File:Good and bad eyesight - and the exercise and preservation of vision (1882) (14597984178).jpg

Identifier: goodbadeyesighte00cart (find matches)
Title: Good and bad eyesight : and the exercise and preservation of vision
Year: 1882 (1880s)
Authors: Carter, Robert Brudenell, 1828-1918
Subjects: Eye Vision Eye Diseases Vision, Ocular
Publisher: Philadelphia : P. Blakiston
Contributing Library: Francis A. Countway Library of Medicine
Digitizing Sponsor: Open Knowledge Commons and Harvard Medical School

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eyeball. The human eye, as shown in an enlarged, diagrammatichorizontal section in Fig I, is a nearly spherical body,which, in the adult, measures a little less than an inchin diameter. It consists chiefly of transparent fluidscontained in membranes, which are called the tunics ofthe eyeball. Of these tunics, which lie over one another concen-trically, like the scales of an onion, the external isdivided into two parts, called respectively the scleroticand the cornea. The sclerotic (s, Fig. i) covers theposterior four-fifths, the cornea (c, Fig. i) covers thefront of the eyeball only. The front portion of thesclerotic is visible through the conjunctiva, or delicateskin covering it, a portion of which is shown at ca,and forms what is called the white of the eye. In i6 EYESIGHT. early childhood it has a bluish tint, and towards thedecline of life it generally becomes somewhat yellow.The back of the sclerotic rests upon a cushion of fatwhich occupies the cavity of the orbit, and which Fig
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surrounds and protects the optic nerve. In texture,the sclerotic is tough, flexible, and opaque. It isthickest at the hinder part of the eyeball, and becomesgradually thinner towards the front. The cornea is inserted into the front margin of the STRUCTURE OF THE EYES. \J sclerotic something in the manner of a watch-glassinto its setting. It is not only as transparent as themost pellucid water, but as smooth as the most polishedmirror; and to these qualities the eye is indebted forits brightness. The transparency, polish, and propercurvature of the cornea, are all of them of essentialimportance to vision. These two membranes, taken together, complete theouter coverings of the eyeball, and they are maintainedby the fluids within their cavities, and by the bloodwhich circulates in the vessels of the internal mem-branes presently to be described, in a certain state oftension or fulness, the degree of which can be estimatedby pressing upon the eye with the points of the fore-fingers throug

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