SENSE ORGANS AND NERVOUS COORDINATION 



579 



canal of Schlemm leads to increased intraocular pressure and the disease 

 glaucoma, in which the pressure flattens and eventually injures the retina. 



The eyeball lies in the orbit of the skull, and six extrinsic ocular 

 muscles, which move the entire eyeball, extend from it to the walls 

 of the orbit. A pair of movable eyelids cover the eyeball and the 

 cornea is kept moist, cleansed and possibly nourished, by the secre- 

 tion of tears from several tear glands. Tears are drained from the 

 median corner of the eye by a lacrimal duct which leads into the 

 nasal cavity. Pigs, cats and many other mammals have a third lid, 

 known as the nictitating membrane, located in the median corner of 

 the eye. It is moved passively over the cornea when the eyeball is 

 retracted slightly, and aids in cleaning and protecting the eye. This 

 membrane is reduced to a vestigial semilunar fold in man. 



Vision. Light that enters the eye is bent toward the optic axis 

 in such a way that it forms a sharp, though inverted image upon the 

 retina (Fig. 29.4 A). The lens is important in bending the light rays 

 but the cornea, humors and the retina itself are also involved. The 

 cornea is the major refractive agent in terrestrial vertebrates, for 

 the difference between the refractive index of air and the cornea is 

 greater than that between any of the other refractive media. The 

 action of the cornea places the image approximately on the retina; 

 the lens brings it into sharp focus. 



When the eye is at rest, distant objects are in focus. The re- 

 fractive power of the eye must be increased in viewing a near ob- 

 ject, or its image would be blurred, for the image would come into 

 sharp focus theoretically at a point behind the retina. Accommoda- 

 tion for near vision is accomplished by the contraction of muscles 



Lioht rays 



Object — 



Optic a:xis 



A 



Convex lens 



Concave lens 

 4^ 



D E 



Figure 29.4. Image formation by the eye. A, Normal eye; B, far-sighted eye; 

 C, far-sighted eye corrected by a convex lens; D, near-sighted eye; E, near-sighted eye 

 corrected by a concave lens. 



