1374 PHYSIOLOGY 



the penis, and of the unstriated muscles of the tunica dartos of the 

 scrotum. In animals which possess a retractor penis muscle, excita- 

 tion of the lumbar nerves causes strong contraction of the muscle. 



The fibres from the sacral nerves can be divided into two classes 

 visceral and somatic. The visceral branches run in the pelvic nerves, 

 or nervi erigentes. Stimulation of these fibres produces active dilata- 

 tion of the arteries of the penis or vulva, and also inhibition of the 

 unstriated muscle of the penis, the retractor muscle of the penis, 

 when present, and of the vulva muscles. The somatic branches 

 supply motor nerves to the ischio- and bulbo-cavernosi, as well as the 

 constrictor urethras. In the female they supply the analogous muscles, 

 namely, the erector clitoridis (ischio-cavernosus) and the sphincter 

 vaginae (bulbo-cavernosus). Both these sets of fibres are therefore 

 involved in the erection of the generative organs which accompanies 

 coitus. 



The internal organs, i.e. the uterus and vagina in the female, and 

 vasa deferentia, seminal vesicles, and uterus masculinus in the male, 

 differ from the external organs in receiving no efferent nerve fibres 

 from the sacral nerves, as has been pointed out by Langley and 

 Anderson. They are supplied with fibres, which pass out through the 

 anterior roots of the third, fourth, and fifth lumbar nerves (in the 

 rabbit and cat), and run through the sympathetic to the inferior 

 mesenteric ganglia, whence they proceed by the hypogastric nerves. 

 On stimulating these fibres, two effects are produced on the uterus and 

 vagina, namely, a contraction of the small arteries, leading to pallor 

 of the organs, and a strong contraction of the muscular coats.* In the 

 vagina the contraction can usually be seen to start from one end and 

 spread to the other. The whole then remains for a time in a state of 

 powerful tonic contraction, which affects both longitudinal as well as 

 circular muscles. In the male stimulation of these nerves excites 

 contraction of the whole musculature of the vasa deferentia and 

 seminal vesicles, which may be strong enough to cause emission of 

 semen from the penis. These effects on the uterus and seminal vesicles 

 are not abolished by injection of atropine. 



The course of the sensory fibres from the generative organs to the 

 lumbo-sacral cord has not yet been fully made out, but it seems 

 probable that it corresponds to the course taken by the efferent fibres. 



An accessory genital muscle, the retractor penis, which is found in the 

 dog, cat, horse, donkey, hedgehog (not in the rabbit or man), presents con- 

 siderable physiological interest. It was first described by Eckhard as the 

 Afterruthenband, and consists of a thin band of longitudinally arranged 

 unstriated muscle (15 to 20 cm. long in a spaniel weighing about 15 kilos), 



* Under some circumstances stimulation of the sympathetic nerves may 

 cause relaxation of the uterus. 



