Elementary anatomy, physiology and hygiene for higher grammar grades . llyable, by a simple operation, to loosen the muscles enoughto straighten the eye. The eyeball is nearly spherical, and has a dense outercoat called the sclerotic coat. The front part of thiscoat is curved outward and transparent, reminding one ofa watch crystal. This is called the cornea, and throughthis the rays of light pass into the eye (Fig. 54).Just within the sclerotic coat is the choroid, which is aloose-meshed tissue, full of blood vessels and coat contains an important muscular body calledthe ciliary m
Elementary anatomy, physiology and hygiene for higher grammar grades . llyable, by a simple operation, to loosen the muscles enoughto straighten the eye. The eyeball is nearly spherical, and has a dense outercoat called the sclerotic coat. The front part of thiscoat is curved outward and transparent, reminding one ofa watch crystal. This is called the cornea, and throughthis the rays of light pass into the eye (Fig. 54).Just within the sclerotic coat is the choroid, which is aloose-meshed tissue, full of blood vessels and coat contains an important muscular body calledthe ciliary muscle, and it is this coat which makes the THE SPECIAL SENSES 233 little colored curtain called the iris. This little curtain,the iris, is like the diaphragm of a ca=aiera, and through itthere is a circular opening called the pupil. The mus-cles which it contains may dilate or contract the pupilaccording to the amount of light, or according to whetherthe object looked at is near or far away, — contracting tosee near objects and dilating to see distant objects. The. Fig. 54. — Horizontal section of the eyeball: d, sclerotic coat; c, cornea; c,choroid coat; i, iris; Aq, aqueous humor; a, crystalline lens; /t, vitreoushumor; b, retina; 0, optic nerve; cm, ciliary muscle; B, place vfheremuscles were attached. light which enters the eye through the cornea, passingthrough the pupil of the eye, must be focused upon thesensitive portion of the eye, the retina. The focusing isdone by the crystalline lens. The crystalline lens is heldin place by little ligaments, which pass out from theciliary muscle. The contraction of this muscle will causethe lens to become more convex, thus focusing the lightfrom near objects upon the retina. 234 PHYSIOLOGY The retina is composed of two cpats, the outer coatconsisting of a sepia-black pigment, which absorbs thelight, while the inner coat (shown white in the figure)consists of the nerve cells of sight. The eye nerve oroptic nerve passes from the b
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