THE FROG 105 



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 nerve, 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 backward 

 to the muscles of the back. 



Find these nerves and trace their branches. 



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 longitudinal 

 nerve are ten enlargements, the sympathetic ganglia, from each of 

 which one or more short branches run to a spinal nerve. 



The first, second, and third of these ganglia lie close to the 

 first, second, and third spinal nerves and are joined with them by 

 short branches. The fourth to the ninth sympathetic ganglia, in- 

 clusive, are situated nearer the median plane than the first three ; 

 they, together with the longitudinal nerve, lie alongside the dorsal 

 aorta, which will be noticed as a median, dark-colored tube. The 

 fourth sympathetic ganglion is the smallest; the ninth is the 

 largest and is joined with the ninth spinal nerve by several 

 branches; the tenth is small and is often wanting. 



Branches from the sympathetic nerves and their ganglia pass 

 to the various viscera. The largest of these branches proceed 

 from the fourth, fifth, and sixth ganglia, and after joining together 

 form a large nerve called the splanchnic nerve, which supplies the 

 intestine and other viscera. 



Study the sympathetic system ; first find the longitudinal nerves 

 and ganglia, then observe their relations to the spinal nerves and 

 ganglia. 



Exercise 12. Make a semidiagrammatic drawing of the spinal nerves 

 and the sympathetic system. Draw first an outline of the spinal 

 column as it appears in the ventral aspect of the opened body 

 cavity ; number the vertebrae ; draw the spinal nerves and then the 

 sympathetic system. 



