The Brain of Man.--In man, the central nervous system consists of a brain and spinal cord inclosed in a bony case. From the brain, twelve pairs of nerves are given off; thirty-one pairs 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_ 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, 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. The Sympathetic Nervous System.--Connected with the central nervous system is that part of the nervous apparatus that controls the muscles 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 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 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 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.