326 THE INTERNAL ENVIRONMENT OF THE BODY Part III 



eleventh century. It has not been copied from the eye but has been built to 

 obtain similar results and automatically resembles the eye. 



Path of Light through the Eye. In order to reach the retina light must 

 penetrate through: (1) the conjunctiva, the outermost covering of the front 

 of the eyeball; (2) the cornea, the transparent front part of the outer, tough 

 or sclerotic coat of the eyeball, actually a most important part of the eye 

 that brings rays of light to a focus (Figs. 17.14 and 17.16); (3) a transparent 

 fluid (aqueous humor) that fills the front chamber of the eye; (4) the 

 crystalline lens, important in accommodation (Fig. 17.14); and (5) the 

 transparent jelly (vitreous humor) that fills the back chamber and keeps 

 the eyeball expanded, and finally (6) the retina with its receptors, the rod and 



EVE MUSCLE 



WASHED 

 BT TEARS 



EVE LIDS 



OPTIC 

 NER\/Es 



EYE MUSCLE 



Fig. 17.15. The eyeball in its socket. It is set deep in a bony socket, packed 

 about by fat, curtained by eyelids, and washed by tears. Each eye is equipped with 

 six muscles by which the front of the ball is moved up or down, from side to side 

 and slightly rotated. (Reprinted from The Machinery of the Human Body by 

 Carlson and Johnson, by permission of The University of Chicago Press. Copy- 

 right 1948.) 



cone cells. These curiously enough are seemingly turned away from the light, 

 a condition that can be explained by their development (Figs. 17.14 and 

 17.16). The back wall of the retina and the choroid coat behind it are heavily 

 pigmented and so absorb excess light. Rays of light pass freely through the 

 pupil which is surrounded by the iris, a circular curtain which is part of the 

 vascular or choroid coat of the eyeball. 



A structure usually located in the choroid coat, called the tapetum lucidum 

 (L., bright carpet), acts as a fight-concentrating mirror and causes the night 

 eyeshine of many animals. Some tapeta, as in many hoofed animals, consist 

 of shimmering connective tissue fibers. In others, the cells are packed with 

 glimmering rodlets, as in the cat's eye, the brilliance of which encouraged the 

 Egyptians to reverence cats which could reflect the light of the sun even at 

 night. 



Accommodation for Near and Far Objects. The image made by means of 

 the cornea and the crystalline lens is a very small picture upside-down on 

 the retina (Fig. 17.14). In sharpening this picture the eye accommodates, 



