Title: Elements of biology; a practical text-book correlating botany, zoology, and human physiology
Identifier: elementsofbiolog00hunt (find matches)
Year: (c1907) ((c10s)
Authors: Hunter, George William, 1873-1948
Subjects: Biology
Publisher: New York, American book company
Contributing Library: NCSU Libraries
Digitizing Sponsor: NCSU Libraries

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424 HUMAN PHYSIOLOGY a transparent tough layer called the cornea. A second cop.t, the choroid, is supplied with blood vessels and cells which bear pigments. It is this coat which we see through the cornea as the colored part of the eye (the iris). In the center of the iris is a small circular hole (the pupil). The iris is under the control of muscles, and may be adjusted to varying amounts of light, the hole becoming larger in dim light, and smaller in bright light. Watch the pupil of a cat's eye, in changing the ani- mal from a dark to a light room. The inmost layer of the eye is called the retina. This is, perhaps, the most delicate layer in the entire body. Despite the fact that the retina is less than -g^^- of an inch in thick- ness, there are no less than eight layers of cells in its composition. The optic nerve
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Longitudinal section through the eye; Sc, sclerotic coat; Ch, cho- roid; O, optic nerve ; C, cornea; /.iris; B, blind spot; con, con- junctiva; R, retina; Y, yellow spot; L, lens; A, anterior chamber, filled with aqueous humor; V, posterior chamber, filled with vitreous humor. enters the eye from behind and spreads out over the surface of the retina. Its finest fibers end in elongated rodlike and cone- like cells. The retina is dark purple in color, this color being caused by a layer of cells next to the choroid coat. This accounts for the black appearance of the pupil of the eye, when we look through the pupil into the darkened space within the eyeball. The retina acts as the sensitized plate in the camera, for on it are received the impressions which are transformed and sent to the brain as sensations of sight. The eye, like the camera, has a lens. This lens is formed of transparent, elastic material. It is found directly behind the iris and is attached to the choroid coat by

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