Heat, and light, and other stimuli, may perhaps exhaust the excitability, by facilitating the combination of oxygen in the fibres with the hydrogen and carbon in the blood.
There are several substances which cause a diminution or exhaustion of the excitability, without producing any previous increased excitement. These substances have by physicians been called sedatives: and though the existence of such bodies is denied by Dr. Brown, yet we are constrained to admit them; nor do their effects seem incapable of being explained on the principle laid down, especially if we call in the aid of chemistry.
Any substance which is capable of combining rapidly with oxygen, and diminishing its quantity, will be a sedative. But the action of some of the animal and vegetable poisons is difficult to explain in the present state of our knowledge; such very minute portions of these produce great exhaustion of the excitability, and even death, that we can scarcely explain their action on the supposition that they combine with the oxygen. They may perhaps act as ferments, and occasion throughout the whole system a new and rapid combination of oxygen with the hydrogenous, carbonic, and perhaps azotic parts of the blood and fluids, and even of the solids, which will speedily destroy the excitability, and even the organization.
Many of the vegetable narcotics, though they will destroy life when given in considerable doses, yet when exhibited in less quantities become very powerful remedies, particularly in cases where the excitability is accumulated, in consequence of which violent spasms and inordinate actions take place, which are very quickly calmed by opium, camphor, musk, asafoetida, ether, &c. medicines that occasion a speedy exhaustion of the excitability. In diseases of exhaustion, however, these remedies are improper. The indication here is to accumulate the irritability, by the introduction of oxygen, and by the diminution of the action of the stimulants which support life. In this idea too I dissent from Dr. Brown, who taught that diseases of exhaustion are to be cured by stimulants, a little less powerful than those which produced the disease. This subject will however be more fully discussed hereafter.
This doctrine of animal life, which I have been attempting to illustrate, and render familiar, exhibits a new view of the manner in which it is constantly supported. It discovers to us the true means of promoting health and longevity, by proportioning the number and force of stimuli to the age, climate, situation, habits, and temperament, of the human body. It leads us to a knowledge of the causes of diseases: these we shall find consist either in an excessive or preternatural excitement in the whole or part of the human body, accompanied generally with irregular motions, and induced by natural or artificial stimuli, or in a diminished excitement or debility in the whole, or in part. It likewise teaches us that the natural and only efficacious cure of these diseases depends on the abstraction of stimuli, from the whole, or from a part of the body, when the excitement is in excess: and in the increase of their number and force when the contrary takes place.
The light which the discoveries of Galvani, and others who have followed his steps, begin to throw on physiology, promises, when aided by the principles of chemistry, and the knowledge of the laws of life, to produce all the advantages that would result from a perfect knowledge of the animal functions.
From what has been said, it does not seem improbable that muscular contraction may depend upon the combination of oxygen with hydrogen and azote, in consequence of a sort of explosion or discharge produced by nervous electricity. According to this hypothesis, animal motion, at least that of animals analogous to man, would be produced by a beautiful pneumatic structure. This hypothesis, though not perhaps at this moment capable of strict demonstration, seems extremely probable, it being countenanced by every observation and experiment yet made on the subject. It accounts likewise for the perpetual necessity of inhaling oxygen, and enables us to trace the changes which this substance undergoes, from the moment it is received into the system, till the moment it is expelled. By the lungs it is imparted to the blood; by the blood to the muscular fibres; in these, during their contraction, it combines with the hydrogen, and perhaps carbon and azote, to form water and various salts, which are taken up by the absorbents, and afterwards exhaled or excreted. We know the necessity of oxygen to muscular motion, and likewise that this motion languishes when there is a deficiency of the principle, as in sea scurvy. Thus a boundless region of discovery seems to be opening to our view: the science of philosophy, which began with remote objects, now promises to unfold to us the more difficult and more interesting knowledge of ourselves. Should this kind of knowledge ever become a part of general education, then the causes of many diseases being known, and the manner in which the external powers, with which we are surrounded, act upon us, a great improvement not only in health, but in morality must be the consequence.
With respect to its influence on the science of medicine, we may observe that, from the time of Hippocrates till almost the present day, medicine has not deserved the name of a science but, as he called it, of a conjectural art. At present however, by the application of the laws of life, and of the new chemistry, there is beginning to appear in physiology and pathology, something like the simplicity and certainty of truth. In proportion as the laws of animal nature come to be ascertained, the study of them will excite more general attention, and will ultimately prove the most popular, as well as the most curious and interesting branch of philosophy.
This must be productive of beneficial consequences to society, since these truths, once impressed upon the mind by conviction, will operate as moral motives, by which the sum of disease and human misery cannot fail to be greatly diminished.