A Civic Biology Part 37

LABORATORY SUGGESTIONS

_Demonstration._--Sensory motor reactions.

_Demonstration._--Nervous system. Models and frog dissections.

_Demonstration._--Neurones under compound microscope (optional).

_Demonstration._--Reflex acts are unconscious acts: show how conscious acts may become habitual.

_Home exercise_ in habit forming.

_The senses.--Home exercises._--(1) To determine areas most sensitive to touch. (2) To determine or map out hot and cold spots on an area on the wrist. (3) To determine functions of different areas on tongue.

_Demonstration._--Show how eye defects are tested.

_Laboratory summary._--The effects of alcohol on the nervous system.

The Body a Self-directed Machine.--Throughout the preceding chapters the body has been likened to an engine, which, while burning its fuel, food, has done work. If we were to carry our comparison further, however, the simile ceases. For the engineer runs the engine, while the bodily machine is self-directive.

Moreover, most of the acts we perform during a day's work are results of the automatic working of this bodily machine. The heart pumps; the blood circulates its load of food, oxygen, and wastes; the movements of breathing are performed; the thousand and one complicated acts that go on every day within the body are _seemingly_ undirected.

[Ill.u.s.tration: The central nervous system.]

Automatic Activity.--In addition to this, numbers of other of our daily acts are not thought about. If we are well-regulated body machines, we get up in the morning, automatically wash, clean our teeth, dress, go to the toilet, get our breakfast, walk to school, even perform such complicated processes as that of writing, without _thinking_ about or _directing_ the machine. In these respects we have become creatures of habit. Certain acts which once we might have learned consciously, have become automatic.

But once at school, if we are really making good in our work in the cla.s.sroom, we begin a higher control of our bodily functions. Automatic control acts no longer, and sensation is not the only guide--for we now begin to make _conscious choice_; we weigh this matter against another,--in short, we _think_.

Parts of the Nervous System.--This wonderful self-directive apparatus placed within us, which is in part under control of our will, is known as the nervous system. In the vertebrate animals, including man, it 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_ and has to do with those bodily functions which are beyond our control. Every group of cells in the body that has work to do (excepting the floating cells of the blood) is directly influenced by these nerves. Our bodily comfort is dependent upon their directive work. The organs which put us in touch with our surroundings are naturally at the _surface_ of the body. Small collections of nerve cells, called _ganglia_, are found in all 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.

Sensations and Reactions.--We have already seen that simpler forms of life perform certain acts because certain outside forces acting upon them cause them to _react_ to the stimulus from without. The one-celled animal responds to the presence of food, to heat, to oxygen, to other conditions in its surroundings. An earthworm is repelled by light, is attracted by food. All animals, including man, are put in touch with their surroundings by what we call the organs of sensation. The senses of man, besides those we commonly know as those of sight, hearing, taste, smell, and touch, are those of temperature, pressure, and pain. It is obvious that such organs, if they are to be of use to an animal, must be at the outside of the body.

Thus we find eyes and ears in the head, and taste cells in the mouth, while other cells in the nose perceive odors, and still others in the skin are sensitive to heat or cold, pressure or pain.

But this is not all. Strangely enough, we do not see with our eyes or taste with our taste cells. These organs receive the sensations, and by means of a complicated system of greatly elongated cell structures, the message is sent inward, relayed by other elongated cells until the sensory message reaches an inner station, in the central nervous system. We see and hear and smell in our brain. Let us next examine the structure of the nerve cells or _neurons_ part of which serve as pathways for these messages.

[Ill.u.s.tration: Diagram of a neuron or nerve unit.]

Neurones.--A nerve cell, like other cells in the body, is a ma.s.s 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 pa.s.s 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 pa.s.s 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: Diagram of the nerve path of a simple reflex action.]

Reflex Actions; their Meaning.--If through disease or for other reasons the cerebrum does not function, no will power is exerted, nor are intelligent acts performed. All acts performed in such a state are known as _reflex actions_. The involuntary brus.h.i.+ng of a fly from the face, or the attempt to move away from the source of annoyance when tickled with a feather, are examples of reflexes. In a reflex act, a person does not think before acting. The nervous impulse comes from the outside to cells that are not in the cerebrum. The message is short-circuited back to the surface by motor nerves, without ever having reached the thinking centers. The nerve cells which take charge of such acts are located in the cerebellum or spinal cord.

Automatic Acts.--Some acts, however, are learned by conscious thought, as writing, walking, running, or swimming. Later in life, however, these activities become automatic. The actual performance of the action is then taken up by the cerebellum, medulla, and spinal ganglia. Thus the thinking portion of the brain is relieved of part of its work.

Bundles of Habits.--It is surprising how little real thinking we do during a day, for most of our acts are habitual. Habit takes care of our dressing, our bathing, our care of the body organs, our methods of eating; even our movements in walking and the kind of hand we write are matters of habit forming. We are bundles of habits, be they good ones or bad ones.

Habit Formation.--The training of the different areas in the cerebrum to do their work well is the object of education. When we learned to write, we exerted conscious effort in order to make the letters. Now the act of forming the letters is done without thought. By training, the act has become automatic. In the beginning, a process may take much thought and many trials before we are able to complete it. After a little practice, the same process may become almost automatic. We have formed a habit. Habits are really acquired reflex actions. They are the result of nature's method of training. The conscious part of the brain has trained the cerebellum or spinal cord to do certain things that, at first, were taken charge of by the cerebrum.

