CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 305 



exertion of effort, there are those special activities \ve know as 

 sight, smell, hearing, and taste, the SPECIAL SENSES, each absolutely 

 peculiar and incapable of merging the one into the other. Finally, 

 our consciousness shows us that accompanying these various states 

 of activity, there may be either one or other of two accompanying 

 conditions which we call respectively PLEASURE and PAIN. 



Certain phenomena which excite the activity of nervous tissue are 

 called STIMULI. 



These stimuli may be mechanical (as tickling, scratching, pinching, 

 cutting, &c.), or thermal (the application of heat or cold, producing 

 a feeling of thermal change) , or chemical (the application of various 

 irritants), or electrical (causing a variation in electrical currents),* 

 or finally, stimuli, natural to nervous tissue, and originating in end 

 organs. These natural stimuli may be of two kinds ; they may be 

 either (1) special (those which affect the organs of special sense, as 

 light, sound, &c.) ; or (2) psychical, i.e., the presence of certain 

 sensations, emotions, or cognitions. 



The presence of any of these stimuli must be of course without 

 effect, unless the nerves acted on by them be in a certain state of 

 excitability or impressionability or, as it is sometimes termed, 

 neuriliiy. The long- continued excitation of a nerve will blunt, and 

 ultimately will for a time destroy its power of action. A too pro- 

 longed repose also diminishes and ultimately destroys its impression- 

 ability, and may at last lead to its transformation into adipose 

 tissue a change which may also ensue if a nerve be separated from 

 its nervous centre while yet remaining within the body. 



We have seen that nervous tissue is of two kinds, fibres and cells, 

 and the activity of the tissue seems also to be of two kinds, namely, 

 conduction (or an activity which conducts influences along nerves), 

 and a more positive kind of activity, comparable with the explosion 

 of a mass of gunpowder, to which a train of the same material has 

 conducted a potent influence. It is the fibres which serve as the 

 agents of conduction, and the cells of the grey matter are commonly 

 supposed to produce the more positive kind of activity by some 

 special powers of receptivity and reaction which they possess. A 

 familiar example of this conduction and suddenly- active result is the 

 application of a heated substance to the skin, with the result of its 

 sudden withdrawal from such substance through the conduction of 

 some influence inwards from the skin to the source of the motor 

 energy of the muscles, which then produce such recoil. Nerve 

 action is altogether invisible. There is no, as yet observed, visible 

 indication of the active state of a nerve analogous to the shorten- 

 ing of a muscle which indicates myological activity. 



What is the NATURE of this NERVOUS ACTIVITY (apart from its 

 results of motion or feeling, or secretion), is a matter of pure 

 speculation. I. It has been compared with the action of electricity, 



* The resistance of nerves to electrical I versely as in the direction of their 

 conduction is five times as great trans- I length. 



