THE greatest blessing we enjoy is health, without it, wealth,
honors, and every other consideration, would be insipid, and even
irksome; the preservation of this state therefore, naturally
concerns us all. In this lecture, I shall not attempt to teach you
to become your own physicians, for when the barriers of health are
once broken down, and disease has established itself, it requires
the deepest attention, and an accurate acquaintance with the
extensive science of medicine, to combat it; to attain this
knowledge demands the labour of years. But, a majority of the
diseases to which we are subject, are the effects of our own
ignorance or imprudence, and it is often very easy to prevent them;
mere precepts however, have seldom much effect, unless the reasoning
upon them be rendered evident; on this account, I shall first
endeavour, in as plain and easy a manner as possible, to explain to
you the laws by which life is governed; and when we see in what
health consists, we shall be better enabled to take such methods as
may preserve it. Health is the easy and pleasant exercise of all the
functions of the body and mind; and disease consists in the uneasy
and disproportioned exercise of all, or some of the functions.
When dead matter acts upon dead matter, the only effects we perceive
are mechanical, or chemical; for though there may appear to be other
kinds of attraction, or repulsion, such as electric and magnetic,
yet these come under the head of mechanical attraction, as producing
motion; we may therefore lay it down as a law, that when dead, or
inanimate bodies act upon each other, no other than mechanical, or
chemical effects are produced; that is, either motion, or the
decomposition, and new combination of their parts. If one ball
strike another, it communicates to it a certain quantity of motion,
this is called mechanical action; and if a quantity of salt, or
sugar, be put into water, the particles of the salt or sugar will
separate from each other, and join themselves to the particles of
the water; the salt and water in these instances, are said to act on
each other chemically; and in all cases whatever, in which
inanimate, or dead bodies act on each other, the effects produced
are, motion, or chemical attraction.
But, when dead matter acts on those bodies which we call living, the
effects are much different; let us take for example a very simple
instance.--Snakes, at least some species of them, pass the winter in
a torpid state, which has all the appearance of death; now heat, if
applied to dead matter, will only produce motion, or chemical
combination; but if it be applied to the snake, let us see what will
be the consequence; the reptile first begins to move, and opens its
eyes and mouth; when the heat has been applied for some time, it
crawls about in search of food, and performs all the functions of
life. Here then, dead matter, when applied to a living body,
produces living functions; for if the heat had not been applied, the
snake would have continued senseless, and apparently lifeless. In
more perfect animals, the effects produced by the action of dead
matter on them, are more numerous, and are different in different
living systems, but are in general the following--sense and motion
in almost all animals, and in many the power of thinking, and other
affections of the mind. The powers, or dead matters, which are
applied, and which produce these functions, are chiefly, heat, food,
and air. The proof that these powers do produce the living
functions, is in my opinion a very convincing one, namely, that when
their actions are suspended, the living functions cease; take away,
for instance, heat, air, and food from animals, and they soon become
dead matter, and it is not necessary that an animal should be
deprived of all these to put a stop to the living functions; if any
one of them be taken away, the body sooner or later becomes dead
matter: it is found by experience, that if a man be deprived of air,
he dies in about three or four minutes; for instance, if he be
immersed under water; if he be deprived of heat, or in other words,
exposed to a very severe degree of cold, he likewise soon dies; or
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