14 



CHAPTER XVII. 



OF VISION. OF THE ANATOMY OF THE EYEBALL, OPTIC NERVES, AND 



APPENDAGES. OF THE PHENOMENA OF VISION. 



IT would appear that an animal may be sensible to light without 

 possessing an organ of vision. Thus, that beautiful little polyp, 

 the hydra, shows a decided predilection for the light side of the 

 vessel in which it is kept. Most animals, too, require the presence 

 of light for the full performance of their functions; and this is not 

 the case with animals alone, but with plants likewise: both, in the 

 great majority of instances, pine away in the dark, or fail to arrive 

 at complete development. 



But the presence of an organ of vision implies something more 

 than the mere power of distinguishing between light and darkness. 

 It must enable the animal to discern something of the colour, or at 

 least of the form, of surrounding objects; and this in a degree pro- 

 portioned to the perfection and complexity of its organization. 



The principle on which the organ is constructed seems to be in 

 all cases the same, viz. that of the camera obscura a dark cham- 

 ber with a small aperture for the admission of light, a quantity of 

 black matter for the absorption of superabundant rays, and a nervous 

 expansion on that wall which receives the rays of light. 



Among the lower invertebrata, the eyes, or ocellij consist only of 

 a nervous point, shielded with a minute quantity of colouring matter. 

 The chief additions which increase the complexity of these organs in 

 the higher animals consist of transparent media and lenses for the 

 refraction of the light, and the production of a more precise image; 

 of an apparatus for the regulation of the quantity of light admitted 

 to the retina; and of other appendages for protection and move- 

 ment. 



The position of the human eye at the upper part of the face and 

 directed forwards, while it gives to the countenance its most impor- 

 tant element of beauty, adds greatly to the utility of the organ, by 

 increasing the visual range. For protection in this exposed situation, 

 it is sunk deeply in a cushion of fat, within a bony cavity, the 

 orbit, the prominent borders of which are well-adapted to receive 



