1116 ACCESSORY MALE ORGANS. [Boon iv. 



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 trabeculse 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 trabeculse 

 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 

 trabeculse 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. 



691. 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 supplied 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 trabeculse, 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 transversus 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 the nervi erigentes will 

 not produce complete erection ; the factors just mentioned are 

 absent, and the blood, though it more or less fills the venous 

 sinuses, flows freely away by the veins. 



The dilating action of the nervi erigentes and the nervous 



