11. What change occurs in the shape of the crystalline lens when we look from distant to near objects? From near to distant objects? Why are these changes necessary? How are they brought about?

12. How does the method of adjustment, or accommodation, of the eyeball differ from that of a telescope or a photographer's camera?

13. With two eyes how are we kept from seeing double?

14. What different purposes are served by the tears. Trace them from the lachrymal glands to the nostrils.

15. Show how the proper lenses remedy short- and long-sightedness.

16. Describe the conjunctiva and give its functions. Why should it be so sensitive?

17. How may eye strain cause disease in parts of the body remote from the eyes?

18. How does "image stimulation" differ from light stimulation in general?

PRACTICAL WORK

*To ill.u.s.trate Simple Properties of Light.*-1. Heat an iron or platinum wire in a clear gas flame. Observe that when a high temperature is reached it gives out light or becomes luminous.

2. Cover one hand with a white and the other with a black piece of cloth, and hold both for a short time in the direct rays of the sun. Note and account for the difference in temperature which is felt.

3. Stand a book or a block of wood by the side of an empty pan in the sunlight, so that the end of the shadow falls on the bottom of the pan.

Mark the place where the shadow terminates and fill the pan with water.

Account for the shadow's becoming shorter.

4. Place a coin in the center of an empty pan and let the members of the cla.s.s stand where the coin is barely out of sight over the edges of the pan. Fill the pan with water and account for the coin's coming into view.

Show by a drawing how light, in pa.s.sing from the water into the air, is so bent as to enter the eye.

5. With a convex lens, in a darkened room, focus the light from a candle flame so that it falls on a white screen and forms an image of the candle.

Observe that the image is inverted. In a well-lighted room focus the light from a window upon a white screen. Show that, as the distance from the window to the screen is changed, the position of the lens must also be changed. (Accommodation.)

6. Hold a piece of cardboard, about eight inches square and having a smooth, round hole an eighth of an inch in diameter in the center, in front of a lighted candle in a darkened room. Back of the opening place a muslin or paper screen (Fig. 157). Observe that a dim image is formed.

Account for the fact that it is inverted. Hold a lens between the cardboard and the screen so that the light pa.s.ses through it also. The image should now appear smaller and more distinct.

[Fig. 166]

Fig. 166-*Diagram* for proving presence of the blind spot.

*To prove the Presence of the Blind Spot.*-Close the left eye and with the right gaze steadily at the spot on the left side of this page (Fig. 166).

Then starting with the book a foot or more from the face, move it slowly toward the eye. A place will be found where the spot on the right entirely disappears. On bringing it nearer, however, it is again seen. As the book is moved forward or backward, the position of the image of this spot changes on the retina. When the spot cannot be seen, it is because the image falls on the blind spot.

*Dissection of the Eyeball.*-Procure from the butcher two or three eyeb.a.l.l.s obtained from cattle. After separating the fat, connective tissue, and muscle, place them in a shallow vessel and cover with water.

Insert the blade of a pair of sharp scissors at the junction of the sclerotic rotic coat with the cornea and cut from this point nearly around the entire circ.u.mference of the eyeball, pa.s.sing near the optic nerve.

Spread open in the water and identify the different parts from the description in the text. Open the second eyeball in water by cutting away the cornea. Examine the parts in front of the lens.

[Fig. 167]

Fig. 167-*Model* for demonstrating the eyeball.

*To ill.u.s.trate Accommodation.*-Paste together the ends of a strip of stiff writing paper (two by five inches) making a ring a little less than three inches in diameter. This is to represent the crystalline lens. Now paste a piece of thin paper (two by seven inches) upon a second strip of the same size, leaving an open place in the middle for the insertion of the paper lens. A flexible piece of cardboard (three by twelve inches) is now bent into the form of a half circle and to its ends are fastened the strips of paper containing the ring. Make a small hole in each of the four corners of the bent cardboard. Through these holes pa.s.s two loops of thread, or fine string, in opposite directions, letting the ends hang loose from the cardboard.

