Читать книгу Pharmacologia - John Ayrton Paris - Страница 62

Substances which directly diminish the force of the circulation, and reduce the heat of the body, without occasioning any diminution of sensibility or nervous energy.

These remedies may be considered either as external and local, or as internal and general. In the first case, there will not be much difficulty in substantiating their claims to be considered Chemical Agents, but in the latter case, the theory of their operation is unsatisfactory and obscure; and even the facts which are adduced to establish the existence of such a class of remedies, are of a very problematical character.

Topical Refrigerants. In the case of external inflammation, refrigeration may be produced by the application of cold substances, such as water, ice, or certain saline solutions, or by the abstraction of heat by means of evaporation, which is very effectually accomplished by the use of lotions composed of spirit or ether. By these methods we are capable of directly diminishing the activity of the vessels of the part; thus, in burns and scalds, the pain is instantly relieved, and the inflammation effectually reduced.

Internal Refrigerants. There are certain saline substances which, by undergoing a rapid solution, and acquiring an increased capacity for caloric, produce a diminution of temperature, and if this takes place in the stomach, the sensation of cold which it will produce is equivalent to a partial abstraction of stimulus; which, being extended by sympathy to the heart, occasions a transient reduction in the force of the circulation, and by this, or by a similar sympathetic affection, causes a sensation of cold over the whole body; in this manner Dr. Murray explains the refrigerant operation of nitre, which after all is of a very doubtful nature. We shall perhaps not feel much difficulty in accepting this theory, and in allowing that general refrigerant effects may be temporarily produced, by occasioning an impression of cold upon the stomach. The theory which is proposed to explain the refrigerant operation of vegetable acids and certain other substances, and which we have now to consider, is derived from those chemical views respecting animal heat, in which the consumption of oxygen in the act of respiration is considered the principal source. Dr. Murray,^[178] who has given a luminous exposition of this theory, says “it is established by numerous experiments and observations, that the quantity of oxygen consumed in the lungs is materially influenced by the nature of the ingesta received into the stomach. When the food and drink are composed of substances which contain a small proportion of oxygen, it is known that the consumption of oxygen in the lungs is increased, and this even in a short time after the aliment has been received; thus Mr. Spalding, the celebrated diver, observed, that whenever he used a diet of animal food, or drank spirituous liquors, he consumed in a much shorter time the oxygen of the atmospheric air in his diving-bell; and therefore he had learned from experience to confine himself to a vegetable diet, and to water for drink, when following his profession.”^[179] During digestion too, it was established by the experiments of Lavoisier and Seguin, that a larger proportion of oxygen than usual is consumed.

But it is known, that the animal temperature is derived from the consumption of oxygen gas by respiration; and, that an increase in that consumption will occasion a greater evolution of caloric in the system, and consequently an increase of temperature in the body, while a diminution in the consumption of oxygen will have an opposite effect. If, then, when the temperature of the body is morbidly increased, we introduce into the stomach substances containing a large proportion of oxygen, especially in a loose state of combination, we may succeed in reducing the general temperature. This we accomplish in part by a vegetable diet, but still more effectually by the free use of the Acids. The vegetable acids in particular, which are found by experience to be the best refrigerants, are readily acted upon by the digestive powers, and assimilated with the food; and as the large quantity of oxygen which they contain is already in a concrete state, little sensible heat can be produced by the combination of that element with the other principles of the food. The nutritious matter which is received into the blood, containing thus a larger proportion of oxygen than usual, will be disposed to abstract less of it from the air in the lungs, and consequently less caloric will be evolved; the temperature of the body will be reduced; and this, again operating as a reduction of stimulus, will lessen the number and force of the contractions of the heart.

Such is the philosophical web which chemical ingenuity has wove for us,—the device is beautiful, but the fabric will be found too frail to endure the touch. The experiments of Dr. Crawford, in proof of the chemical origin of animal heat, are highly ingenious and plausible, but it is now generally admitted that the temperature of animals depends upon the living principle^[180] which animates them, and that although the absorption of oxygen, in the act of respiration, may directly contribute something to its production, yet that its chief action is that of serving as a stimulus to the living power in generating it; for, as Sir Gilbert Blane^[181] remarks, oxygen plays an interesting and active part as an exciting power throughout all nature, both animate and inanimate. If the heat of the body depended on respiration alone, any one might, by a voluntary effort of quick, deep, and prolonged respiration, increase the temperature of his body at will; the effect also of the emotions of the mind, in generating both heat and cold, adds Sir Gilbert, is proof sufficient of temperature depending on a vital, and not on a chemical cause.