Myopia (commonly called near-sightedness,) in its various phases / by Julian J. Chisholm.

  • Chisolm, Julian John, 1830-1903.
Date:
[1880]
Catalogue details

Licence: Public Domain Mark

Credit: Myopia (commonly called near-sightedness,) in its various phases / by Julian J. Chisholm. Source: Wellcome Collection.

Provider: This material has been provided by The Royal College of Surgeons of England. The original may be consulted at The Royal College of Surgeons of England.

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    part of the sensitive nerve layer, and is known as the fovea cen- tralis or the yellow spot of Soemmering. It is on this little spot that all perfect microscopic pictures of whatever we look at are condensed by the focusing lens. This yellow spot in a good eye, capable of any kind of work, near or far mustbe fixed at a determined distance from the back of the lens so as to catch the concentrated light. It must undergo no change of place whatever, and hence it is supported by a very heavy backing of the thickest part of the eye-walls. The entrance of the optic nerve into the eye-ball is within two lines of the yellow spot. During the accommoda- tion of the eye for near objects and while the changes in the shape of the lens, needful for this act, are being produced through the contraction of the ciliary muscles the posterior pole of the lens never moves from its fixed position, however much the curved sides of the marginal edges of the lens may vary in place. In the working of the eye as an optical apparatus, the adjust- ing of the lens to focus light at varied distances is the especial action of the muscular elements of the ciliary bodies. When an eye-ball has been cut in half midway between the cornea and the optic nerve entrance, the curious collarett which is seen approach- ing the circular edge of the kns, is the accommodating apparatus for changing the focal power of the lens. These radiating bodies of prismatic shape, and about sixty in number, which form this deeply pigmented circular layer, are made up chiefly of blood vessels and muscular fibres hid away under and between masses of pigment cells. It is the muscular element of these ciliary bodies which, in the living eye, plays the part of the adjusting screw of the opera glass. By the varied muscular contractions of these bodies, acting upon the lens, during our waking hours, . all light passing through the pupil is at once concentrated upon • the yellow spot to form a perfect diminutive picture of whatever is placed before us, regardless ofthe various distances from which the light comes, or the object upon which the normal eye may be directed. The working of the eye is a muscular act. Muscles, when over- worked, will exhibit fatigue, which is often expressed by painful -.sensations. They can also become irritable when over-used, and