464 THE SENSORIAL FUNCTIONS. 



tercept such rays as would have fallen on parts 

 of the crystalline lens less fitted to produce their 

 regular refraction, the object of which will be 

 better understood when we have examined the 

 functions of this latter part. But, before engag- 

 ing in this inquiry, it will be proper to complete 

 this sketch of the anatomy of the eye by 

 describing the principal parts of the apparatus 

 belonging to that organ, which are exterior to 

 the eye-ball, and may be considered as its ap- 

 pendages. 



The purposes answered by the parts exterior 

 to the eye-ball are chiefly those of motion, of 

 lubrication, and of protection. 



As it is the central part of the retina which is 

 endowed with the greatest share of sensibility, 

 it is necessary that the images of the objects to 

 be viewed should be made to fall on this part ; 

 and consequently that the eye should be capa- 

 ble of having its axis instantly directed to those 

 objects, wherever they maybe situated. Hence 

 muscles are provided within the orbits, for effect- 

 ing the motions of the 

 eye-ball. A view of these 

 muscles, with their attach- 

 ments to the ball of the 

 eye, but separated from 

 the other parts, is given 

 in Fig. 416. Four of these 

 proceed in a straight course from the bottom of 



