266 



HUMAN PHYSIOLOGY 



CHAP. 



pleura and pericardium. The capsule and cushion of 

 fat protect the eyeball and permit it to turn freely in all 

 directions with very little friction. 



The eyeball has three coats or layers. The outer is a 

 firm and strong coat, white in color, that gives the eye its 

 shape. The part at the back and sides does not permit 

 light to pass through. This portion is called the sclerotic 



coat. The part in 

 front is very trans- 

 parent and forms 

 a round window 

 known as the cor- 

 nea, through which 

 light enters the 

 eye. The second 

 or middle is a more 

 delicate though 

 thicker coat, deep 

 black in color, that 

 serves to absorb 

 all the extra light 

 which would other- 

 wise interfere with good clear vision. In this layer there 

 are many blood vessels and nerves. This coat, known 

 as the choroid, has the little self-regulating curtain just 

 back of the cornea, which by changing the size of the 

 small opening at its center, determines the amount of 

 light that shall enter the eye. This part of the choroid 

 coat is the iris, and the little window is the pupil. The 

 iris gives color to the eyes, as it may be black, brown, 



FIG. 141. A section through the eyeball 

 (Zuppke). 



