352 BODY CONTROL AND HABIT FORMATION 



cerebrum is thrown into folds or convolutions which give a large surface, 

 the cell bodies of the neurons being found in this part of the cerebrum. 

 Holding the cell bodies and fibers in place is a kind of connective tissue. 

 The inner part (white in color) is composed largely of fibers which pass 

 to other parts of the brain and down into the spinal cord. Under the 

 cerebrum, and dorsal to it, hes the httle brain, or cerebellum. The two 

 sides of the cerebellum are connected by a band of nerve fibers which 

 run around into the lower hindbrain or medulla. This band of fibers is 

 called the pons. The medulla is, in structure, part of the spinal cord, and 

 is made up largely of fibers running longitudinally. 



The Sympathetic Nervous System. — Connected with the central ner- 

 vous system is that part of the nervous apparatus that controls the mus- 

 cles of the digestive tract and blood vessels, the secretions of gland cells, 

 and all functions which have to do with life processes in the body. This 

 is called the sympathetic nervous system. 



Functions of the Parts of the Central Nervous System of the 

 Frog. — From careful study of living frogs, birds, and some mam- 

 mals 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 is 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 move- 

 ments. If the cerebrum is separated from the rest of the nervous 

 system, the frog seems to act a little differently from the normal 

 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. 

 But it manifests no hunger or fear, and is in every sense a machine 

 which will perform certain actions after certain stimulations. Its 

 movements are automatic. If now we watch the movements of 

 a frog which has the brain uninjured in any way, we find that it 

 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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