176 COMPARATIVE ZOOLOGY. 



and intestines. Their apparent office is to stimulate these 

 organs to constant activity, but is little understood. 



1. The Senses. 



Sensation is the consciousness of impressions on the 

 sensory nerves. These impressions produce some change 

 in the brain ; but what that change is, is a darkness on 

 which no hypothesis throws light. Obviously, we feel 

 o-nly the condition of our nervous system, not the objects 

 which excite that condition. 87 



All animals possess a general sensibility diffused over 

 the greater part of the body. 88 This sensibility, like as- 

 similation and contractility, is one of the primary physio- 

 logical properties of protoplasm. But, besides this (save 

 in the very lowest forms), they are endowed with special 

 nerves for receiving the impressions of light, sound, etc. 

 These nerves of sense, as they are called, although struct- 

 urally alike, transmit different sensations : thus, the Ear can- 

 not recognize light, and the Eye cannot distinguish sounds. 

 In the Vertebrates, the organs of sight, hearing, and smell 

 are situated in pairs on each side of the head ; that of 

 taste, in the mucous membrane covering the tongue; 

 while the sense of touch is diffused over the skin. 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, hearing, and 

 touch 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 an- 

 imal is without it, at least in the form of general sensibility. 

 It is likewise the most positive and certain of the senses. 

 In the Sea-anemone, Snail, and Insect, it is most acute in 

 the " feelers" (tentacles, horns, and antennae), 89 in the Oys- 



