OF HEARING. 55 



in proportion as the pieces are smaller and more nu- 

 merous. 



89. Compound eyes are destitute of the optical apparatus 

 necessary to concentrate the rays of light, and cannot adapt 

 themselves to the distance of objects ; they see at a certain 

 distance, but cannot look at pleasure. The perfection of 

 their sight depends on the number of facettes or cones, 

 and the manner in which they are placed. Their field of 

 vision is wide, when the eye is prominent ; it is very limited, 

 on the contrary, when the eye is flat. Thus the dragon-flies, 

 on account of the great prominency of their eyes, see equally 

 well in all directions, before, behind, or laterally ; whilst 

 the water-bugs, which have the eyes nearly on a level with 

 the head, can see to only a very short distance before them. 



90. If there be animals destitute of eyes, they are either 

 of a very inferior rank, such as most of the polypi, or else 

 they are animals which live under unusual circumstan- 

 ces, such as the intestinal worms. Even among the ver- 

 tebrates, there are some that lack the faculty of sight, as 

 the Myxine glutinosa^ which has merely a rudimentary eye 

 concealed under the skin, and destitute of a crystalline lens. 

 Others, which live in darkness, have not even rudimentary 

 eyes, as, for example, that curious fish (Aniblyopsis spelceiis,} 

 which lives in the Mammoth Cave, and which appears to 

 want even the orbital cavity. The craw-fishes, (Astacus 

 pettucidus,) of this same cave, are also blind ; having 

 merely the pedicle for the eyes, without any traces of 

 fa9ettes. 



2. Hearing. 



91. To hear, is to perceive sounds. The faculty of per- 

 ceiving sounds is seated in a peculiar apparatus, the EAR, 

 which is constructed with a view to collect and augment the 

 sonorous vibrations of the atmosphere, and convey them to 



