. Principles of modern biology. Biology. 422 - Multicellular Animals, Especially Man MUSCLE CILIARY) CONJUNCTIVA. NERVE OPTIC) Fig. 23-5. Section of the human eye. makes the eyeball more opaque and prevents a scattering of light not absorbed by the retina. And lastly, a tree rotation of the eye- ball within its bony socket is effected by six small muscles, which permit the vision to be directed according to the situation (see Fig. 23-5). The lens of the eye accommodates for near and distant vision by changes of curvature; and these curvature changes are controlled partly by the elasticity of t
. Principles of modern biology. Biology. 422 - Multicellular Animals, Especially Man MUSCLE CILIARY) CONJUNCTIVA. NERVE OPTIC) Fig. 23-5. Section of the human eye. makes the eyeball more opaque and prevents a scattering of light not absorbed by the retina. And lastly, a tree rotation of the eye- ball within its bony socket is effected by six small muscles, which permit the vision to be directed according to the situation (see Fig. 23-5). The lens of the eye accommodates for near and distant vision by changes of curvature; and these curvature changes are controlled partly by the elasticity of the lens itself, and partly by the action of the ciliary muscle (Fig. 23-5). This circular band of muscle surrounds the margins of the lens, exerting a tension on the lens by means of the sus- pensory ligament (Fig. 23-5). Accommoda- tion for near vision results when the ciliary muscle contracts, reducing the tension on the suspensory ligament and lens. When it is released of tension, the elastic lens assumes its unstrained form, which is quite thick and nearly spherical. For distant vision, on the other hand, the ringlike ciliary muscle relaxes, increasing its circumference. This restores the strain upon tlie suspensory liga- ment and forces the lens to become flatter and broader. The refractive power of the lens, which effects a precise focusing of the image on the retina, is augmented by the curvature of the cornea. The space in front of the lens con- tains a relatively nonviscous fluid, the aque- ous humor; whereas the region behind the lens is filled with a viscous liquid, the vitreous humor. The retina, or true photosensory surface of the eye, is composed mainly of a prodigious number of specialized receptor cells, called the rods and cones (Fig. 23-6). In each human retina there are more than 100 million rods and about 6 million cones. The cones are more numerous in the central area of the retina; they are concerned with ordinary "bright-light vision," in which
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