increase the erection. In the dog even powerful stimulation of 



962 



