380 THE NERVOUS SYSTEM OF THE FROG. 
branches, and divides just above the elbow into 
the radial and ulnar nerves which supply the 
forearm and hand. 
4, 5, and 6: the fourth, fifth, and sixth spinal 
nerves are small, and supply the muscles and 
skin of the body-wall. They leave the verte- 
bral canal between the fourth and fifth, fifth 
and sixth, and sixth and seventh vertebree 
respectively. 
7, 8, and 9: the seventh, eighth, and ninth spinal 
nerves together form the sciatic plexus. The 
roots of these three nerves within the vertebral 
canal form the main part of the cauda equina, 
The seventh nerve leaves the canal between 
the seventh and eighth vertebre, the eighth 
nerve between the eighth and ninth vertebre, 
and the ninth nerve between the ninth or 
sacral vertebra and the urostyle. Outside the 
vertebral canal the three nerves unite together 
opposite the middle of the urostyle to form 
the sciatic plexus, from which branches are 
given to the large intestine, bladder, oviducts, 
etc. Just before joining the plexus the seventh 
nerve gives off the ileohypogastric and crural 
nerves supplying the muscles and skin of the 
abdomen and thigh. Beyond the plexus is the 
large sciatic nerve, which runs down the thigh, 
giving branches to it, and dividing a short distance 
above the knee into the tibialand peroneal nerves 
supplying the leg and foot. 
10: The coccygeal or tenth spinal nerve emerges 
through a small hole in the side of the urostyle 
near its anterior end. It gives branches to the 
bladder, cloaca, and other adjacent parts. 
II. The Sympathetic Nervous System. Consists of a 
longitudinal nervous band on each side of the body connected 
by branches with the several spinal nerves. The two main 
sympathetic trunks lie in front close to the dorsal surface and 
alongside the vertebral column: further back they are in close 
relationship with the dorsal aorta alongside which they rune 