Importance of Forming Right Habits.--Among the habits early to be acquired are the habits of studying properly, of concentrating the mind, of learning self-control, and, above all, of contentment. Get the most out of the world about you. Remember that the immediate effect in the study of some subjects in school may not be great, but the cultivation of correct methods of thinking may be of the greatest importance later in life. The man or woman who has learned how to concentrate on a problem, how to weigh all sides with an unbiased mind, and then to decide on what they believe to be best and right are the efficient and happy ones of their generation.

"The h.e.l.l to be endured hereafter, of which theology tells, is no worse than the h.e.l.l we make for ourselves in this world by habitually fas.h.i.+oning our characters in the wrong way. Could the young but realize how soon they will become mere walking bundles of habits, they would give more heed to their conduct while in the plastic state. We are spinning our own fates, good or evil, and never to be undone. Every smallest stroke of virtue or of vice leaves its never-so-little scar. The drunken Rip Van Winkle, in Jefferson's play, excuses himself for every fresh dereliction by saying, 'I won't count this time!' Well! he may not count it, and a kind Heaven may not count it; but it is being counted none the less. Down among his nerve cells and fibers the molecules are counting it, registering and storing it up to be used against him when the next temptation comes. Nothing we ever do is, in strict scientific literalness, wiped out. Of course this has its good side as well as its bad one. As we become permanent drunkards by so many separate drinks, so we become saints in the moral, and authorities in the practical and scientific, spheres by so many separate acts and hours of work. Let no youth have any anxiety about the upshot of his education, whatever the line of it may be. If he keep faithfully busy each hour of the working day, he may safely leave the final result to itself. He can with perfect certainty count on waking up some fine morning, to find himself one of the competent ones of his generation, in whatever pursuit he may have singled out."--JAMES, _Psychology_.

Some Rules for Forming Good Habits.--Professor Horne gives several rules for making good or breaking bad habits. They are: "First, _act on every opportunity_. Second, _make a strong start_. Third, _allow no exception_.

Fourth, _for the bad habit establish a good one_. Fifth, summoning all the man within, _use effort of will_." Why not try these out in forming some good habit? You will find them effective.

[Ill.u.s.tration: The effect of fatigue on nerve cells. _a_, healthy brain cell; _b_, fatigued brain cell.]

Necessity of Food, Fresh Air, and Rest.--The nerve cells, like all other cells in the body, are continually wasting away and being rebuilt.

Oxidation of food material is more rapid when we do mental work. The cells of the brain, like muscle cells, are not only capable of fatigue, but show this in changes of form and of contents. _Food_ brought to them in the blood, plenty of _fresh air_, especially when engaged in active brain work, and _rest_ at proper times, are essential in keeping the nervous system in condition. One of the best methods of resting the brain cells is a change of occupation. Tennis, golf, baseball, and other outdoor sports combine muscular exercise with brain activity of a different sort from that of business or school work. But change of occupation will not rest exhausted neurones. For this, sleep is necessary. Especially is sleep an important factor in the health of the nervous system of growing children.

Necessity of Sleep.--Most brain cells attain their growth early in life.

Changes occur, however, until some time after the school age. Ten hours of sleep should be allowed for a child, and at least eight hours for an adult.

At this time, only, do the brain cells have opportunity to rest and store food and energy for their working period.

Sleep is one way in which all cells in the body, and particularly those of the nervous system, get their rest. The nervous system, by far the most delicate and hardest-worked set of tissues in the body, needs rest more than do other tissues, for its work directing the body only ends with sleep or unconsciousness. The afternoon nap, s.n.a.t.c.hed by the brain worker, gives him renewed energy for his evening's work. It is not hard application to a task that wearies the brain; it is _continuous_ work without rest.

THE SENSES

Touch.--In animals having a hard outside covering, such as certain worms, insects, and crustaceans, minute hairs, which are sensitive to touch, are found growing out from the body covering. At the base of these hairs are found neurones which send axons inward to the central nervous system.

[Ill.u.s.tration: Nerves in the skin: _a_, nerve fiber; _b_, tactile papillae, containing a tactile corpuscle; _c_, papillae containing blood vessels.

(After Benda.)]

Organs of Touch.--In man, the nervous mechanism which governs touch is located in the folds of the dermis or in the skin. Special nerve endings, called the _tactile corpuscles_, are found there, each inclosed in a sheath or capsule of connective tissue. Inside is a complicated nerve ending, and axons pa.s.s inward to the central nervous system. The number of tactile corpuscles present in a given area of the skin determines the accuracy and ease with which objects may be known by touch.

If you test the different parts of the body, as the back of the hand, the neck, the skin of the arm, of the back, or the tip of the tongue, with a pair of open dividers, a vast difference in the accuracy with which the two points may be distinguished is noticed. On the tip of the tongue, the two points need only be separated by 1/24 of an inch to be so distinguished. In the small of the back, a distance of 2 inches may be reached before the dividers feel like two points.

Temperature, Pressure, Pain.--The feeling of temperature, pressure, and pain is determined by different end organs in the skin. Two kinds of nerve fibers exist in the skin, which give distinct sensations of heat and cold.

These nerve endings can be located by careful experimentation. There are also areas of nerve endings which are sensitive to pressure, and still others, most numerous of all, sensitive to pain.