THE NERVOUS SYSTEM 



387 



Sight and hearing are stimulated, each by one agent 

 only ; while touch, taste, and smell may be excited by 

 various substances. The agents awakening sight, hear- 

 ing, touch, and the sense of temperature are physical ; 

 those causing taste and smell are chemical. Animals 

 differ widely in the numbers and keenness of their 

 senses. But there is no sense in any one which does 

 not exist in some other. 



Touch is the simplest and the most general sense ; no 

 animal is without it, at least in the form of general sen- 

 sibility. It is likewise the 

 most positive and certain 

 of the senses. In the sea 

 anemone, snail, and in- 

 sect, it is most acute in 

 the " feelers" (tentacles, 

 horns, and antennae) ; M3 

 in the oyster, the edge 

 of the mantle is most 

 sensitive ; in fishes, the 

 lips ; in snakes, the tongue ; 

 in birds, the beak and 

 under side of the toes ; in quadrupeds, the lips and 

 tongue ; and in monkeys and man, the lips and the tips 

 of the tongue and fingers. In the most sensitive parts 

 of birds and mammals, the true skin is raised up into 

 multitudes of minute elevations, 

 called papilla, containing loops of 

 capillaries and nerve filaments. At 

 the ends of the latter are the es- 

 sential organs of touch, the tactile 

 corpuscles and the touch cells. 

 There is a correspondence between 

 the * delicacy of touch and the development of intelli- 

 gence. The cat and dog are more sagacious than 



FIG. 344 Antennae of various Insects 

 (magnified). 



FIG. 345. Papillae of Human 

 Palm, x 35, the cuticle being 

 removed. 



