22 THE FROG 



as necessary. The first nerve is a small one just in front of the sec- 

 ond, to which it sends a branch. The third is a small nerve which 

 comes close to the second and sends off a communicating branch. 

 These connections between the first three nerves make up the brachial 

 plexus. The fourth, fifth, and sixth nerves are very slender and 

 run obliquely outward over the muscles of the back. What region do 

 they seem to supply? The seventh, eighth, and ninth nerves are larger 

 and run almost directly backward. They have various communicating 

 branches which form the sciatic plexus, as a continuation of which the 

 sciatic nerve will be seen. Follow it in one hind leg as far as you 

 can, noticing the blood vessels, the sciatic artery and sciatic vein, 

 which accompany the nerve. The tenth nerve is small and may be 

 hard to find. It comes out through an opening in the urostyle, the 

 elongated bone which terminates the spinal column, and runs almost 

 directly backward to supply the bladder and cloaca. What is the 

 function of nerves? Why are some of the spinal nerves larger than 

 others? What is the significance of the brachial and sciatic plexuses? 

 Make an outline (X 3) of the central portions of the spinal column. 

 Add to this the spinal nerves, representing accurately their origin, 

 courses, and relative sizes. Whatever part you have seen of the sym- 

 pathetic system should be included in the drawing. 



Exercise 20. — The Central Nervous System and the Origin of 

 the Nerves. 



(d) Cut off the fore legs close to the body. Remove the hind 

 legs together by cutting across the posterior part of the trunk, and 

 save them for a study of the muscles. The following dissection will 

 be made more readily if the head and trunk are held in one hand 

 with the axial skeleton supported by the forefinger. Remove the 

 skin and muscles from the back with scissors and scalpel until the 

 skull and vertebral column can be seen. Find the opening at the 

 posterior end of the skull where the most anterior vertebra is at- 

 tached. Insert the scissors into this opening in such a way as not to 

 damage the soft brain, and make two short cuts in the form of a 

 "V." The cut bone can be broken away with the forceps and the 

 breaking continued forward until the entire brain, covered by its 

 membrane, is exposed. Do not damage the membrane. Continue the 

 dissection posteriorly by cutting off the dorsal part of each vertebra 

 with a scalpel. To see the spinal cord more clearly break the vertebra? 

 right and left with the fingers; it is also covered by a membrane. 

 What is the importance of the large blood supply to the central nerv- 

 ous system as indicated by the numerous blood vessels in the mem- 



