TO SPECIAL SENSES. 



a manner, that only those rays -which are parallel to the axes 

 can reach the retina (A) ; all those which enter obliquely are 

 lost ; so that of all the rays which proceed from the points a 

 and b, only the central ones in each pencil act upon the 

 optic nerve, d : the others strike against the walls of the 

 cones. To compensate for the disadvantage of such an ar- 

 rangement, and for the want of motion, the number of fa- 

 cettes is greatly multiplied, so that no less than 25,000 have 

 been counted in a single eye. The image on the retina, in this 

 case, may be compared to a mosaic, composed of a great num- 

 ber of small images, each of them representing a portion of 

 the figure. The entire picture is, of course, more perfect, in 

 proportion as the pieces are smaller and more numerous. 



143. Compound eyes are destitute of the optical appa- 

 ratus necessary to concentrate the rays of light, and cannot 

 adapt themselves to the distance of objects ; they see at a cer- 

 tain 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 direc- 

 tions, 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. 



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

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

 they are animals which live under unusual circumstances, 



V 



such as the intestinal worms. Even among the vertebrata, 

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

 ylutinosa, 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 ex- 

 ample, that curious fish (AmUyopsis spelceus), which lives in 

 the Mammoth cave, and which appears to want even the 

 orbital cavity. The crawfishes (Astacus pelhicidus) of this 

 same cavern are also blind ; having merely the pedicle for the 

 eyes, without any traces of facettes. 



2. OF HEARING. 



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

 ceiving sounds is seated in a peculiar apparatus, the EAE, which 