When everything is in position, the tension from the cardboard flattens the paper lens, while pulling the strings releases this tension and permits the lens to become more rounded. With this simple device the changes in the curvature of the lens for near and distant vision are easily shown.

CHAPTER XXIII - THE GENERAL PROBLEM OF KEEPING WELL

"To cure was the voice of the Past: to prevent is the divine whispering of To-day."

As stated in the introduction to our study, the fundamental law of hygiene is the law of harmony: _Habits of living must harmonize with the plan of the body._ Having acquainted ourselves with the plan of the body, we may now review briefly those conditions that help or hinder its various activities. The hygiene already presented in connection with the study of the various organs may be condensed into general rules, or laws, as follows:

1. Of exercise: Exercise daily the important groups of muscles.

2. Of form: Preserve the natural form of the body.

3. Of energy: Observe regular periods of rest and exercise and avoid exhaustion.

4. Of nutriment: Eat moderately of a well-cooked and well-balanced diet and drink freely of pure water.

5. Of respiration: Breathe freely and deeply of pure air and spend a part of each day out of doors.

6. Of nervous poise: Suppress wasteful and useless forms of nervous activity, avoid nervous strain, and practice cheerfulness.

7. Of cleanliness: Keep the body and its immediate surroundings clean.

8. Of restraint: Abstain from the unnecessary use of drugs as well as from the practice of any form of activity known to be harmful to the body.

9. Of elimination: Observe all the conditions that favor the regular discharge of waste materials from the body.

Obedience to these laws is of vast importance in the proper management of the body. They should, indeed, be so thoroughly impressed upon the mind as to become fixed habits. There are, however, other conditions that relate to this problem, and it is to these that we now turn. These conditions have reference more specifically to

*The Prevention of Disease.*-While the average length of life is not far from thirty-five years, the length of time which the average individual is capable of living is, according to some of the lowest estimates, not less than seventy years. This difference is due to disease. People do not, as a rule, die on account of the wearing out of the body as seen in extreme old age, but on account of the various ills to which flesh is heir. It is true that many people meet death by accident and not a few are killed in wars, but these numbers are small in comparison with those that die of bodily disorders. The prevention of disease is the greatest of all human problems. Though the fighting of disease is left largely to the physician, much is to be gained through a more general knowledge of its causes and the methods of its prevention.

*Causes of Disease.*-Disease, which is some _derangement of the vital functions_, may be due to a variety of causes. Some of these causes, such as hereditary defects, are remote and beyond the control of the individual. Others are the result of negligence in the observance of well-recognized hygienic laws. Others still are of the nature of influences, such as climate, the house in which one lives, or one's method of gaining a livelihood, that produce changes in the body, imperceptible at the time, but, in the long run, laying the foundations of disease. And last, and most potent, are the minute living organisms, called microbes or germs, that find their way into the body. Although there are two general kinds of germs, known as _bacteria_ (one-celled plants) and _protozoa_ (one-celled animals), most of our germ diseases are caused by bacteria.

*Effects of Germs.*-While there are many kinds of germs that have no ill effect upon the body and others that are thought to aid it in its work, there are many well-known varieties that produce effects decidedly harmful. They gain an entrance through the lungs, food ca.n.a.l, or skin, and, living upon the fluids and tissues, multiply with great rapidity until they permeate the entire body. Not only do they destroy the protoplasm, but they form waste products, called _toxins_, which act as poisons. Diseases caused by germs are known as infectious, or contagious, diseases.(129) The list is a long one and includes smallpox, measles, diphtheria, scarlet fever, typhoid fever, tuberculosis, la grippe, malaria, yellow fever, and others of common occurrence. In addition to the diseases that are well p.r.o.nounced, it is probable that germs are responsible also for certain bodily ailments of a milder character.(130)

*Avoidance of Germ Diseases.*-The problem of preventing diseases caused by germs is an exceedingly difficult one and no solution for all diseases has yet been found. One's chances of avoiding such diseases, however, may be greatly enhanced: