A Treatise on Anatomy, Physiology, and Hygiene (Revised Edition) by Calvin Cutter (lightweight ebook reader txt) 📖
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925. Different degrees of density, as already mentioned, modify the refractory power of any transparent medium. It is found, on examination, that the cornea, the vitreous, the 407 crystalline, and the aqueous humors, have each, severally, various degrees of density: and that the crystalline lens, at its circumference, is less dense than at its centre. These circumstances modify the direction of the refraction of the rays of light, in their passage from the cornea to the retina.
922. What is the use of the cornea, aqueous, crystalline, and vitreous humors? 923. What is the office of the retina? 924. What is the function of the optic nerve? How is sensibility conferred on this organ? Give the 1st observation in this connection. The 2d observation.
926. The refracting powers of the plane, convex, concave, plano-convex, plano-concave, and concavo-convex lenses,[22] are different. The cornea and aqueous humors are convexo-concave, the vitreous humor is concavo-convex, while the crystalline humor is a convexo-convex medium. (Fig. 139.)
Fig. 142.
Fig. 142. The forms of the different lenses. 1, A plane lens. 2, A globe lens. 3, A convexo-convex lens. 4, A plano-convex lens. 5, A concavo-concave lens. 6, A plano-concave lens. 7, Meniscus. 8, A concavo-convex lens.
925. Have the cornea and the humors of the eye different degrees of density? What is said of the crystalline lens? What effect has the different density of the parts of the eye upon the light admitted to this organ? 926. What kind of lenses do the humors exhibit? 927. What modifies the refracting powers of transparent mediums? How does this principle apply to the humors of the eye?
927. The different degrees of convexity or concavity also modify the refracting character of transparent mediums. The crystalline lens is of different degrees of convexity on its two sides. The convex surfaces of the aqueous and vitreous humors are segments of circles, of different diameters from their concave surfaces. (Fig. 139.) All these 408 circumstances still further influence the refracting character of the visual organ. The achromatic arrangement of the transparent refracting mediums of the eye, remedies the aberration of refraction in the different portions of the eye.
928. Again, the refracting power of lenses is modified by their convexity or concavity. The more convex a lens is, the shorter the distance from the refracting medium, where the different refracted rays converge to a focus. To adapt the eye to view objects at different distances, requires a change in the refracting power of some of the transparent mediums of the eye.
929. Both surfaces of the crystalline lens are oval, not spherical, and the refraction of the rays of light is mainly effected in this portion of the eye. Change the inclination of this lens, so that different portions of its anterior surface shall be directly behind the pupil, and its refracting power is increased or diminished, as the surface presented is more or less convex.
930. To view objects at a distance, a less convex lens is needed than in examining articles very near the eye; and this organ, from its structure, has the power of adaptation to different distances. It is supposed that the muscular substance of the ciliary body and processes changes, by its contraction, the inclination of the crystalline lens. Without this, or some other adapting power, a picture of objects at different distances would not be formed upon the retina, and the vision of every person would be defective, except in reference to objects at certain definite distances from the eye.
928. What modifies the refracting power of lenses? What is necessary to adapt the eye to view objects at different distances? 929. Where is the refraction of the rays of light mainly effected? 930. When we view objects at a distance, what kind of lens is required? Has the eye the power of adapting itself to different distances? How is it effected?
409Observation. It is well known that a separate image is formed on each eye, and, if they are not in the same direction, the objects will appear double. This is proved by pressing one eye, so that the rays of light cannot enter it in the same direction as they do in the other; consequently, the vision is double.
931. By the action of the muscles of the eye, it is turned in different directions, so that objects can be examined upon each side, as well as in front, without turning the body. By the slight or intense action of the straight muscles, the eye is more or less compressed, and the form of the globe is changed, together with the relative positions of the different humors. This modification also adapts the eye to view objects at different distances.
Fig. 143.
Fig. 143. 1, A pen, an inverted image of which is painted on the retina of the eye, at 2. The image of all objects upon the expansion of the optic nerve, is inverted by the crossing of the rays of light from objects as they traverse the pupil.
Observation. If the eye is fixed for a time on some object which is distinguished with difficulty, there is a painful sensation, similar to that experienced by other muscles of the body when used too long. This is called “straining the eye.”
932. When the refraction of the rays of light is too great, as in over-convexity of the cornea, or the crystalline lens, or the vitreous humor, or all of them, the image is formed a 410 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.
What does fig. 143 represent? 931. Why can we see objects at the side as well as in front of the eye, without turning the body? What is the effect when the eye is fixed on an object that is indistinctly seen?
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.
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 particularly 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. 411 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.
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.
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 “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?
412938. 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, require that 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 lachrymal 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 413 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.
938. What is the effect of repeatedly using
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