166 8ENSORIAL FUNCTIONS. 



Fig. JOG. tractile fibres, Fig. 100. One of these 

 layers form concentric circles, the other 

 being disposed in the form of radii 

 reaching from the pupil to the circum- 

 ference. 



The delicate fibres of which these 

 layers are composed, perform the part of muscles in 

 varying the size of the pupil. When those forming 

 concentric circles act, the pupil is diminished ; when 

 those forming radii contract, the margin of the pupil is 

 brought nearer the circumference, and of course the 

 aperture is enlarged. 



No piece of mechanism can excel this in simplicity, 

 beauty, and perfection. By its contractions and dila- 

 tions the quantity of light admitted into the eye is regu- 

 lated. When its intensity would be injurious to the 

 sensibility of the retina, then the circular muscles act 

 and close the aperture, sometimes almost to a point ; 

 while, if the quantity of light flowing from objects be 

 too small as during the evening, or on going into a dark 

 room, the radiating fibres contract, and opening the 

 aperture, admit as large a quantity of light as possible. 

 We shall see hereafter also, that the contraction of the 

 pupil serves to exclude such rays as would otherwise 

 fall upon parts of the crystalline lens which are unfitted 

 to bring them to a focus, and in which case vision would 

 be confused and imperfect. 



PHYSIOLOGY OF VISION. 



Having thus described the mechanism of the eye, we 

 will next explain the optical principles involved in the 

 visual function. 



The passage of light, through the air, or through any 

 other medium of equal density, throughout, is always 

 in straight lines. But when a ray of light passes into, 

 or out of, a medium of a different density, it is bent, or 



What is the structure of the iris 7 "What are the different uses of the 

 t<*o coats ] When a ray of light passes obliquely into, or out of a 

 rarer, or a denser medium, how is the ray effected 1 In what direction is 

 the refraction when it passes from a rarer into a denser medium 7 



