PROPERTIES OF NERVOUS SUBSTANCE. 219 



cal stimuli also excite the nerves: whether these are ideational, emo- 

 tional, or volitional, they proceed from the brain, being themselves 

 sometimes induced by external causes, and sometimes originating pri- 

 marily in the great nervous centres, from the operations of the instinct, 

 the memory, the reason, or the will. 



When a stimulus of any kind, whether mechanical, chemical, elec- 

 trical, or vital, acts upon the living nervous substance, be it composed 

 of nerve-fibres, or of nerve-cells, it produces an impression on that 

 nerve-substance, and excites within it some particular change ; and 

 the property, by virtue of which this takes place in the nerve-substance, 

 whether composed of fibre or cell, has been called its excitability or 

 neurility. But the nerve-substance, whether vesicular or fibrous, not 

 only receives such an impression from a stimulus, and is excited to 

 such a change, but it possesses the property of conducting that im- 

 pression, or the change produced by it, in certain definite directions ; 

 and this property might be spoken of as conductility. When such an 

 impression, or excited change, is thus conducted, or propagated, 

 simply along a nerve-fibre, or through .a nerve-cell on to a nerve-fibre, 

 and thence to a muscle, it induces or excites, as we have seen, the 

 contraction of that muscle, and so exercises what is called a motor 

 function ; but when such impression, or change, is excited in, or prop- 

 agated along, a nerve-fibre simply, or through nerve-cells also, up to 

 the common sensorium of the body, it then exercises a sensory func- 

 tion, and ends in the production of a sensation. The anatomical seat 

 of such sensation, so far as we are at present able to trace it, is exclu- 

 sively in the nerve-cells, which therefore may be said to possess a 

 peculiar kind of receptivity. Hence, though both the nerve-fibre and 

 the nerve-cell are excitable, and may be said to possess excitability, 

 and though both can also conduct or propagate onwards, changes ex- 

 cited in them by stimuli, and therefore possess conductility, yet only 

 the nerve-cells, so far as we know, possess receptivity, or true sensi- 

 bility, or, as already said, can become the anatomical seats of sensation. 



It must therefore be understood, that the term excitability, employed 

 in a general sense, includes simple excitability, conductility, and sen- 

 sibility properly so called. Furthermore, the nerve-fibre is wholly 

 incapable of being acted upon directly, by mental stimuli, whether 

 these be ideational, emotional, or volitional ; for the reaction of these 

 mental states upon the nervous system, takes place exclusively upon, 

 or within, the gray matter of the nervous centres, and therefore, it is 

 fair to presume, upon or in the nerve-cells, of which that gray matter 

 is principally composed. Hence, these nerve-cells appear to possess 

 beyond the simple excitability to general stimuli, conductility, and the 

 peculiar receptivity, which is essential to sensation, a special or more 

 exalted kind of excitability, which is called into play under mental or 

 psychical stimuli, by the changes produced in the gray matter, in the 

 formation of ideas, emotions, and will. 



The excitability of a nerve remains for a time, after its separation, 

 by cutting or bruising, from its nervous centre ; but its conductility is 

 of course immediately destroyed. The excitability of a divided motor 

 nerve, is at first even slightly exalted ; but it then slowly diminishes, 



