PHYSIOLB(4Y. lIaving described the anatom ical structure cf the eye, we are able to pro ceed to the consideration of the uses of the va rion, parts of this organ. Assuming a general knowledge of the ordinary laws of geometriva I optics (see LIGHT; LENS; etc. I we will trace the course of the rays of proceed hie from any luminous body through tie different media on which they impinge. Tf a luminous object—as, for example. a lighted candle—he placed at about the ordinary distance of distinct vision (about 10 inches) from the front of the eye, some rays fall on the sclerotic. and being reflected, take no part in vis ion. Tip more central ones fall upon the cornea, and of these some also are reflected, giving to the surface of the eye its beautiful glistening appear ance: while others pass through it, are converged by it are enter the aqueous humor, which prob ably exerts 110 perceptible effect on their direc tion. Those which fall on and pass through the outer or circumferential part of the cornea are stopped by the iris, and are either reflected or ab sorbed by it : while those which fall upon its more central part pass through the pupil and are con eerred in vision. In consequence of its refractive power, the rays passing through a comparatively large surface of the cornea are converged so as to pass through the relatively small pupil and im pinge upon the lens, which. by the convexity of its surface and Icy its greater density toward the centre, very much increases the convergence of the rays passing through it. They then traverse the vitreous humor, whose principal use appears to be to afford support to the expanded retina, and are brought to a focus upon that tunic, forming there an exact but inverted image of the object.
This inversion of the image may he easily ex hibited in the eye of a white rabbit or other albino animal, after removing, the muscles, etc.. from the beck part of the globe. The flame of a candle Lehi he lure the cornea may be seen inverted at the back of the eye, increasing. in size as the candle is brought :sear, as it retires, and alway;. maring in a direction opposite to that of the flame.
The adaptation of the eye to distinct. vision at every distance beyond that of a few inches is ex tremely remarkable. The process to which the structures of the eye lend themselves in accom plishing this is known as accommodation. The essential factor in the process is the contraction of the ciliary muscle, which, as previously noted, allows the suspensory ligament of the lens to relax. with a resultant bulge of its anterior sur
face and a decrease in its focal length. Accord ing to Helmholtz. the radius of curvature of the anterior surface of the lens diminishes on turning the eye to a near object from ten to six milli meters (from about 0.4 to 0.24 of an inch), while the' most projecting point of the same surface is brought forward about 0.2 of an inch. Accord ing to the observations of Hueck, the focal dis tance may be changed about three times in a sec ond. The accommodation from a near to a dis tant object is effected much more rapidly than the converse process.
There are two well-known forms of defective vision in which this power of adaptation is very much limited—viz. short-sightedness, or m yopia, and long-sightedness, or hy perm etropia. The lim itation, however, is not due to a defect in the muscular apparatus to which we have referred, but to an abnormality either in the curves of the refracting media or to congenital or acquired changes in the antero-posterior diameter of the ball. In short-siyhtcdness from too great a re fractive power from either cause, the rays from objects at the ordinary range of distinct vision arc brought too soon to a focus, so as to cross one another. and begin to diverge before they fall on the retina, the eye in this case being able to bring to the proper focus on the retina only those rays which were previously diverging at a large angle from a very near object. The correction for this deficiency is accomplished by interpos ing between the eye and indistinctly seen objects a conca rc lens, with a curvature just sufficient to throw the images of external objects at the ordi nary distance of distinct vision backward 11114)11 the retina. In f ar-s; ah ed ness, on the other hand, there is an abnormal diminution of the refractive power from too flat a cornea, a deficient aqueous humor, or a flattening of the lens, so that the fo cus is behind the retina. This defect is corrected by eon err lenses, which increase the convergence of the rays of light. Presbyopia, as its name in dicates, usually comes on at a comparatively ad vanced period of life, and is Otte to senile changes affecting the elasticity of the lens and its attach ments.