THE SENSES. 



593 



by light and to transmit by means of the optic nerve, of which it is the 

 terminal expansion, the impression of the stimulation to the brain, in 

 which it excites the sensation of vision; (2) An apparatus consisting of 

 certain refracting media, cornea, crystalline lens, aqueous and vitreous 



FIG. 401. Section through the macula lutea and fovea centralis of human retina, a, fovea; 6, 

 descent of the macula towards fovea. The numbers indicate the layers of the retina. (Kuhnt.) 



humor, the function of which is to collect together into one point, the 

 different divergent rays emitted by each point of every external body and 

 of giving them such directions that they are exactly focussed upon the 



FIG. 402. Meridional section through the lens of a rabbit. 1, Lens capsule; 2, epithelium of 

 lens; 3, transition of the epithelium into the fibres; 4, lens fibres. (Bubuchin.) 



retina, and thus produce an exact image of the object from which they 

 proceed. For as light radiates from a luminous body in all directions, 

 when the media offer no impediment to its transmission, aluminous point 



FIG. 403. Laminated structure of the crystalline lens. The laminae are split up after hardening 

 in alcohol. 1, the denser central part or nucleus; 2, the successive external layers. 4/1. (Arnold..) 



will necessarily illuminate all parts of a surface, such as the retina opposed 

 to it, and not merely one single point. A retina, therefore, without any 

 optical apparatus placed in front of it to separate the light of different 

 objects, would not allow of distinct vision, but would merely transmit such 

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