104 VERTEBRATE ZOOLOGY 



organs and tissues of the body. The sympathetic nervous system 

 lies in the body cavity in connection with the cranial and spinal 

 nerves and innervates certain important viscera. 



Remove the urogenital system from the body. Raise it care- 

 fully with forceps, and with fine scissors cut it loose from the 

 dorsal body wall. Note the spinal column projecting into the 

 body cavity, and lying ventral to it note a large blood vessel, 

 the dorsal aorta ; this must not be disturbed. The spinal column 

 is made up of nine vertebrae and a long terminal bone called the 

 urostyle. Identify them. 



We shall study first the spinal nerves and the sympathetic sys- 

 tem. Each spinal nerve is joined with the spinal cord by two roots, 

 a dorsal and a ventral root, and passes out from the neural 

 canal of the spinal column through a space between two verte- 

 brae called the intervertebral foramen. At the point where these 

 two roots meet, the dorsal root bears a large ^ganglion called the 

 spinal ganglion. This ganglion is embedded in a prominent white 

 body, present between the vertebrae, called the calcareous body. 

 The ten pairs of spinal nerves will be seen in the body cavity, 

 where they appear as white strands which lie against the dorsal 

 body wall on each side of the vertebral column. The most con- 

 spicuous ones are the seventh, eighth, and ninth nerves, which 

 lie close together in the hinder part of the abdominal cavity. 

 They emerge on each side from the intervertebral foramina, be- 

 tween the seventh and eighth vertebrae, the eighth and ninth, and 

 the ninth and the urostyle respectively, and proceed straight back 

 almost parallel with the spinal column. These nerves are joined 

 with one another by short connecting branches and form a net- 

 work, or plexus, called the sciatic plexus. From this plexus issue a 

 number of nerves which proceed to the hinder quarters of the body 

 and the hind legs. Of these the largest are the sciatic nerve, which 

 goes to the hind leg, and the crural and iliohypogastric nerves, 

 which supply the muscles and skin of the abdomen and thigh. 



Find the sciatic plexus and these nerves. Follow the sciatic 

 nerve into the leg ; trace the other two as far as possible. 



In the forward part of the abdominal cavity, on each side, is 

 another nerve plexus called the brachial plexus, which is composed 



