THE NERVOUS SYSTEM 



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from the left optic lobs to the right eyeball. This rroRsing of the optic 

 nerves is found in man The region just under and behind the optic lobes 

 is the hindhrain; it consists of the cerebellum (seen as a little ridge post^irior 

 to the optic lobes) and the medulla (the hindmost part 

 of the brain). Notice the nerves leaving it laterally. 

 By carefully removing the bone surrounding the spinal 

 cord, you may be able to see some of the spinal nerves 

 and the cord itself. Make a drawing of the brain of 

 the frog, naming all the parts. Now turn the frog 

 over. After removing all the organs from the body 

 cavity, trace the course of some of the white spinal 

 nerves. There are ten in all. They leave the spinal 

 cord by two branches known as the dorsal and ven- 

 tral roots. These roots unite under a series of yellow- 

 ish white patches {ganglionic glands). The dorsal 

 roots enter collections of nerve cells known as the 

 spinal ganglia. Connected with that part of the cen- 

 tral nervous system just described is the sympathetic 

 nervous system. Part of this may, in favorable speci- 

 mens, be found as a row of nerves and ganglia lying 

 along each side of the spinal cord in the body cavity. 

 The sympathetic nervous system supplies all the or- 

 gans of the body cavity, and is connected with the 

 spinal and cranial (brain) nerves. In man the sym- 

 pathetic nervous system has practically the same posi- 

 tion and function as it has in the frog. It has the 

 control of the organs of digestion, circulation, respira- 

 tion, excretion, and reproduction, the so-called vege- 

 tative functions. 



Functions of the Parts of the Central Ner- 

 vous System of the Frog. — From careful study of 

 living frogs, birds, and some mammals we have learned 

 much of what we know of the functions of the parts 

 of the central nervous system in man. 



It has been found that if the entire brain of a frog is destroyed and 

 separated from the spinal cord, "the frog will continue to live but with a 

 very peculiarly modified activity." It does not appear to breathe nor does 

 it swallow. It will not move or croak, but if acid be placed upon the skin 

 so as to irritate it, the legs make movements to push away and to clean off 

 the irritating substance. The spinal cord is thus shown to be a center for 

 defensive movements. If the forebrain be separated from the rest of the 

 nervous system, the frog seems to act a little differently from the norinal 

 animal. It jumps when touched and swims when placed in water. It will 

 croak when stroked or swallow if food be placed in its mouth. Hut it 

 manifests no hunger or fear, and is in every sense a machine which will 

 perform certain actions after certain stimulations. Its lnoveInent^s arc 

 automatic. If now we watch the movements of a frog which has the brain 

 uninjured in any way, we find that the frog acts spontaneously. It tries to 

 escape when caught. It feels hungry and seeks food. It is capable of 

 voluntary action It acts like a normal individual. 



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Diagram of a iiouron or 

 nerve unit. 



