176 PHYLUM CHORD ATA 



vertebrae. At the point where these two roots meet, the dorsal 

 root bears a large ganglion called the spinal ganglion. This gan- 

 glion 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 between the seventh and eighth ver- 

 tebra, 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 network, 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 is the sciatic nerve, which goes to the 

 hind leg. 



Find the sciatic plexus. Follow the sciatic nerve into the leg as 

 far as possible. 



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

 another much smaller nerve plexus called the brachial plexus, 

 which is composed of the first three spinal nerves. Of these the 

 second, which is the largest and most conspicuous, is a large white 

 cord lying at right angles to the spinal column and emerging from 

 between the second and third vertebrae ; it is joined by a small 

 branch from the first and one from the third spinal nerves, and 

 passes to the foreleg. 



The fourth, fifth, and sixth pairs of spinal nerves are delicate 

 cords which emerge from between the fourth and fifth, fifth and 

 sixth, and sixth and seventh vertebrae, and pass obliquely back- 

 ward to the muscles of the back. 



Find these nerves. 



The sympathetic system consists of a pair of delicate longitudi- 

 nal nerves which lie in the abdominal cavity on either side of the 

 spinal column, close to the dorsal body wall. In each longitu- 

 dinal nerve are ten enlargements, the sympathetic ganglia, from 



