404 THE ORGANS OF SPECIAL SENSE 



and nerve impulses are developed which are conveyed 

 to the brain. 



When one becomes conscious of seeing an object, 

 it is due to the fact that an image has been formed 

 by the rays of light passing through the pupil and 

 coming in contact with complex structures which 

 are termed the refracting apparatus, which consists 

 of the cornea, aqueous humor, crystalline lens, and 

 vitreous humor. The rays of light must pass through 

 these structures, and as they pass from one to the 

 other they are changed in direction by their surfaces 

 and are narrowed to a single point of focus on the 

 retina. Thus if the rays of light coming from various 

 directions and distances were not focussed, the same as 

 an object on the lens of a camera, they would simply 

 be thrown on the retina and create the sensation of 

 diffused light rays, and form an indistinct image on 

 the retina. 



Accommodation. This is the term expressing the 

 power possessed by the eye of adjusting itself to 

 vision at different distances; or the power of focussing 

 rays of light on the retina, which come from different 

 distances at different times (Brubaker's Physiology). 

 In other words, the eye cannot see two different objects 

 at different distances, and both be' distinctly seen. 

 If the eye looks at the distant object the near object 

 is not clear, and then when the eye focusses on the near 

 object the distant one is indistinct. 



Accommodation is regulated mainly by the changes 

 in the shape of the lens produced by the action of the 

 ciliary muscle; and the pupil becoming decreased or 

 increased in circumference. 



The change in the shape of the lens is the means 

 by which the eye accommodates itself to vision. 

 How this mechanism of accommodation is produced 

 is not definitely settled. However, it is supposed to 

 be due to the anatomic relation between the ciliary 



