direct many of our everyday doings. Our attention is thus set free to carry on other kinds of work.

As every one knows, it takes a soldier a long time to learn his drill—for instance, to put himself into the attitude of 'attention' at the instant the word of command is heard. But, after a time, the sound of the word gives rise to the act, whether the soldier be thinking of it, or not. There is a story, which is credible enough though it may not be true, of a practical joker, who, seeing a discharged veteran carrying home his dinner, suddenly called out 'Attention!' whereupon the man instantly brought his hands down, and lost his mutton and potatoes in the gutter. The drill had been thorough, and its effect had become embodied in the man's nervous structure." 1

237. Importance of habit. The tremendous importance of making our habits our allies instead of our enemies cannot be emphasized too strongly.

"The hell to be endured hereafter," says Professor James, "of which theology tells, is no worse than the hell we make for ourselves in this world by habitually fashioning 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,

"Huxley's "Lessons in Elementary Physiology," Macmillan Company.

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."

238. Conditions necessary for a healthy nervous system. - In studying the hygiene of other parts of the body, we found that four conditions were necessary for healthy activity. That the nervous system, too, may develop as it should and that it may do its work properly, the same four conditions are essential; namely, food, fresh air, various kinds of activity, and periods of rest.

239. Food and air.-In the nervous system of a human being there are millions of nerve cells. Each of these cells must be supplied with nutritious food and pure air, or it becomes stunted in its growth and unable to do its proper work. These busy cells are constantly giving off carbon dioxid, water, and other wastes, and if these are not removed and fresh oxygen supplied, one feels a drowsiness and headache, and is unable to think clearly. Well-ventilated rooms, both by day and by night, are of prime importance in the hygiene of the nervous system.

240. Varied activity. To develop a well-balanced brain one must be active along many lines. Experience tells us, too, that we cannot work successfully at the same task hour after hour without some change. Hence, varied activity is an important principle in sound education. The young child must, of necessity, turn, after a short time, from one lesson to another, and all lessons must finally give way to the relaxation of play. Unfortunate is the boy who fails to find exhilaration in baseball, bicycle riding, or general athletics, for these sports, when properly regulated, besides developing strong lungs and vigorous muscles, are important means of educating the nerve cells and fibers.

Not only in youth, but throughout life, must the student, the business man, or the laborer, at the end of a day's employment, find relaxation in other forms of activity. If he fails to do this, not only will he become weary of his work, but he will also finally come to lose the power of enjoying the pleasures he has been neglecting. In the later years of his life, the great naturalist, Charles Darwin, wrote as follows: "My mind seems to have become a kind of machine for grinding general laws out of large collections of facts. . . . If I were to live my life again, I would have made a rule to read some poetry and listen to some music at least once every week; for perhaps the parts of my brain now atrophied would thus have been kept alive through use. The loss of these tastes is a loss of happiness, and may possibly be injurious to the intellect, and more probably to the moral character, by enfeebling the emotional part of our nature."

241. Rest. Experiments with animals show a striking difference in the appearance of nerve cells before and after vigorous exercise. In the nerve cells of a bird that has been flying all day, the protoplasm has a distinctly granular appearance, which is not seen in the cells before exercise. Tired nerve cells can be restored by rest alone. In childhood and youth an abundance of sleep is absolutely essential for healthy development. Late hours of evening entertainment or of study should never be allowed to keep growing boys or girls from having at least nine hours of sleep.

242. Effect of alcohol on the nervous system. "The effect of alcohol appears to be, as it were, to shave off the nervous system, layer by layer, attacking first the highest developed faculties and leaving the lowest to the last, so that we find that a man's judgment may be lessened, though at the same time some lower faculties, such as the imagination and emotions, may appear to be more active than before. . . . Thus you find that after a man has taken alcohol his judgment may be diminished, but he may become more loquacious and more jolly than before. Then after a while his faculties become dull; he gets stupid and drowsy. . . . Later on it affects the motor centers, probably the cerebellum, so that the man is no longer able to walk, and reels whenever he makes the attempt. At this time,

M

however, he may still be able to ride (on horseback), and a man who is so drunk that he cannot walk and cannot speak may ride perfectly well. . . . Later on the further anaesthetic action of the alcohol abolishes sensation, and its paralyzing action destroys the power of the spinal cord, so that the man is no longer able even to ride; but still the respiratory center in the medulla will go on acting, and it is not until enormous doses of alcohol have been given that respiration becomes paralyzed.

"Alcohol . . . makes all the nervous processes slower, but at the same time it has the curious effect of producing a kind of mental anæsthesia, so that these processes seem to the person himself to be all quicker than usual, instead of being, as they really are, much slower. Thus a man, while doing things much more slowly than before, is under the impression that he is doing things very much more quickly. What applies to these very simple processes applies also to the higher processes of the mind; and a celebrated author once told me that if he wrote under the influence of a small quantity of alcohol, he seemed to himself to write very fluently and to write very well, but when he came to examine what he had written next day, after the effect of the alcohol had passed off, he found that it would not stand criticism." 1

V. THE EYES

243. Protection for the eye. The delicate organs of vision, the eyes, are protected in a wonderful manner. In the first place, the eyeballs are set far back in bony sockets, in such a way that, even if one falls forward or if the face is struck with a large object, there is little danger that the eyes themselves will be hit. Again, each eyeball is covered by two movable lids that involuntarily close at any threatened danger. And, finally, the curving eyelashes on the edge of each lid protect the eyeball to a considerable extent from dust and dirt.

244. Structure of the eye. Each eye is nearly spherical in shape (Fig. 52). Its outer surface is covered with a tough coat which

1 T. Lauder Brunton, London, "Lectures on the Action of Medicine," pp. 190, 191, 194.

is white in color, except in front, where it becomes the transparent

cornea.

Inside of the outer coat is second layer which is seen beneath the cornea as a colored ring known as the iris. In the center of the iris is a circular opening, the pupil, which is black in appearance. Through the pupil enter the rays of light into the interior of the eyeball. If one comes suddenly from a dark room into the light, it is possible to see this opening quickly decrease in size. The inner lining of the eyeball is extremely thin and black in color; it is known as the retina, and connected with it are the many nerve fibers that carry messages to the brain.

Behind the iris is a beautiful transparent object, the crystalline lens, both surfaces of which are convex. The space within the eyeball in front of this lens is occupied by a liquid, and behind the lens is a jellylike substance.

V. H.

FIG. 52. Section of the eye.

C=

= Cornea.

I Iris.

L = Crystalline lens.

ON

Ꭱ

Optic nerve.
Retina.

V. H. Jelly like substance.

245. The eye as a camera. Any one who is at all familiar with a camera knows that by means of a lens, or a combination of lenses, the scene to be photographed is made to appear upside down on the ground glass plate at the back of the camera. If the image is not clear, it is brought into focus by moving the lens nearer to, or farther from, the object.

In the eye, too, we have an arrangement similar to that of a camera, since the convex surfaces of the cornea and crystalline lens (Fig. 53) focus the rays of light so that an image is formed on the sensitive retina at the back of the eye. Since, however, the lenses within the eye cannot be moved backwards and forwards, as in a camera, focusing or accommodation of the eye must be accomplished