This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
The first effect of heat is to excite its own peculiar sensation in the seat of application. In a moderate degree, this may not be unpleasing; indeed, when the temperature has been depressed below the normal standard, it is often highly grateful; but, if increased, it begins at length to become painful, and in its higher degrees is often extremely so. The excitant influence is speedily extended to the capillaries, which dilate under the stimulation, admitting a larger amount of blood, and thus reddening the surface, and producing a greater or less distension of the tissue generally. The blood of the part, thus increased in amount, and at the same time flowing more rapidly with the increased action of the vessels, receives an increment of heat from without, which it carries through the system, to produce everywhere a degree of the same stimulant effect first produced upon the surface of application. The impression upon the nerves, moreover, is transmitted to the cerebral and spinal centres, exciting them to increased action. A universal stimulant influence is thus exerted. The heart beats more rapidly and energetically; and the pulse is consequently fuller, stronger, and more frequent. The respiration is hurried. The general temperature, under the universal excitation, is elevated in a degree much greater than would result from the mere addition of caloric from without. The secretions are promoted; all of them probably at first, under a gentle operation of the stimulant: but, when the heat is considerable and continued, the mucous membranes and the kidneys appear to be irritated beyond the point of free secretion, as indicated by a dryish state of the mouth, and diminished discharge of urine; while the secretory functions of the liver and skin are augmented, so that there is usually a freer flow of bile, and greater or less increase of perspiration. An explanation of this difference in effect, and of its uses, will be given hereafter under the head of diaphoretics. At present it is sufficient to observe that, in some instances, even the liver and skin appear to be so much irritated as to be unable to perform their functions, and the patient consequently suffers with hepatic congestion, and general heat and dryness of the surface. Along with the exaltation of the organic functions, the nervous also become excited. A moderate influence of the cause is usually attended with agreeable effects. Sensibility is rendered more acute, muscular motion invigorated, and the intellect, imagination, and feelings more or less exalted. But these functions, under a higher degree of the excitation, become deranged, and at a still higher are impaired; so that a feeling of fulness and even painful distension of the head, vertigo, and other abnormal cerebral sensations, and hebetude of the intellectual and emotional functions, are often experienced. This result is simply in conformity with the general law, that, by moderate over-excitation or irritation, the functions are increased in a normal direction, by a greater amount of it, are deranged, and by a still greater, are diminished or suppressed. The generative functions obey the same general law; and the sexual feelings are excited, the menstrual act promoted, and the capacity for conception probably increased, under the genial influence of a moderately elevated temperature; while they may be perverted or suppressed by its excess. Should the heat be very considerable, or continued too long, a universal acute derangement of the functions may take place, constituting fever; and this is the condition of system existing in the disease called sun-stroke, and for which the name of heat-fever has been proposed, under this idea of its nature.* after the subsidence of the general excitement produced by the hot bath. Hence, too, the enfeebled state of system, the habitual languor and lassitude, experienced after a prolonged exposure to the heats of summer, or a residence of some months in a tropical climate.
As in the case of almost every other stimulant, the excitement produced by heat in the functions, is, followed by a proportionate depression. The excitability, exhausted by excessive exercise, now fails to respond to the ordinary vital stimuli, and action is of course diminished. As before stated, this may be considered as a nearly universal law of stimulation. Hence the soft, compressible, and often feeble pulse, the cool, pale, and relaxed skin, the muscular debility, the lassitude or indisposition to exertion, the mental languor, and the tendency to sleep.
* See the article on Heat-fever in the sixth edition of my Treatise on the Practice of Medicine. (Note to the third edition).