54 THE FROG 
occur are sometimes said to constitute a brachial plexus. There 
is considerable variation in the cross-branching in different speci- 
mens of the frog, but the brachial plexus is never so well de- 
veloped as in the mammals where it takes the form of a com- 
plicated network. 
(c) The third spinal nerve is small and emerges from between 
the third and fourth vertebre ; for some distance it runs along the 
posterior edge of the brachial nerve with which it is connected 
by one or more slender branches, and finally supplies some 
muscles of the body wall. 
(d) The fourth, fifth and sixth spinal nerves are small and 
run obliquely backward. Determine their distribution. 
(ce) The seventh, eighth and ninth spinal nerves pass almost 
directly backward and anastomose with each other to form the 
sciatic plexus. The eighth and ninth unite to form the large 
sciatic nerve which may be followed by separating the great 
muscles of the dorsal surface of the leg. 
(f) The tenth pair of spinal nerves are very small and pass 
almost directly backward. They innervate the bladder and the 
cloaca. 
Add the spinal nerves to your drawing of the central nervous 
system; represent them twice natural size. 
3. The sympathetic system centers in two delicate chains of 
ganglia extending longitudinally, one on each side of the body, 
lying close to the dorsal aorta and systemic arches. Turn the 
viscera to one side and lift up the kidney; this will expose the 
sympathetic system of one side. A nerve trunk unites the 
ganglia of each chain. About how many ganglia are there? 
Are the sympathetic ganglia united with the spinal nerves? If 
so, how? The sympathetic system innervates the internal organs 
generally. Branches of the sympathetic unite to form plexuses, 
the largest of which, the solar plexus, sends branches to the 
stomach and adjacent organs. 
Add the main trunks of the sympathetic system to your draw- 
ing of the central nervous system; show also the connections 
between the sympathetic system and the spinal nerves. 
B. The Finer Structure of the Nervous System.—Nervous 
ussue is made up of two different elements: (a) the nervous ele- 
