as the result of much observation and experiment. Travelers in Arctic regions and others exposed to intense cold agree that those who use no alcohol whatever are far better able to resist the cold than are those who indulge in it. Physiologists show by careful experiments that though the temperature of the body rises. during digestion of food, it is lowered for some hours when alcohol is taken. The flush which is felt upon the skin after a drink of wine or spirits is due in part to an increase of heat in the body, but also to the paralyzing effect of the alcohol upon the capillary walls, allowing them to dilate, and so permitting more of the warm blood of the interior of the body to reach the surface. There it is cooled by radiation, and the general temperature is lowered." — MACY, Physiology.

Effect of Alcohol. - Alcohol lowers the temperature of the body by dilating the blood vessels of the skin. It does this by means of its influence on the nervous system. It is, therefore, a mistake to drink alcoholic beverages when one is extremely cold, because by means of this more bodily heat is allowed to escape.

"Alcohol and Heat. The amount of heat in the body depends upon the balance between its production and its loss. The rapid destruction of alcohol, in all probability, yields heat too rapidly to be utilized by the body. The most constant effect of taking alcohol is to dilate the arteries of the skin, so that an extra amount of heat is lost. More heat is always lost than is produced. Alcohol lessens the power of the body to endure cold. On a cold day when the arteries of the skin are contracted so that there is but little blood to warm its nerves, alcohol may send the blood to these nerves and produce an agreeable sense of warmth, but in reality. this feeling of warmth is due only to the heat which is passing off from the interior of the body." OVERTON, Applied Physiology.

XXXV. THE NERVOUS SYSTEM

General Functions of the Nervous System. We have seen that, in the simplest of animals, one cell performs the functions necessary to its existence. In the more complex animals, where groups of cells form tissues, each having a different function, a nervous system is developed. The functions of the nervous system are: (1) the providing of the man with sensation, by means of which he gets in touch with the world about him; (2) the giving to the human being a will, a provision for thought; (3) the connection of organs in different parts of the body so that they act as a united and harmonious whole. Coöperation in word and deed is the end attained. We are all familiar with examples of the coöperation of organs. You see food; the thought comes that it is good to eat; you reach out, take it, raise it to the mouth; the jaws move in response to your will; the food is chewed and swallowed; while digestion and absorption of the food are taking place, the nervous system is still in control. The nervous system also regulates pumping of blood over the body, respiration, secretion of glands, and, indeed, every bodily function.

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 most other cells in the body, is a mass of protoplasm containing a nucleus. 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 protoplasm dendrites. One of these processes, the axis cylinder process, is much longer than the others and ends in a muscle or organ of sensation. The axis cylinder process forms the pathway over which nervous impulses travel to and from the nerve centers.

Nerves consist of bundles of such tiny axis cylinder processes, bound together by 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. 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 Cerebro-spinal Nervous System of the Frog.-For this exercise use frogs that have been preserved in alcohol. Those previously used for other work on the anatomy of the frog will do perfectly well.

With a sharp knife or scalpel, cut away the skin from the top of the head and along the back. Cut carefully through the top of the cartilaginous skull. The brain will then be exposed, lying in a bony cavity surrounded by a watery fluid, the function of which is to protect the delicate brain from shock. Notice the white elongated hemispheres of the forebrain or cerebrum. The two anterior projections of the cerebrum are called the olfactory lobes. It is by means of the olfactory lobes that odors are perceived. Posterior to the cerebrum and connected with it is the midbrain. The dorsal side is enlarged to form a pair of optic lobes. How do the optic lobes compare in size with the cerebrum? Insert the blunt end of a scalpel, and turn the optic lobes slightly so as to find the optic nerves. Notice that they cross each other, the one from the right optic lobe going to the left eyeball, the one

Dendrites

--Cell Body

Axis Cylinder

Process

from the left optic lobe to the right eyeball. This crossing of the optic nerves is found in man. The region just under and behind the optic lobes is the hindbrain; it consists of the cerebellum (seen as a little ridge posterior to the optic lobes) and the medulla (the hindmost part of the brain). Notice the nerves leaving it laterally. By carefully removing the bone surrounding the spinal cord, you may be able to see some of the spinal nerves and the cord itself. Make a drawing of the brain of the frog, naming all the parts. Now turn the frog over. After removing all the organs from the body cavity, trace the course of some of the white spinal nerves. There are ten in all. They leave the spinal cord by two branches known as the dorsal and ventral roots. These roots unite under a series of yellowish white patches (ganglionic glands). The dorsal roots enter collections of nerve cells known as the spinal ganglia. Connected with that part of the central nervous system just described is the sympathetic nervous system. Part of this may, in favorable specimens, be found as a row of nerves and ganglia lying along each side of the spinal cord in the body cavity. The sympathetic nervous system supplies all the organs of the body cavity, and is connected with the spinal and cranial (brain) nerves. In man the sympathetic nervous system has practically the same position and function as it has in the frog. It has the control of the organs of digestion, circulation, respiration, excretion, and reproduction, the so-called vegetative functions.

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.

Nerve-ends

Diagram of a neuron or nerve unit.

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 be 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 be 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.