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 
only 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 and that of temperature are dif¬ 
fused over the skin, including the mucous membrane of 
the mouth, throat, and nose. Sight and hearing are stim¬ 
ulated, each by one agent only; while touch, taste, and 
smell may be excited by various substances. The agents 
awakening sight, hearing, touch, and the sense of tempera¬ 
ture are physical; those causing taste and smell are chem¬ 
ical. 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 
