150 > ON THE EXTERNAL SENSE ^' 



That insects in general hear is unquestionable, but it is highly question- 

 able by what organ they obtain the sense of hearing. The antennas, and 

 perhaps merely because we do not know their exact use, have been sup- 

 posed by many naturalists to furnish the means ; it appears fatal, however, 

 to this opinion to observe, that spiders hear though they have no true anten- 

 nas, and that other insects which possess them naturally seem to hear as 

 correctly after they are cut off. / 



The sense of vision exhibits perhaps more variety in the different classes 

 of animals than any of the external senses. In man, and the greater num- 

 ber of quadrupeds, it is guarded by an upper and lower eye-lid ; both of 

 which in man, but neither of which in most quadrupeds, are terminated by 

 the additional defence and ornament of cilia or eye-lashes. In the ele- 

 phant, opossum, seal, cat kind, and various other mammals, all birds, and 

 all fishes, we find a third eye-lid, or nictitating membrane, as it is usually | 

 called, arising from the internal angle of the eye, and capable of covering 

 the pupil with a thin transparent veil, either wholly or in part, and hence of 

 defending the eyes from danger in their search after food. In the dog this 

 membrane is narrow ; in oxen and horses it will extend over half the eye-* 

 ball ; in birds it will easily cover the whole ; and it is by means of this 

 veil, according to Cuvier, that the eagle is capable of looking directly 

 against the noon-day sun. In fishes it is almost always upon the stretch, as 

 in their uncertain element they are exposed to more dangers than any other 

 animal. Serpents have neither this nor any other eye-lid : nor any kind 

 of external defence whatever but the common integument of the skin. 



The largest eyes in proportion to the size of the animal belong to the 

 bird tribes, and nearly the smallest to the whale ; the smallest altogether 

 to the shrew and mole ; in the latter of which the eye is not larger than a 

 pin's head. 



The iris, with but few exceptions, partakes of tlie colour of the hair, 

 and is hence perpetually varying in different species of the same genus. 

 The pupil exhibits a very considerable, though not an equal variety in its 

 shape. In man it is circular ; in the lion, tiger, and indeed all the cat 

 kind, it is oblong ; transverse in the horse and in ruminating animals ; 

 and heart-shaped in the dolphin. 



In man, and the monkey tribes, the eyes are placed directly under the 

 forehead ; in other mammals, birds, and reptiles, more or less laterally ; 

 in some fishes, as the genus pleuronectes, including the turbot and flounder 

 tribes, both eyes are placed on the same side of the head; in the snail 

 they are situated on its horns, if the black points on the extremities of the 

 horns of this worm be real eyes, of vt^hich, however, there is some doubt : 

 in spiders the eyes are distributed over different parts of the body, and in 

 ilifferent arrangements, usually eight in number, and never less than six. 

 The eyes of the sepia have lately been detected by M. Cuvier ; their con- 

 struction is very beautiful, and nearly as complicated as that of vertebra- ^ 

 ted animals.^ Polypes and several other zoophytes appear sensible of the 

 presence of light, and yet have no eyes ; as the nostrils are not in every 

 animal necessary to the sense of smell, the toague to that of taste, or the 

 ears to that of sound. A distinct organ is not always requisite for a dis- 

 tinct sense. In man himself we have already seen this in regard to the 

 sense of touch, which exists both locally and generally ; the distinct organ 

 of touch is the tips of the tongue and of th© fingers, but the feehng is also 



*Ije Regce Animale di5tribi;,e d'apres snn Orgatjj?.afsor>; 4 fomes, 8ro. Paris I81T 



