162 SKNSORIAL FUNCTIONS. 



tractible fibres, Fig. 100. One of these lay- Fi S- 10 - 

 ers form concentric circles, the other being 

 disposed in the form of radii reaching from 

 the pupil to the circumference. 



487. The delicate fibres of which these 

 layers are composed, perform the part of mus- 

 cles 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. 



488. No piece of mechanism can excel this in simpli- 

 city, beauty, and perfection. By its contractions and dila- 

 tions, the quantity of light admitted into the eye is reg- 

 ulated. When its intensity wonld 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 



489. Having thus described the mechanism of the eye, 

 we will next explain the optical principles involved in the 

 visual function. 



490. The passage of light through the air, or through 

 any other medium of equal density through at, is always 

 in straight lines. But when a ray of lig 1 passes into, 

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



What is the structure of the iris ? What are the different uses of the 

 two coats ? When a ray of light passes obliquely into, or out of a rarer, 

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

 refraction when it passes from a rarer into a denser medium ? 



