OF HEARING. 31 



in proportion as the pieces are smaller and more nume- 

 rous. 



89. Compound eyes, being destitute of the optical appara- 

 tus necessary to collect the rays of light, cannot adapt 

 themselves to the distance of objects ; they see, but cannot 

 look. The perfection of their sight depends on the number 

 of fa^ettes 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. Those animals which are destitute of eyes are 

 either of a very inferior rank, such as most of the polypi, 

 or .else they comprise animals which live under unu- 

 sual circumstances, such as the intestinal worms. Even 

 among the vertebrates, there are some which lack the fac- 

 ulty of sight, as the Myxine glutinosa, which has merely a 

 rudimentary eye concealed under the skin, and destitute of a 

 crystalline. Others, which live in darkness, have not even 

 rudimentary eyes, as for example, the fishes which live in 

 the Mammoth Cave, (Amblyopsis spelceus), and which 

 appear to want even the orbital cavity. The craw-fishes, 

 (Astacus pellucidus}) of this same cave, are also blind ; 

 having merely the pedicle for the eyes, without any 

 traces of 



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 



