This fact is demonstrated in the most satis factory manner by the delicate experiments of Messrs. Becquerel and Breschet as follows. When one of the joinings (rowlures) is kept uniformly at a temperature of 36° c. F.). and the other is inserted into the biceps muscle with the arm extended, the magnetic needle was found to deviate about .10 of a degree. On the arm being bent, however, the amount of deviation was observed to increase suddenly to the extent of from one to two degrees. Waiting till the oscillation of the needle and its return are completed, if the arm be bent anew so as to give a fresh impulse to the needle for several times in succession, a deviation of fifteen degrees is obtained at length, equivalent to a difference of five degrees in comparison with the original deviation, and corresponding to an increase of about half a degree of tem perique as measured by the centigrade scale. If the needle be inserted into the biceps, and the arm be used in the action of sawing for about five minutes, the temperature is observed to rise considerably, sometimes to the amount of a degree centigrade.
In these researches, then, we have evidence of facts of which we could not have acquired any precise information by our ordinary means of investigation. Every one, indeed, knows that exercise warms the body ; but every one also CO that in producing this effect, besides the contraction of the voluntary muscles, exercise is accompanied by an acceleration in the mo tions of the heart and organs of respiration. In this simultaneous concurrence of a variety of phenomena, it was impossible to distin guish the share which each had in the general result. Such an analysis could only be made by an experiment of the delicate and ingenious description of that which has been detailed.
It would appear that it is by the repetition of the muscular contraction after each relaxa tion that the highest evolution of heat is ob tained, each contraction producing a slight in crease of temperature, which, with the addition of that which follows, mounts to a certain limited point which it cannot pass. Let us remark, however, that the mere persistence of a primary contraction ought to have the effect of causing or maintaining a temperature higher than that which is evolved by a contraction followed immediately by relaxation ; indeed it is now known that a permanent muscular con traction is but a series and succession of smaller and imperceptible contractions, following each other with extreme rapidity.
It were well to observe here, that neighbour ing parts must increase in temperature at the same time much less in consequence of the direct communication of heat in virtue of con tiguity than by the afflux of blood, which, transmitted to the muscles in larger quantity, must also be more copiously than usual dis tributed to adjacent tissues. The relaxation of the muscles ought, on the other hand, to have a tendency to reduce the temperature, and this by so much the more as the relaxation is more complete. From all this it follows that the
attitude and state of the body will be favoura ble to cooling in the ratio of the general relax ation of the muscles, and of the degree in which each of them in particular is in a state of qui escence. This is what happens in sleep, of which we shall speak by-and-bye.
In the rise of temperature observed along with muscular contraction, we have in the first place only considered the action of the blood ; but neither contraction of the muscles nor the afflux of a larger quantity of blood could take place without the nervous influence ; for it is the will which determines the muscular contrac tion, and the will only acts through the me dium of the nerves which are distributed to the muscles. From this consideration it follows equally as from general relations pre viously exposed, that whatever lessens the nervous influence will likewise tend to reduce the temperature. Here we are, then, reverting to the two general conditions which we had already found to be the most influential in ca lorificatiun, namely, the arterial blood and the nervous system.
This examination of the relative tempera tures of the different parts of the body has led us, by the immediate comparison of the super ficial and deeper layers, to the consideration of the Influence of external temperature.—An in ert or inanimate body of higher temperature than the surrounding medium will of neces sity cool faster at its surface than in its internal parts. A living body, likewise, having within itself a permanent source of heat, which we shall suppose equally distributed through it, will lose more caloric from its surface than from its interior. This loss will become apparent by the cooling of the surface, so long as the source of heat remains everywhere equal. If, on the contrary, it be unequally distributed, if it be greater towards the surface, so as to compensate the greater loss which takes place there, the surface will have the same temperature as the interior. Without such a supposition it were necessary that the surface of the body should be lower in tem perature than the interior. This, indeed, is the actual state of the case. The external parts of living bodies are colder than the internal parts, because on the one hand the focus of heat is less, by reason of the nature of the component tissues, and on the other because the loss of heat there is greater. When the external tem perature falls, then the outer layers will tend to sink in temperature also, and will, in fact, sink so long as the internal source of heat remains the same. This partial refrigeration will be propagated internally, and the general tem perature will be lessened unless the economy provides against such an occurrence by an in crease of activity in its calorific powers.