ORGANS OF SPECIAL SENSE THE EYE 



343 



'USC/P 



Muscle 



Olanct 



constantly used, not alone, but to adjust the movements of 

 the rectus muscles more accurately. 



The extreme delicacy with which these muscles work is 

 not usually appreciated. 

 Especially remarkable is 

 the accuracy with which 

 they move when one is 

 reading a printed page. 

 The eye is perfectly di- 

 rected to a certain, letter, 

 to comma or period, and 

 then as suddenly turned, 

 it may be only a hair's 

 breadth, and the process 

 repeated thousands of 

 times in an hour, yet 

 always with the most re- 

 markable precision. 



STRUCTURE 

 OF THE EYE PROPER 



The Sclerotic and Cho- 

 roid Coats. On the ex- FlG - 166. A SECTION THROUGH THE 

 terior of the front of the UPPER EYELID 



. . Highly magnified. (Fuchs) 



eye is an extremely thin 



layer of tissue called the conjunctiva. This is continuous 

 with the lining of the eyelids, and its transparency 

 makes it imperceptible. Beneath the conjunctiva is a 

 thicker layer which extends over the whole eyeball, 

 serving to protect it and keep it in shape. In front, 

 this also is perfectly transparent; but beginning with the 

 "white" and passing about the rest of the sphere, it is tough 

 and opaque. The transparent portion in front is called the 

 cornea, the rest the sclerotic coat; Fig. 167, 



