must have some property different from dead matter, which renders them capable of being acted upon by these external powers, so as to produce the living functions; for if they had not, the only effects which these powers could produce, would be mechanical, or chemical.  Though we know not exactly in what this property consists, or in what manner it is acted on, yet we see, that when bodies are possessed of it, they become capable of being acted upon by external powers, and thus the living functions are produced; we shall therefore call this property excitability, and in using this term it is necessary to mention, that I mean only to express a fact, without the least intention of pointing out the nature of that property which distinguishes living from dead matter, and in this we have the example of the great Newton, who called the property which causes bodies in certain situations to approach each other, gravitation, without in the least hinting at its nature; yet, though he knew not what gravitation was, he investigated the laws by which bodies were acted on by it, in the same manner, though we are ignorant of excitability, or the nature of that property which distinguishes living from dead matter, we can investigate the laws by which dead matter acts on living bodies through this medium.  We know not what magnetic attraction is, and yet we can investigate its laws; the same holds good with regard to electricity; if we ever should attain a knowledge of the nature of this property, it would make no alteration in the laws which we had before discovered.

I shall now proceed to the investigation of the laws by which the excitability is acted on; but I must first define some terms which it will be necessary to use, to avoid circumlocution, and at the same time to give us more distinct ideas on the subject.

When the excitability is in such a state as to be very susceptible of the action of external powers, I shall call it abundant, or accumulated; but when it is found not very capable of receiving their action, I shall say, it is deficient, or exhausted.  I would not wish however, to have it thought, that by these terms I mean in the least to hint at the nature of excitability, nor that it is really one while increased, and at another diminished in quantity, for the abstract question is in no shape considered; we know not whether the excitability, or the vital principle, depends on a particular arrangement of matter, or from whatever cause it may originate; by the terms here used, I mean only to say, that the excitability is easily acted on when I call it abundant, or accumulated; at other times the living body is with more difficulty excited, and then I say, the vital principle is deficient, or exhausted.

The laws by which external powers act on living bodies, will, on a careful examination, be found to be the following—­

First, when the powerful action of the exciting powers ceases for some time, the excitability accumulates, or becomes more capable of receiving their action, and is more powerfully affected by them.