in a different way, namely, by making the elastic lens more or less

convex.

246. Sensations of sight. We shall now try to see how it is that the eye helps us to get sensations of sight. If an object, say an arrow, is held in front of the eye, rays of light pass in a great many directions from every part of the arrow tip. A considerable number of these rays strike the convex surface of the cornea and the crystalline lens, and are thereby focused, or made to converge upon a point on the retina. In the same way the light rays from every other part of the arrow are brought to focus on the inner surface of the retina. By this means a smaller, inverted image, of the arrow (Fig. 53) is

B

B

FIG. 53. Formation of an image on the retina.

projected on the inner lining of the eye. The influence of these light rays then passes through the layers of the retina, and when these so-called "messages traverse the nerve fibers and reach the brain, we become conscious of sensations of sight.

247. Defective eyes. A normal, healthy eye has the power of adjusting itself so that objects become visible which are within five to ten inches, or as far away as a distant horizon. Many people, however, find that they can see objects near at hand much more clearly than those at a distance; in other words, they are nearsighted. Others, on the other hard, are farsighted. These defects in vision are due to imperfect formation of the eye, and can be corrected only by the use of proper eyeglasses or spectacles, which should be purchased only on the recommendation of a competent eye specialist.

Another very common defect of the eye is known as a-stig'ma-tism. Many people, on looking with each eye separately at

Figure 54, find that some of the radiating lines stand out sharply de fined, while others are indistinct or blurred. In reality, all the lines are equally distant from each other, and the indistinctness referred to above is due to the fact that some of the rays of light are not brought to a focus. Astigmatism, like nearsightedness and farsightedness, should be corrected by the use of proper glasses, otherwise constant eyestrain is likely to

cause headaches and other disorders

of the body.

Some people, too, are unable to distinguish clearly various colors; thus, red and green may appear the same to them. In other words, such people are color blind. Color blindness cannot be corrected by glasses, but may be to some extent by training.

FIG. 54.-Test for astigmatism.

248. Hygiene of the eyes. The eyes have, as we know, wonderful powers of adapting themselves to varying conditions. This adaptability often leads us to abuse them. Thus, we frequently read when the light is insufficient, we look steadily at objects until we suddenly find that we cannot see clearly, and we read or study while riding in swiftly moving trains. In these and other ways we compel our eyes to make adjustments under trying conditions, and more or less eyestrain is sure to follow.

When we read, we should make sure that the light is sufficient, that it is steady, and that it comes over the left shoulder. The type on the printed page should be little, if any, smaller than that in which most of this book is printed, the lines should not be close together, and the paper should not have a glossy surface to reflect the light into the eyes. One should remember, too, that the eyes, like other organs of the body, need frequent periods of rest. Hence study. hours should be followed by periods in which the eyes are allowed to relax. Pupils who have defective eyesight should at once secure proper glasses.

VI. THE EAR

249. The external ear. Attached to each side of the head is an oval, more or less flattened expansion, composed largely of cartilage and connective tissue. The irregular surface of this outer portion of the ear doubtless helps somewhat, like an ear trumpet, to catch and converge the sound waves into the funnel-like canal which is about an inch long, and leads to the interior of the head.

FIG. 55. Middle and inner ear, greatly enlarged.

In the lining of this canal are certain wax glands; these secrete a thin fluid which, on thickening, hardens into a yellow paste, the earwax. Across the inner end of this tube of the external ear is stretched a thin membranous partition, known as the eardrum, or tym'pa-num (Latin tympanum drum (Fig. 55)).

=

It is never safe for one to thrust into the canal of the ear any hard object, because of the danger of puncturing the eardrum. Ordinarily the canal cleans itself, but if it is necessary to remove bits of wax or dirt, this should be done with a tightly rolled corner of a piece of cloth. It is dangerous, too, to punish a child by boxing the ears,

because the sudden compression of the air is likely to injure the drum. Earache is often relieved by hot applications; never should laudanum or other substances be put into the ear without the advice of a physician.

250. The middle ear. Beyond the tympanum is a small cavity, known as the middle ear. From this cavity a narrow tube (the Eustachian tube) about an inch and a half long, communicates with the upper part of the throat cavity (Fig. 55). If one were to go up on a high mountain, he would find that the pressure of the air on the outside of the body, and therefore on the exterior of the eardrum, would become less, and if some of the air in the middle ear were not to escape, the eardrums would be forced outward, and hence would be ruptured. If, on the other hand, one should go into a deep mine, the increased pressure on the outside of the drums would force them inward. All these accidents are prevented by the presence of the Eustachian tubes, through which air can pass into and out from the middle ear, and so the pressure on both sides of the tympanum can be equalized. In severe head colds, the opening from the throat cavity into the Eustachian tubes becomes temporarily closed and we are then conscious of a ringing sensation in the ears. Catarrh sometimes closes the Eustachian openings and causes deafness. If the hearing seems to be at fault in any way, a specialist should be consulted.

251. Sensations of sound. - - When a stone is dropped into water, the ripples move outward over the surface in circular waves. In a similar manner sound waves are transmitted in all directions from a given body, for instance, a vibrating bell. When some of these sound waves enter the tube of the external ear, they cause the eardrum to vibrate, and this vibration is transmitted across the middle ear by a chain of tiny bones, and so reaches the complicated inner ear, which is a series of canals imbedded in solid bone. The inner ear contains a large number of sensitive cells which transfer the vibrations to nerves communicating with the brain. When the brain cells receive and interpret these impulses, we get sensations of sound.

GREAT BIOLOGISTS

252. Library studies of biologists. Select for study one or more of the following men who have made great contributions to our knowledge of biology: Agassiz, Aristotle, Audubon, Darwin, Harvey, Huxley, Jenner, Koch, Lamarck, Leeuwenhoek, Linnæus, Lister, Pasteur, Spencer, Wallace. Consult Locy's "Biology and its Makers," Williams's "A History of Science," Encyclopedias or other works of reference as to (1) the important events in the life of the biologist, and (2) his contributions to biological science.

LOUIS PASTEUR 1 (See Frontispiece)

I. Interesting Features of his Biography.

1. Parents.

a. Father (Jean Joseph), a tanner-sergeant major in Napoleon's army decorated with Legion of Honor.

b. Mother (Jeanne Rogui) of middle class family.

2. Birth, at Dôle (in Eastern France), Dec. 27, 1822.

3. Education.

a. In colleges near his birthplace (Arbois and Besançon) early evidences of remarkable ability in concentrating his mind in study.

b. In colleges at Paris - much influenced by the scientists

Dumas and Biot.

1 The ability to prepare logical outlines of library or laboratory studies is of great value to students (1) because in this form the principal facts can be stated more briefly than is possible in continuous paragraphs, and (2) because the various interrelations of the facts may be more clearly shown. In preparing such outlines the student should first select the most important division topics, all of which should be of equal value and expressed in similar form. Each of the various subordinate topics should be an organic part of the main division topic under which it is placed; each should be stated in a brief form, and as far as possible words or phrases should be used and verbs, clauses, or sentences avoided.

The outline on the life and works of Louis Pasteur is inserted (1) because of the importance of Pasteur's work, and (2) as a suggestive form for biology records.