SUPPORT, MOTION, AND SENSATION 



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are attached to the outer coat of the eyeball and to the bony wall 

 around the eye. 



The wall of the eyeball is made up of three coats. The outer tough 

 white coat of connective tissue is called the sclerutic coat. In front, 

 where the eye bulges out a little, the outer coat becomes transparent, 

 forming the cornea. A second coat, the choroid, is supplied with blood 



pupil 

 conjuTictV;^ 

 Cornea 



vitreous 

 humor 



Sagittal section of a inaiiimalian eye. 



vessels and cells containing considerable quantities of black pigment. 

 The iris, which shows through the cornea as the colored part of the 

 eye, is a part of this coat. In the center of the iris there is a small 

 circular hole, the -pupil. The iris is under the control of involuntary 

 muscles, and may be adjusted to varying amounts of light, the hole 

 becoming larger in dim light and smaller in bright light. The inmost 

 layer or coat of the eye, called the retina, is double, consisting of an 

 outer pigmented and an inner sensory part. This is perhaps the 

 most delicate layer in the entire body. Despite the fact that the 

 retina is less than ^^ of an inch in thickness, it is composed of several 

 layers of cells. The optic nerve, made up of a chain of relaying 

 neurons, enters the eye from behind and spreads out over the surface 

 of the retina. At its point of entry a cross section of the optic nerve 

 shows that the nerve consists only of axons of neurons, and conse- 

 quently this "blind spot" is not sensitive to light. The ultimate 

 photoreceptors are numerous elongated cells, called rods and cones. 

 The function of the rods is a highly specialized sensitivity to light, 

 and of the cones the perception of color. In the optical center of the 

 H. w. H. — 24 



