THE NERVOUS SYSTEM. 173 
column. Irom these ganglia nerves are given off, which, 
instead of going to the skin and muscles, like the spinal 
nerves, form net-works about those internal organs over 
which the will has no control, as the heart, stomach, and 
intestines. Their apparent oftice is to stimulate these or- 
gans to constant activity. By some anatomists, the gan- 
glia are considered as reservoirs of nervous force. 
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. 
All animals possess a general sensibility diffused over 
the greater part of the body.” But, besides this (save in 
the very lowest forms), they are endowed with special 
nerves for receiving the impressions of light, sound, ete. 
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 can not distinguish 
sounds. In the higher animals, the organs of sight, hear- 
ing, 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, hear- 
ing, 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 only universal sense; no 
animal is without it. It is likewise the most positive and 
