SPINAL NERVES OF THE FROG 1 



1. Pin your specimen in the dissecting pan as before. Remove any of 

 the viscera which may be present in the body cavity. Examine the dor- 

 sal wall of the body cavity and note the paired spinal nerves which 

 may be seen as small, white cords running laterally from either side of 

 the vertebral column. They arise in the spinal cord and emerge from 

 between the vertebrae through the light-colored calcareous rodies. 



2. Beginning at the anterior end, identify and trace the ten pairs of 

 spinal nerves as follows : 



(1) A small pair which emerges from between the first and second verte- 

 brae just anterior to the fore limbs, usually giving off a small branch to 

 the second nerve before continuing laterally into the muscles. 



(2) A large pair which supplies the fore limbs after usually having 

 received branches from the first and third pairs. This union forms the 



RRACHIAL PLEXUS. 



(3) A small pair which, after giving off a branch to the second pair for 

 the brachial plexus, can be traced laterally into the muscles. 



(4), (5), and (6) are small pairs which can be traced into the muscles of 

 the body wall. 



(7), (8), and (9) are larger pairs which run almost directly posteriorly 

 and anastomose to form the sciatic plexus, from which arises the large 

 sciatic nerve. By careful dissection, follow the sciatic nerve from the 

 plexus down through the leg, noting the various branches which are given 

 off. Note also a branch of the seventh nerve, which is given off anterior 

 to the sciatic plexus. 



(10) A small pair, each of which emerges from openings near the ante- 

 rior end of the urostyle and supplies chiefly the urogenital organs. 



3. Note the two small, ganglionated chains of the autonomic nervous 

 system ; one lying on either side of the mid-dorsal line. Note the attach- 

 ment by a fine nerve fiber to each of the spinal nerves. 



4. Make a drawing to show the location and course of the spinal nerves 

 and the autonomic chains. 



iB. pp. 213-215. 



341 



