THE SENSE OF VISION. 311 



applied to muscular fibres of this sort may be misleading. The voluntary 

 character of the act of accommodation is not affected by the circumstance that 

 the will needs, as a rule, to be assisted by visual sensations. The fact that 

 most persons cannot affect the necessary change in the eye unless they direct 

 their attention to some near or far object is only an instance of the close rela- 

 tion between sensory impressions and motor impulses, which is further exem- 

 plified by such phenomena as the paralysis of the lip of a horse caused by 

 division of the fifth nerve. It is found, moreover, that by practice the 

 power of accommodating the eye without directing it to near and distant 

 objects can be acquired. The nerve-channels through which accommodation 

 is affected are the anterior part of the nucleus of the third pair of nerves 

 lying in the extreme hind part of the floor of the third ventricle, the most 

 anterior bundle of the nerve-root, the third nerve itself, the lenticular ganglion, 

 and the short ciliary nerves (see diagram p. 323). 



The mechanism of accommodation is affected in a remarkable way bv drugs, 

 the most important of which are atropia and physostigmin, the former para- 

 lyzing and the latter stimulating the ciliary muscle. As these drugs exert a 

 corresponding effect upon the iris, it will be convenient to discuss their action 

 in connection with the physiology of that organ. 



The changes occurring in the eye during the act of accommodation are 

 indicated in the following table, which shows, both for the actual and the 

 reduced eye, the extent to which the refracting media change their form and 

 position, and the consequent changes in the position of the foci : 



Accommodation for 

 Actual Eye. distant objects. near objects. 



Radius of cornea 8 mm. 8 mm. 



Radius of anterior surface of lens 10 6 " 



Radius of posterior surface of lens 6 " 5.5 " 



Distance from cornea to anterior surface of lens . . 3.6 " 3.2 " 



Distance from cornea to posterior surface of lens . 7.2 " 7.2 " 



Reduced Eye. 



Radius of curvature 5.02 " 4.48 " 



Distance from cornea to principal point 2.15 " 2.26 " 



Distance from cornea to nodal point 7.16 " 6.74 " 



Distance from cornea to anterior focus 12.918 " 11.241 " 



Distance from cornea to posterior focus 22.231 " 20. 248 " 



It will be noticed that no change occurs in the curvature of the cornea, and 

 next to none in the posterior surface of the lens, while the anterior surface of 

 the lens undergoes material alterations both in its shape and position. 



Associated with the accommodative movements above described, two other 

 changes take place in the eyes to adapt them for near vision. In the first 

 place, the axes of the eyes are converged upon the near object, so that the 

 images formed in the two eyes shall fall upon corresponding points of* the 

 retinas, as will be more fully explained in connection with the subject of 

 binocular vision. In the second place, the pupil becomes contracted, thus 

 reducing the size of the pencil of rays that enters the eye. The importance 

 of this movement of the pupil will be belter understood after the subject of 



