A Lecture on the Preservation of Health | Page 5

thing distinctly for some time afterwards.
Let us next consider the matter of heat: suppose water to be heated lukewarm, if you put one hand into it, it will feel warm; if you now put the other hand into water, heated for instance to 120 degrees or 130 degrees, and keep it there some time, we will say, two minutes; if then you take it out, and put it into the lukewarm water, that water will feel cold, though still it will seem warm to the other hand; for, the hand which had been in the heated water, has had its excitability exhausted by the application of heat. Before you go into a warm bath, the temperature of the air may seem warm and agreeable to you, but after you have remained for some time in a bath that is rather hot, when you come out, you feel the air uncommonly cool and chilling.
Let us now examine the effects of substances taken into the stomach; and as the effects of spirituous, and vinous liquors, are a little more remarkable than food, we shall make our observations upon them.
A person who is unaccustomed to drink these liquors, will be intoxicated by a quantity that will produce no effect upon one who has been for some time accustomed to take them; and when a person has used himself to these stimulants for some time, the ordinary powers which in common support life, will not have their proper effects upon him, because his excitability has been in some measure exhausted by the stimulants.
The same holds good with respect to tobacco and opium; a person accustomed to take opium will not be affected by a quantity that would completely intoxicate one not used to it; because the excitability has been so far exhausted by the use of that drug, that it cannot be acted on by a small quantity.
These facts, with innumerable others, which will easily suggest themselves to you, prove the truth of our second proposition, namely, that when the exciting powers have acted violently, or for a considerable time, the excitability is exhausted, or less fit to be acted on.
This exhaustion of the excitability, may, however, be either finite, or temporary; we see animals, while the exciting powers continue to act, at first appear in their greatest vigour, then gradually decay, and at last come into that state, in which, from the long continued action of the exciting powers, the excitability is entirely exhausted, and death takes place.
We likewise see plants in the spring, while the exciting powers have acted on them, moderately, and for a short time, arrayed in their verdant robes, and adorned with flowers of "many mingling hues;" but, as the exciting powers which support the life of the plant, continue to be applied, and some of them, for instance heat, as the summer advances become increased, they first lose their verdure, then grow brown, and at the end of summer cease to live; because their excitability is exhausted by the long continued action of the exciting powers; and this does not happen merely in consequence of the heat of summer decreasing, for they grow brown and die, even in a greater degree of heat than that which in spring made them grow luxuriantly.
These are examples of the finite, or irreparable exhaustion of the excitability, but we find also, that it may be exhausted for a time, and accumulated again. Though the eye has been so dazzled by the splendour of light, that it cannot see an object moderately illuminated, yet, if it be shut for some time, the excitability of the optic nerve accumulates again, and we are again capable of seeing with an ordinary light.
We find, that we are not always equally capable of performing the functions of life. When we have been engaged in any exertion, either mental or corporeal, for some hours only, we find ourselves fatigued, and unfit to pursue our labours much longer; if in this state, several of the exciting powers, particularly light and noise, be withdrawn; and if we are laid in a posture which does not require much muscular exertion, we soon fall into that state which nature intended for the accumulation of the excitability, and which we call Sleep. In this state, many of the exciting powers cannot act upon us, unless applied with some violence, for we are insensible to their moderate action. A moderate light, or a moderate noise, does not affect us, and the power of thinking, which exhausts the excitability very much, is in a great measure suspended. When the action of these powers has been suspended for six or eight hours, the excitability is again capable of being acted on, and we rise fresh, and vigorous, and fit to engage in our occupations.
Sleep then, is