CHAP. IX.] XEIiVOUS SYSTEM AXD ORGANS OF SENSE. 283 



round the head of the fibula beneath the peroneus longus and 

 divides into two branches. The first of these, called the musculo- 

 cnt((iteous nerve, descends between the extensor longus digitorum and 

 the peronei muscles to the dorsum of the foot, where it ramifies. It 

 supplies the peronei and skin of the front of the leg and dorsum of 

 the foot. The second division of the external popliteal nerve is 

 called the anterior tibial. It passes obliquely inwards beneath the 

 extensor longus digitorum, and descends with the anterior tibial 

 vessels to the anlde, where it divides, one part continuing on to the 

 first interosseous space, the other passing outwards obliquely beneath 

 the extensor brevis. It supplies the tibialis anticus and the extensors 

 both long and short, as well as the skin of part of the dorsum of 

 the foot. 



Of the smaller branches from the sacral plexus, the pudic nerve 

 is that which supplies the generative organs and adjacent parts. 

 The other nerve, the origin of which has been mentioned, is the 

 small sciatic nerve, which arises behind the pyriformis, descends 

 beneath the gluteus maximus, and gives branches to that muscle 

 and to the skin of the lower part of the buttock and back of the 

 thigh. 



§ 20. The NERVES of the tail come from the sacral plexus, 

 which gives origin to a lateral nerve which runs along each side of 

 the tail, giving off branches to the muscles. The more anterior 

 part of the tail is also supplied by branches of the cauda equina, 

 which are continued into it. 



§ 21. The SYMPATHETIC SYSTEM cousists of an immense number 

 of small nerves (of pale fibres), with many ganglia scattered through 

 the body, and specially connected with the viscera and blood-vessels, 

 the whole system being connected with the spinal system of nerves 

 by two elongated, ganrjliated cords, which, extend from before 

 backwards, one on each side of the ventral aspect of the skeletal 

 axis, from the pre-sphenoid to the tail. It is with these two longi- 

 tudinal cords that the several filaments already noticed as passing 

 from the spinal nerves close to their roots, unite. The sympathetic 

 visceral nerves, in passing to the organs which they supply, traverse 

 those folds of membrane (the mesenteries) which, as we have seen, 

 suspend the viscera from the backbone. 



In the trunk, the sympathetic nerves and ganglia are here and 

 there congregated together, forming great plexuses, whence other 

 nerves proceed. 



In the head, filaments of the sympathetic communicate with 

 all the cranial nerves (except those nerves of special sense, the 

 " optic" and " olfactory " nerves), and where these unions take 

 place, certain ganglia are developed. 



The conDS, placed symmetrically one on each side of the ventral 

 aspect of the vertebral column, from the base of the skull to the 

 tail, each developes ganglia, which in the trunk correspond in 

 number with the dorsal and lumbar vertebra). These cords are 

 connected in front with sympathetic nerves of the skull, while 



