little in front of the retina. Persons thus affected cannot see distinctly, except at a very short distance. This infirmity is called near, or short-sightedness. This defect is in a great measure obviated by the use of concave glasses, which scatter the luminous rays, and thus counterbalance the too strong refracting force of the eye.

933. When the different parts of the eye are not sufficiently convex, the image is formed beyond the retina, and thus only distant objects are distinctly seen. This defect is called long-sightedness. The feebleness in the refracting power of the eye may be caused by disease; but usually it is a consequence of old age, and is remedied by wearing spectacles with convex glasses.

HYGIENE OF THE ORGANS OF VISION.

934. The eye, like other organs of the body, should be used, and then rested. If we look intently at an object for a long time, the eye becomes wearied, and the power of vision diminished. The observance of this rule is particular. ly needful to those whose eyes are weak, and predisposed to inflammation. On the contrary, if the eye is not called into action, its functions are enfeebled.

935. Sudden transitions of light should be avoided. The iris enlarges or contracts, as the light that falls upon the eye is faint or strong; but the change is not instantaneous.

932. What is short-sightedness? How is the defect remedied? 933. What is long-sightedness? How is the defect remedied? 934-942. Give the hygiene of the organs of vision. 934. Do the same principles apply to the use of the eye as to other organs? What is the effect if the eye is fixed intently on an object for a long time? What results if the eye is not called into action? 935. Why should sudden transitions of light be avoided?

Note. Let the anatomy, and physiology of the eye be reviewed from figs. 139 and 143, or from anatomical outline plate No. 10.

Hence the imperfect vision in passing from a strong to a dim light, and the overwhelming sensation experienced on emerging from a dimly-lighted apartment to one brilliantly illuminated. A common cause of am-aur-o'sis, or paralysis of the retina, is, using the eye for a long time in a very intense light.

936. Long-continued oblique positions of the eye should be avoided, when viewing objects. If the eye is turned obliquely for a long time in viewing objects, it may produce an unnatural contraction of the muscle called into action. This contraction of the muscle is termed stra-bis'mus, or cross-eye. The practice of imitating the appearance of a person thus affected, is injudicious, as the imitation, designed to be temporary, may become permanent.

Observation. The vision of a 66 cross-eye " is always defective. In general, only one eye is called into action, in viewing the object to which the mind is directed. This defect can be remedied by a surgical operation, which also corrects the position of the eye.

937. Children should be trained to use the eye upon objects at different distances. This is necessary, in order that the vision may be correct when objects at various distances are viewed. Any action unnatural to the muscles, if frequently repeated, may and will modify the character and action of the parts so operated upon. If a limb, as the arm, be kept flexed for a long time, one set of muscles will be relaxed and elongated, and another will be shortened, and its contractile power will be increased. The same principle is true of

the eye.

What causes palsy of the retina? 936. Why should we avoid oblique positions of the eye in viewing objects? What is said of the practice of imitating persons thus affected? What is said in reference to the vision of a cross-eye"? 937. Why should children be trained to use the eye upon objects at different distances? What is the effect if an unnatural action of the muscles is frequently repeated? Does the same principle apply to the eye?

938. In viewing objects very near the eye, the ciliary processes are called into action to produce a proper inclination of the crystalline lens, so that the rays of light may be properly refracted to form a perfect image on the retina. In looking at objects at a great distance, the ciliary processes are called into a different action, to produce a different inclination of the lens. Let either of these actions be repeated, again and again, for weeks and months, and they will become natural, and the acquired inclination will be permanent.

939. From the preceding principle, a person becomes short or long sighted, as the objects to which the eye is usually directed are near or remote. This is one reason why scholars, watchmakers, and artisans, who bring minute objects near the eye to examine them, are short-sighted, and why hunters and sailors, who are habituated to view objects at a distance, are long-sighted.

Observation. In the management of children, whether in the nursery or school-room, it is very important that their books, or articles upon which they may labor, should be held at an appropriate distance from the eye. Were this attended to by the parent or instructor, we should not see so many persons with defective vision.

940. Cleanliness, as well as the health of the eye, requie hat it be bathed every morning with pure water, either cold or tepid, accompanied with as little rubbing or friction as possible. In all instances, the secretion from the lachrymnal glands, that sometimes collects at the angle of the eye, should be removed, as it contains saline matter.

941. When small particles, or dust, get upon the eye, they produce much inconvenience, which is often increased by

938. What is the effect of repeatedly using the eye in one direction? 939. Why are artisans and scholars generally short-sighted? Why are sailors and hunters long-sighted? How can defective vision in a great degree be prevented? 940. What reasons are there for bathing the eye? 941. How can dust and other small particles be removed from the eye?

harsh attempts to remove them. The individual should be placed before a strong light, the lids held open with one hand, or by another person, and the particles removed with the corner of a fine linen or silk handkerchief.

942. Sometimes the substance is concealed under the upper eyelid, and it may then be exposed by turning back the lid in the following manner: Take a knitting-needle, or small, slender piece of stick, which is perfectly smooth, and place it over the upper lid, in contact with, and just under the edge of the orbit; then, holding it firmly, seize the eyelashes with the fingers of the disengaged hand, and gently turn the lid back over the stick or needle. The inner side of the lid can then be examined, and any substance removed that may have been there concealed. Too many trials ought not to be made, if unsuccessful, as much inflammation may be induced; but a surgeon should be consulted as soon as possible.

Observation. Eyestones ought never to be placed in the eye, as they often cause more pain and irritation than the evil which they are intended to remedy.

942. How removed from the upper eyelid? Why should not eyestones be used?

85

CHAPTER XLVI.

THE SENSE OF HEARING.

943. THE sense of hearing is next in importance to that of vision. Through this sense we are enabled to perceive sounds, that not only subserve to our comfort and pleasure, but are instrumental in promoting our intellectual enjoyments. The organ of hearing, or the ear, is one of the most complicated in the human body.

ANATOMY OF THE ORGANS OF HEARING.

944. The EAR is composed of three parts: 1st. The Ex ternal Ear. 2d. The Tym'pan-um, or middle ear. 3d. The La'by-rinth, or internal ear.

945. The EXTERNAL EAR is composed of two parts: The Pin'na, (pavilion of the ear,) and the Me-a'tus Aud-it-o'ri-us Ex-ter'nus, (auditory canal.)

946. The PINNA is a cartilaginous piate which surrounds the entrance of the auditory canal. It presents many ridges and furrows, arising from the folds of the cartilage that form it.

Observation. The pinna, in many animals, is movable; in those that pursue their prey, it is generally directed forward; in timid animals, as the hare and rabbit, it is directed

943. What is said of the importance of hearing? Is the ear complicated in its structure? 944-962. Give the anatomy of the organs of hearing. 944. Of how many parts is the ear composed? Name them. 945. Give the parts of the external ear. 946. Describe the pinna. What is said in reference to the pinna of many animals?