S28 THE SENSORIAL FUNCTIONS. 



former act, the pupil is contracted; 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 deli- 

 cate 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 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 appendages. 



The purposes answered by the parts exterior to the eye- 

 ball are chiefly those of motion, of lubrication, and of pro- 

 tection. 



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 ca- 

 pable of having its axis instantly directed to those objects, 

 wherever they may be situated. Hence, muscles are pro- 

 vided within the orbits, for effecting the motions of the eye- 

 ball. A view of these muscles, with their attachments 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 the orbit, arising 

 from the margin of the aperture 

 through which the optic nerve 

 passes, and being inserted by a 

 broad tendinous expansion into the 



