Relations in point of temperature between external parts.—We have only data for insti tuting comparisons in regard to the hand, the axilla, the groin, and the feet among the external parts of the body. In a moderate summer-heat the hand appears to be the part which is most susceptible of showing a high, the feet the parts most susceptible of exhibiting a low temperature. The axilla and groin gene rally exhibit nearly the same degree of tempe rature; and the amount in which they differ in this particular from the mouth may be stated at about 1°, 75.
When we direct our inquiries with a view to ascertaining any general relation in the tempe ratures of different parts of the body, whether external or internal, we soon discover, as has been already stated, that this has no connexion of an inverse kind with their distance from the heart. At the same time there is a general condition discovered influencing the tempera ture of the diflbrent parts. This is their situa tion in reference to the surface or inside of the body. The temperature is higher, for instance, within the trunk than on its outside. What ever other reason may be assigned for this, there is one which is purely physical, that must influence it powerfully. It is obvious that the surface of the body and limbs must cool much more rapidly than the interior of the body. So that, supposing the temperature at first to be every where uniform, the difference in the rate of cooling would very soon suffice to cause a notable reduction on the exterior beyond that which took place in the interior of the body. This cause, however, can only be charged with its own share of influence ; there are others which must act with considerable effect, and among these especially the one upon which the production of heat depends. We have seen that the condition of the functions of nutrition most closely in relation with animal heat was connected with the arterial blood. Now inas much as the arterial blood is that which is most intimately connected with the production of caloric among animals, we might fairly expect that the temperature generally would be rather above that of the venous blood. And we have seen that there was actually a difference be tween the temperatures of the two ventricles, that of the left being the higher. Experiment has also shown that there was a corresponding difference in the temperatures of the two kinds of blood circulating in the arteries and veins ; arterial blood is actually higher in temperature than venous blood to the extent of a degree of Fahrenheit's scare. We shall add here, and in
conformity with the same principle, that it is to this difference of temperature of the two kinds of blood that the difference in the tempe rature of the right and left ventricle of the heart is owing. We need not be hindered in adopt ing this conclusion from the circumstance of the blood of either ventricle being found in a slight degree inferior in temperature to the ven tricle itself, inasmuch as the blood abstracted from the canals that contain it, and exposed to the air, begins to evaporate, and loses heat ra pidly. Nevertheless it is not demonstrated that the difference in temperature of the blood out of and of the blood in the ventricles of the heart depends on this cause. There may be another at work ; the influence of muscular contraction for instance, a point which we shall examine generally by-and-bye. New means of estimating variations of temperature have been lately discovered, by which changes that en tirely escaped us as judged of by the thermo meter are made abundantly obvious; by which, indeed, the temperature of parts inaccessible in their natural and normal condition to the ther mometer are now investigated without diffi culty. In using the thermometer as the means of estimating temperature, it is evident that this instrument could not he introduced into the external parts without injuring the tissues, without incisions, &c., which would necessarily alter them materially, and produce so much dis turbance in their functions, that an increase or diminution of temperature must almost neces sarily have been the consequence. The ther mometer, besides, however small its dimen sions, has the inconvenience of always either absorbing or giving out a considerable quantity of heat according to circumstances, before it gets into equilibrium with the parts with which it is brought into contact. A necessary fall or rise in the absolute temperature of these parts is the natural consequence of this. Further, the thermometer is incapable of showing sudden variations in temperature; several minutes must always elapse before it. gets into a state of equi librium in regard to temperature with the parts or medium surrounding it. If a thermometer, for instance, be placed in the mouth, three or four minutes must elapse before it will cease to show any increase of temperature. Now if any calorific phenomena of short duration were de veloped in that time, it is evident that all idea of their occurrence would escape us.