414 THE SENSORIAL FUNCTIONS. 



margin.* When the former act, the pupil is con- 

 tracted ; when the latter act, the breadth of the iris 

 is diminished, and the pupil is, of course, dilated. 

 By varying the size of the pupil, the quantity of light 

 admitted into the interior of the eye is regulated, 

 and accommodated to the sensibility of the retina. 

 When the intensity of the light would be injurious 

 to that highly delicate organ, the pupil is instantly 

 contracted, so as to exclude the greater portion ; 

 and, on the contrary, when the light is too feeble, it 

 is dilated, in order to admit as large a quantity as 

 possible. The iris also serves to intercept 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 engaging in this inquiry, it will be 

 proper to complete the sketch of the anatomy of 

 the eye by describing the principal parts of the 

 apparatus belonging to that organ, which are exte- 

 rior to the eye-ball, and may be considered as its 

 appendages. 



The purposes answered by the parts exterior to 

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

 tion, 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 capable of 

 having its axis instantly directed to those objects, 

 wherever they may be situated. Hence muscles are 



* See Fig. 47, vol. i, p. 125. 



