636 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



Course and origin of post-ganglionic fibres. — In the main the 

 sympathetic fibres producing these effects have the same general arrange- 

 ment as the nerves to the skin — that is, they leave the sympathetic 

 by grey rami, pass to the spinal nerves, and follow the branches of 

 these to the periphery. The branches followed in this case are the 

 pudic nerves, which give rise to the nervus dorsalis penis, the genito- 

 anal nerve or nerves, and the scrotal nerve. It must be noticed that 

 the nervus dorsalis and the genito-anal nerves cause contraction of one or 

 more of the striated intrinsic muscle's of the penis. 



The fibres of the grey rami which run to the pudic nerve follow, as 

 regards their origin, the general rule of the fibres of the grey rami which 

 run to the skin, i.e. they arise from the nerve cells of the corresponding 

 ganglion. The ganglia in the cat and dog are the three sacral ganglia. 

 But the ganglia in the sacral region often fuse together, so that a grey 

 ramus is less restricted to a corresponding ganglion here than elsewhere ; 

 and the first sacral grey ramus not infrequently receives from the 

 seventh lumbar ganglia some fibres for the external generative organs. 



The pudic nerves vary in origin, and in correspondence with this 

 there is some variation in the particular sympathetic ganglia, from which 

 the fibres for the external generative organs arise. 



Whilst the great majority of the sympathetic fibres for the external generat- 

 ive organs have the same general arrangement as the sympathetic fibres for 

 the skin, a few have a different arrangement, passing by the inferior mesenteric 

 ganglion (cf. p. 646), i.e. by a pre- vertebral ganglion, and reaching their 

 destination by the hypogastric nerve and the pelvic plexus. The origin of 

 these fibres from the spinal cord is the same as that of the fibres which run 

 by way of the vertebral ganglia, and as they are few in number, we need not 

 enter into any detailed account of them. 



Origin of pre-ganglionic fibres from spinal nerves. — We have seen, 

 in treating of the sympathetic fibres which supply the skin of the body 

 generally, that the sacral ganglia receive pre-ganglionic fibres from the 

 thirteenth thoracic and the first three lumbar nerves (cf. table, p. 634) 

 in the cat. Since the sacral ganglia send fibres to the external generat- 

 ive organs, we might expect that the spinal nerves which influence the 

 external generative organs, by way of the sympathetic, would be the 

 thirteenth thoracic and the first three lumbar nerves; and we might 

 expect that the second and third lumbar would have a greater influence 

 than the first lumbar, and the first lumbar than the thirteenth thoracic 

 nerve. And this, in fact, is the case. But there is an additional fact, 

 which could not be deduced simply from the connections of the cutaneous 

 fibres of the sacral ganglia, namely, that the fourth lumbar nerve, which 

 has no effect on the hairs of the tail or on the blood vessels or glands of 

 the foot, causes marked contraction of the musculature of the external 

 generative organs. This difference is constant. Thus when, in the cat, 

 the arrangement of nerves is posterior, and the fourth lumbar nerve 

 sends some fibres to the tail and to the foot, the fibres for the external 

 generative organs are shifted also, and the fifth lumbar nerve will cause 

 retraction of the penis. 



It is natural enough that the external generative organs should have 

 a more extensive nervous supply than the sweat glands of the foot, or 

 the hairs of the tail. But the fact that it is so, involves a selective 

 arrangement of pre-ganglionic fibres with particular nerve cells, which 



