The Eye : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 24 of 28)

How we see Objects. - To see an object at least four things must happen:

1. Light must pass from the object into the eye. Objects cannot be seen where there is no light or where, for some reason, it is kept from entering the eye.

2. The light from the object must be focused (made to form an image) on the retina. In forming the image, an area of the retina is stimulated which corresponds to the form of the object.

3. Impulses must pass from the retina to the brain, stimulating it to produce the sensations.

4. The sensations must be so interpreted by the mind as to give an impression of the object.

Focusing Power of the Eyeball. - The eyeball is essentially a device for focusing light. All of its transparent portions are directly concerned in this work, and the portions that are not transparent serve to protect and operate these parts and hold them in place. Of chief importance are the crystalline lens and the cornea. Both of these are lenses. The cornea with its enclosed liquid is a plano-convex lens, while the crystalline lens is double convex. Because of the great difference in density between the air on the outside and the aqueous humor within, the cornea is the more powerful of the two. The crystalline lens, however, performs a special work in focusing which is of great importance. The iris also aids in focusing since it, through the pupil, regulates the amount of light entering the back chamber of the eyeball and causes it to fall in the center of the crystalline lens, the part which focuses most accurately.

Accommodation. - A difficulty in focusing arises from the fact that the degree of divergence of the light waves entering the eye from different objects, varies according to their distance. Since the waves from any given point on an object pass out in straight lines in all directions, the waves that enter the eye from distant objects are at a different angle from those that enter from near objects. In reality waves from distant objects are practically parallel, while those from very near objects diverge to a considerable degree. To adjust the eye to different distances requires some change in the focusing parts that corresponds to the differences in the divergence of the light. This change, called accommodation, occurs in the crystalline lens. In the process of accommodation, changes occur in the shape of the crystalline lens, as follows:

1. In looking from a distant to a near object, the lens becomes more convex, rounder and thicker. This change is necessary because the greater divergence of the light from the near objects requires a greater converging power on the part of the lens.

2. In looking from near to distant objects, the lens becomes flatter and thinner. This change is necessary because the less divergent waves from the distant objects require less converging power on the part of the lens.

The method employed in changing the shape of the lens is difficult to determine and different theories have been advanced to account for it. The following, proposed by Helmholtz, is the theory most generally accepted:

The lens is held in place back of the pupil by the suspensor ligament. This is attached at its inner margin to the membranous capsule, and at its outer margin to the sides of the eyeball, and entirely surrounds the lens. It is drawn perfectly tight so that the sides of the eyeball exert a continuous tension, or pull, on the membranous capsule, which, in its turn, exerts pressure on the sides of the lens, tending to flatten it. This arrangement brings the elastic force of the eyeball into opposition to the elastic force of the lens. The ciliate muscle plays between these opposing forces in the following manner:

To thicken the lens, the ciliate muscle contracts, pulling forward the suspensor ligament and releasing its tension on the membranous capsule. This enables the lens to thicken on account of its own elastic force. To flatten the lens, the ciliate muscle relaxes, the elastic force of the eyeball resumes its tension on the suspensor ligament, and the membranous capsule resumes its pressure on the sides of the lens. This pressure, overcoming the elastic force of the lens, flattens it.

Movements of the Eyeballs. - In order that the light may enter the eyeballs to the best advantage, they must be moved in various directions. These movements are brought about through the action of six small muscles attached to each eyeball. Four of these, named, from their positions, the superior, inferior, internal, and external rectal muscles, are attached at one end to the sides of the eyeball and at the other end to the back of the orbit. These, in the order named, turn the eyes upward, downward, inward, and outward. The other two, the superior and inferior oblique muscles, aid in certain movements of the rectal muscles and, in addition, serve to rotate the eyes slightly. The movements of the eyeballs are similar to those of ball and socket joints.

Binocular Vision. - In addition to directing the eyeballs so that light may enter them to the best advantage from different objects, the muscles also enable two eyes to be used as one. Whenever the eyes are directed toward the same object, an image of this object is formed on the retina of each. Double vision is prevented only by having the images fall on corresponding places in the two eyes. This is accomplished by the muscles. In each act of seeing, it becomes the task of the superior and inferior rectal muscles to keep the eyes in the same plane, and of the external and internal rectal muscles to give just the right amount of convergence. If slight pressure is exerted against one of the eyes, the action of the muscles is interfered with and, as a consequence, one sees double. The advantages of two eyes over one in seeing lie in the greater distinctness and broader range of vision and in the greater correctness of judgments of distance.

D.C. Heath and Co. - Publishers
Original copyright 1909