XXVIII. THE NERVOUS SYSTEM AND ORGANS OF SENSE Problem LV. A study of the nervous system, reactions to stimuli, and habit formation. (Laboratory Manual, Prob. LV.) Divisions of the Nervous System. The control of a number of activities for the attainment of a definite end is the function of the nervous system in the lowest as well as the highest of animals. In the vertebrate animals, the nervous system consists of two divisions. One includes the brain, spinal cord, the cranial and spinal nerves, which together make up the cerebro-spinal nervous system. The other division is called the sympathetic nervous system. The activities of the body are controlled from nerve centers by means of fibers which extend to all parts of the body, there ending in the muscles. The brain and spinal cord are examples of such centers, since they are largely made up of nerve cells. Small collections of nerve cells, called ganglia, are found in other parts of the body. These nerve centers are connected, to a greater or less degree, with the surface of the body by the nerves which serve as pathways between the end organs of touch, sight, taste, etc., and the centers in the brain or spinal cord. Thus sensation is obtained. Nerve Cells and Fibers. - 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 axis cylinder process, is much longer than the others and ends in a muscle A nerve cell from the brain of a monkey, showing a great number of tiny protoplasmic projections or dendrites. or organ of sensation. The axis cylinder process forms the pathway over which nervous impulses travel to and from the nerve centers. A nerve consists of a bundle of such tiny axis cylinder processes, bound together by a connective tissue. As a nerve ganglia is a center of activity in the nervous system, so a nerve cell is a center of activity which may send an impulse over this thin strand of protoplasm (the axis cylinder process) prolonged into a nerve fiber many hundreds of thousands of times the length of the cell. Some nerve cells in the human body, although visible only under the compound microscope, give rise to axis cylinder processes several feet in length. Dendrites Cell Body Neuraxon -Medulla -Node of Ranvier Neurilemma Because some nerve fibers originate in organs that receive sensations and send those sensations to the central nervous system, they are called sensory nerves. Other axis cylinder processes originate in the central nervous system and pass outward as nerve fibers; such nerves produce movement of muscles and are called motor nerves. The Brain of Man. -In man, as in the frog, the central nervous system consists of a brain Diagram of a neuron or and spinal cord inclosed in a bony case with the Nerve-ends nerve unit. nerves leaving it. From the brain, twelve pairs of nerves are given off; Cerebrum Cerebellum - C Pons Medulla The brain, with parts separated to show each clearly. HUNT. ES. BIO.- -26 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 sense 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 ganglia connected with each other and with the central nervous system through some of the spinal and cranial nerves, especially 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 processes 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 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 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 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. different parts of the brain in man agree with those functions we have already ob Parietal Region Motion Feeling Frontal Region Thought Cerebrum Occipital Region Sight our sensations. Spinal Cord Reflex Actions Respiration Region the body has its definite Regions of the head and action of the different localized center in the parts of the brain. cerebrum. Experiments have been performed on monkeys, and these, together with observations made on persons who had lost |