PART III.

THERMIC AGENTS.

THERMIC agents may be defined as those which are measured by degrees of temperature. When they are brought in contact with other bodies a tendency toward the establishment of a temperatureequilibrium is developed, and the laws which govern this process are the same for animate and inanimate bodies. Animate objects, however, differ in these important respects that, on the one hand, they maintain life only within certain limits of temperature, and, on the other hand, that they possess the power of combating the causes which tend to disturb their normal conditions of body-heat.

All organized structures are destroyed by extremes of temperature; but when properly employed both heat and cold become valuable therapeutic agents.

At present only those comparatively moderate thermic variations. which are brought into play in the practice of balneotherapy and hydrotherapy will be considered.

The human body, on account of its constant temperature and unprotected skin, is very sensitive to thermic agents, even if the temperature approaches that of the part to which they are applied; and for this reason general applications produce very marked effects, which are of a stimulating character. The law which regulates this stimulation is that the stimulation or physiological reaction is in direct proportion to the strength of the stimulus or the abruptness of its application. Thus, if we expose a motor nerve and heat or cool it gradually, no reaction-i. e., muscular contraction-is produced, even if the application be continued to complete disorganization of the nerve. But if, on the other hand, a breath of cold or hot air is suddenly directed against a motor nerve, it will be stimulated and a muscular contraction will follow.

The procedures of balneotherapy and hydrotherapy belong to the class of external stimuli which excite local and remote reflex effects

through the mediation of the nervous system. The changes produced are principally vasomotor in character, and manifest themselves locally and at a distance by alterations in the distribution of the blood and in the local circulation, in blood-pressure, in the phenomena of endosmosis and exosmosis, and in the secretory functions, chiefly of the skin and kidneys. An influence may also be exerted upon the respiratory centres, upon the pneumogastric nerves, and upon the body-temperature; whereas, if purely mechanical stimuli are employed, the temperature-variations are nearly always localized and depend exclusively upon modifications in the distribution of the blood.

These powers enable thermic stimuli to exercise a very strong influence upon the nerve-centres, which respond with various phenomena; some of which are readily investigated in the laboratory, while others elude the scientist and are recognizable only by the clinician at the bedside. The clinical results obtained by the use of thermic agents are of great practical importance, and they consist in an exaltation or depression of nervous energy, so that we speak of the effects as stimulant or tonic, or as depressant or sedative.

Thermic agents employed in the treatment of disease also possess the peculiarity that, while stimulating the cutaneous nerves, they at the same time convey to or abstract from the economy a certain amount of heat, a feature which separates them sharply from all other modes of peripheral excitation.

Since thermic agents are represented chiefly by air and water, it is important to remember that the former has a very much smaller absorbing power and capacity for heat than the latter. For this reason we are better able to endure variations in the temperature of the air than of water.

The naked body can remain indefinitely in an atmosphere of 90° F., while a bath in the same temperature produces a sense of cold or even a chill. In order that the conditions be equivalent the bath must have a temperature of 104° to 112° F. Conversely, a bath at 115° F. is hotter and of greater influence than an atmosphere of the same temperature; and it is upon these facts that we base our judgment as to the temperature of thermic applications in disease.

Passing on from these theoretical conclusions to their practical application, we find that as the mean peripheral temperature of man is 98.5° F., the temperature of a temperate bath should be 100° F.

Baths having a temperature of less than 90° F. are considered cold; the ordinary cold bath has a temperature of 75° F. (Liebermeister). Hot baths have usually a temperature of from 112° to 120° F., the last being rarely exceeded. Above 120° F. a bath becomes both insupportable and dangerous.

In respect to the temperature of douches, it is important to distinguish between the projection of water in drops and the effect of immersion proper, since, among other differences, the former affects the body-temperature far less than the latter. The classification varies with different authors, but we can well adopt that of Fleury, slightly modified by Beni-Barde, who states that the very cold douche should be 52° to 60°; the cold, 60° to 70°; the cool, 70° to 80°; the temperate, 80° to 90°; the hot, 90° to 100°; the very hot, 100° to 120°.

Here, also, as in the case of the bath, a temperature of 125° is seldom surpassed. It is evident, however, that the water may be colder for the douche than for the bath. In the douche the mass of water is far smaller than in the bath, and the effects of percussion facilitate the reaction of the organism. The duration, moreover, is always brief, and very brief if the water used is very cold. For limited applications the temperature of the water may be very low; and even ice-water may be used.

Hot-air baths are divided into those in which the air is moist and has a temperature varying from 115° to 145°, and those in which the air is dry and the temperature 145° or less, though it may rise to 200° or even higher.

A very important part of the application of thermic agents in disease is the condition of the individual, which is necessarily quite variable and must be carefully noted. Even in health we note appreciable differences in reaction to thermic stimuli, whether these be hot or cold. The differences depend, in the main, upon the state of the circulation, the activity of the sweat-secretion, and the excitability of the nervous system. Sometimes they are the consequences of habits formed in childhood. In disease the variations are more marked and the effects produced are intensified.

By combining different forms of application, as the alternate use of hot and cold water, the addition of mechanical irritation, the degree of stimulation and the intensity of the reaction may be varied within wide limits. The employment of thermic agents in order to secure the best results is an art which, though in a measure depend

ing upon a knowledge of certain scientific facts, can, in the end, be acquired only by practical experience.

In our description of the practical employment of thermic agents, we shall begin by considering the effects of cold water. For convenience the subject may be divided into two parts-a general description of the effects of cold applications and a description of the special procedures.

The sensations which we perceive at the points touched are specific and distinct from other modes of cutaneous sensation. Physiologists have discovered that there are points on the surface where heat and cold, but not pain, is felt. These are the thermic points of Blix and of Goldscheider. The nerve apparatus of the thermic sense is double; i.e., there are nerve-endings sensitive only to cold, others that respond only to heat (frigorific and calorific points).

According to Herzen, the sensation of cold is, as are tactile impressions, transmitted to the brain by the posterior columns of the spinal cord, while heat-sensations, as well as those of pain, pass upward by the anterior columns. These theories, however, still lack positive proof, and all that is definitely known is the fact that the human skin is very sensitive to temperature-variations, and that the stimulation resulting from such variations is conducted to the central nervous system.

Intense cold lowers the acuteness of cutaneous sensation, and may, therefore, be employed for the production of local anesthesia. Cold applications of short duration, the water having a temperature of from 50° to 80° F., exalt sensibility, a fact that may be demonstrated by means of the æsthesiometer. At the point of application the skin becomes pale, on account of a vascular spasm, and presents the phenomenon of "goose-flesh," the latter resulting from a constriction of the smooth muscular fibres of the integument. These effects are manifested with a rapidity and intensity proportionate to the strength of the stimulus. At the periphery, below the point of application, the arterioles are contracted, their calibre being reduced to one-half or one-fourth the normal, or even entirely obliterated, a fact that serves to explain the styptic action of cold,

Delmas and Winternitz have shown by the use of the sphygmograph that under the influence of cold applications to the arm or forearm the elevation to which the lever of the instrument rises diminishes as the strength of the thermic stimulus increases. The surface of the part touched becomes cold, since it strives to place