A Lecture on the Preservation of Health

by Thomas Garnett

If we examine separately the different exciting powers, which act on the body, we shall find abundant confirmation of this law. Let us first consider Light; if a person be kept in darkness for some time, and be then brought into a room in which there is only an ordinary degree of light, it will be almost too oppressive for him, and appear excessively bright; and if he have been kept for a considerable time in a very dark place, the sensation will be very painful. In this case, while the retina, or optic nerve, was deprived of light, its excitability accumulated, or became more easily affected by light; for if a person goes out of one room, into another which has an equal degree of light, he will feel no effect. You may convince yourselves of this law by a very simple experiment—shut your eyes, and cover them for a minute or two with your hand, and endeavour not to think of the light, or of what you are doing; then open them, and the day-light will for a short time appear brighter. If you look attentively at a window, for about two minutes, and then cast your eyes upon a sheet of white paper, the shape of the window-frames will be perfectly visible upon the paper; those parts which express the wood-work, appearing brighter than the other parts. The parts of the optic nerve on which the image of the frame falls, are covered by the wood-work from the action of the light; the excitability of these portions of the nerve will therefore accumulate, and the parts of the paper which fall upon them, must of course appear brighter. If a person be brought out of a dark room where he has been confined, into a field covered with snow, when the sun shines, it has been known to affect him so much, as to deprive him of sight altogether.

Let us next consider what happens with respect to heat; if heat be for some time abstracted, the excitability accumulates; or in other words, if the body be for some time exposed to cold, it is more liable to be affected by heat, afterwards applied; of this also you may be convinced by an easy experiment—put one of your hands into cold water, and then put both into water which is considerably warm; the hand which has been in cold water, will feel much warmer than the other. If you handle some snow with one hand, while you keep the other in your bosom, that it may be of the same heat as the body, and then bring both within the same distance of the fire, the heat will affect the cold hand infinitely more than the warm one. This is a circumstance of the utmost importance, and ought always to be carefully attended to. When a person has been exposed to a severe degree of cold for some time, he ought to be cautious how he comes near a fire, for his excitability will be so much accumulated, that the heat will act violently; often producing a great degree of inflammation, and even sometimes mortification. We may by the way observe, that this is a very common cause of chilblains, and other inflammations. When the hands, or any other parts of the body have been exposed to violent cold, they ought first to be put into cold water, or even rubbed with the snow, and exposed to warmth in the gentlest manner possible.

Exactly the same takes place with respect to food, if a person have for some time been deprived of food, or have taken it in small quantity, whether it be meat or drink; or if he have taken it of a less stimulating quality, he will find, that when he returns to his ordinary mode of living, it will have more effect upon him than before he lived abstemiously.

Persons who have been shut up in a coal-work from the falling in of the pit, and have had nothing to eat for two or three days, have been as much intoxicated by a bason of broth, as a person in common circumstances with two or three bottles of wine; and we all know that spirituous, or vinous liquors affect the head more in the morning, than after dinner.

This circumstance was particularly evident among the poor sailors who were in the boat with Captain Bligh after the mutiny. The captain was sent by government to convey some plants of the bread-fruit tree from Otaheite, to the West-Indies; soon after he left Otaheite, the crew mutinied, and put the captain and most of the officers, with some of the men, on board the ship's boat, with a very short allowance of provisions, and particularly of liquors, for they had only six quarts of rum, and six bottles of wine, for nineteen people, who were driven by storms about the south-sea, exposed to wet and cold all the time, for nearly a month; each man was allowed only a tea-spoon full of rum a-day, but this tea-spoon full refreshed the poor men, benumbed as they were with cold, and faint with hunger, more than twenty times the quantity would have done those who were warm, and well fed; and had it not been for the spirit having such power to act upon men, in their condition, they never could have outlived the hardships they experienced. All these facts, and many others which might be brought, establish beyond a doubt the truth of the law I have mentioned, namely, that when the powerful action of the exciting powers ceases for some time, the excitability accumulates, or becomes more capable of receiving their actions.

The second law is, that when the exciting powers have acted with violence, or for a considerable time, the excitability becomes exhausted, or less fit to be acted on, and this we shall be able to prove by a similar induction. Let us take the effects of light upon the eye; when it has acted violently for some time upon the optic nerve, it diminishes the excitability of that nerve, and renders it incapable of being affected by a quantity of light that would at other times affect it. When you have been walking out in the snow, if you come into your room, you will scarcely be able to see any thing for some minutes. Look stedfastly at a candle for a minute or two, and you will with difficulty discern the letters of a book, which you were before reading distinctly; and if you happen to cast your eyes upon the sun, you will not see any 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.