402 THE NERVOUS SYSTEM AND ORGANS OF SENSE 



thirty-one more leave the spinal cord. The brain has three divisions. 

 The cerebrum makes up the largest part. In this respect it differs from the 

 cerebrum of the frog and other vertebrates. It is divided into two lobes, 

 the hemispheres, which are connected with each other by a broad band 

 of nerve fibers. The outer surface of the cerebrum is thrown into folds or 

 convolutions. The outer layer, seen in section, is gray in color, and is made 

 up of nerve cells and supporting material (the neuroglia, 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, lies the little 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. 



Sensory and Motor Nerve Fibers. — Nerves which are connected with 

 the central nervous system may be made up of fibers bearing messages 

 from sens, organs in the skin or elsewhere to the central nervous system, 

 the sensory fibers, or of other fibers which carry impulses from the 

 central nervous system to the outside, the motor fibers. Some nerves 

 are made up of both kinds of fibers, in which case they are called mixed 

 nerves. 



The Sympathetic Nervous System. — The sympathetic nervous system 

 consists of a series of gangha connected with each other and with the cen- 

 tral nervous system through some of the spinal and cranial nerves, espe- 

 cially the vagus (tenth cranial). The sympathetic system, both in the frog 

 and man, controls the muscles of the digestive tract and blood vessels, the 

 secretions of gland cells, and all functions which have to do with life pro- 

 cesses in the body. 



Functions of the Parts'of the Central Nervous 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 wiU 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 movements. If the forebrain 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 



