242 



ANIMAL PHYSIOLOGY. 



the anterior surface of the lens; and a third, small and clear, 

 and inverted, is from the hinder surface of the lens. When 

 the focus of the eye is changed from distant to near objects, 

 without altering the direction, the inverted image retains its 

 form and position, while that from the front of the lens 

 becomes smaller, and approaches the corneal image. From 

 this it is known that the posterior surface of the lens remains 

 unchanged, while the anterior surface is made more, con vex; 

 and by examination of the eye in profile, the lens has even 

 been seen to project through the pupilary aperture when 

 adjusted to short distances. The cause of this change of 

 shape is not thoroughly understood, but the principal agent 

 in effecting it would appear to be the ciliary muscle. 



Fig. 123. ACCOMMODATTON TO DISTANCES. F, Lens accommodated to 

 far objects ; N, to near objects ; a, anterior chamber ; 6, posterior 

 chamber ; c, canal of Petit ; d, ciliary muscle ; e, ciliary process. 



Another change which takes place in adjustment to short 

 distances is lessening of the pupil; and it has been suggested 

 that the contraction of the circular fibres of the iris presses 

 on the sides of the lens so as to alter its form. But this 

 supposition is disproved, not only by the fact that the faculty 

 of adjustment has been found unimpaired when the iris has 

 been wanting, but by the size of the pupil being diminished 

 still more by increase of light than by looking-at near objects. 

 The iris seems to act simply as a diaphragm, cutting off the 

 lateral rays; and this is specially required in a bright light, 

 to save the retina from undue stimulation, and in looking at 

 near objects, because the rays from them are so exceedingly 

 divergent. 



17S. When objects are looked at with both eyes, the muscles 

 which move the eyeballs are brought into requisition. These 



