if he be deprived of food, his death is equally certain, though more slow. It is sufficiently evident then, that the living functions are owing to the action of these external powers upon the body. What I have here said, is not confined to animals, but the living functions of vegetables are likewise caused by the action of dead matter upon them. The dead matters, which by their action produce these functions, are principally heat, moisture, light, and air. It clearly follows therefore, from what I have said, that living bodies 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.