THE SENSE OF SIGHT. 141 



receive the image and transmit the luminous impressions through the optic 

 nerve to the brain. 4. A contractile structure, the ciliary muscle, which 

 can so manipulate the lens as to enable external objects to be seen at near 

 or far distances. 



The Refracting Apparatus, by which parallel rays of light are brought 

 to a focus on the retina, consists mainly of the crystalline lens, though 

 aided by the cornea and aqueous humor. A ray of light passing through 

 the pupil is refracted and concentrated by the lens at a given point pos- 

 terior to it. For the correct perception of images of external objects, the 

 rays of light must be accurately focused on the retina ; in order that this 

 may be accomplished, the lens must have a certain density, and a proper 

 curvature of its surfaces. When the lens is too convex, its refracting power 

 is greatly increased, the rays of light are brought to a focus in front of the 

 retina, and the visual perception becomes dim and confused. When it is too 

 flat, the rays are not focused at all, and the resulting perception is. the 

 same. 



The Crystalline lens, therefore, produces a distinct perception of the 

 outline and form of external objects. 



Action of the Iris. The iris, consisting of contracting and dilating 

 fibres, transmits and regulates the quantity of light passing through its 

 central aperture, the pupil, which is necessary for distinct vision. 



If the light be too intense or excessive, the circular fibres contract under 

 the stimulus of the 3d pair of nerves, and the aperture is diminished in 

 size ; if the quantity of light be insufficient, the dilating fibres contract 

 under the stimulus of the sympathetic, and the pupillary aperture is 

 increased in size. 



The Retina, which is formed partly by the expansion of the optic 

 nerve, and partly by new nervous structures, is the membrane which 

 receives the impressions of light. Its posterior surface, which is in con- 

 tact with the choroid, and 'especially the layer of rods and cones, is the 

 sensitive portion, in which the rays of light produce their effects. 



The point of most distinct vision is in the macula lutea, and especially 

 in its central depression, the fovea, which corresponds to the central axis 

 of the eye ; it is situated about ^ of an inch to the outside of the 

 entrance of the optic nerve. It is at this point that images of external 

 objects are seen most distinctly, while all around it the perceptions are 

 more or less obscure ; at the macula all the layers disappear except the 

 layer of rods and cones. 



Blind Spot. At the point of entrance of the optic nerve is a region 



