A Civic Biology, Presented in Problems

by George W. Hunter

Neurones.—A nerve cell, like other cells in the body, is a mass of protoplasm containing a nucleus. But the body of the nerve cell is usually rather irregular in shape, and distinguished from most other cells by possessing several delicate, branched protoplasmic projections called dendrites. One of these processes, the axon, is much longer than the others and ends in a muscle or organ of sensation. The axon forms the pathway over which nervous impulses travel to and from the nerve centers.

A nerve consists of a bundle of such tiny axons, bound together by connective tissue. As a nerve ganglia is a center of activity in the nervous system, so a cell body is a center of activity which may send an impulse over this thin strand of protoplasm (the axon) prolonged many hundreds of thousands of times the length of the cell. Some neurones in the human body, although visible only under the compound microscope, give rise to axons several feet in length.

Because some bundles of axons originate in organs that receive sensations and send those sensations to the central nervous system, they are called sensory nerves. Other axons originate in the central nervous system and pass outward as nerves producing movement of muscles. These are called motor nerves.

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.

Diagram to show the parts of the brain and action of the different parts of the brain.

Functions of the Cerebrum.—In general, the functions of the different parts of the brain in man agree with those functions we have already observed in the frog. The cerebrum has to do with conscious activity; that is, thought. It presides over what we call our thoughts, our will, and our sensations. A large part of the area of the outer layer of the cerebrum seems to be given over to some one of the different functions of speech, hearing, sight, touch, movements of bodily parts. The movement of the smallest part of the body appears to have its definite localized center in the cerebrum. Experiments have been performed on monkeys, and these, together with observations made on persons who had lost the power of movement of certain parts of the body, and who, after death, were found to have had diseases localized in certain parts of the cerebrum, have given to us our knowledge on this subject.