[55] In order to ascertain the cause of the differences in the cooling of the body after the various kinds of death, it is necessary to examine what general conditions can have an influence in the cooling of a body left to itself. Of these there are three principal ones.
Under the same external circumstances, a body will cool so much the slower. 1st. As its temperature at the beginning of the experiment, shall be higher in relation to that of surrounding bodies; 2d. As its surface shall be less in relation to its size; 3d. And as its exterior parts shall be less perfect conductors of heat.
In order to see how the first condition is modified in different cases, it is necessary to recollect what is the source of animal heat. The blood is warmed in passing through the lungs in consequence of the chemical phenomena of respiration; and as from the lungs it is carried to all parts of the body, it yields to the different organs a portion of the heat which it has received. Hence the general temperature of the body will be higher in proportion to the temperature of the blood, to the frequency with which this fluid is renewed in the organs and to the quantity of it that is brought to them, at each pulsation.
Now in diseases of long duration, the volume of blood is considerably diminished, the activity of the heart is lessened, and respiration is performed in an imperfect manner. Thus the body of the patient who sinks under these circumstances has less heat to lose than that of the man who dies suddenly, when all these functions were performed in perfection.
Let us pass now to the second condition. The cooling, as we have said, takes place so much the quicker as the surface of the body is the more extended in proportion to its size; now, in the emaciation which accompanies almost all diseases that are protracted, the size decreases much more rapidly than the surface. Thus then, when even at the moment of death the general temperature of the body may be as high as in a state of health, the cooling would however take place more quickly.
It remains for us now only to examine under what circumstances the third condition is fulfilled in the most advantageous manner. When an individual in full health dies, the sub-cutaneous cellular texture usually contains a greater or less quantity of fat. Now we know that it is one of the characters of all fat substances to be very bad conductors of heat. Hence then a third reason which should render cooling more slow after sudden deaths. Sometimes after a disease, this last condition is fulfilled in another manner. In certain derangements of the circulation, the cellular texture is filled with serum; and as all aqueous fluids are bad conductors of caloric, though the temperature of the body may not be very high at the moment of death, the heat is yet preserved for a long time.
To the different causes which we have just mentioned, there is sometimes added another which is peculiar to one kind of sudden death. It is observed that in the midst of the same external circumstances, the blood does not always cool with the same quickness, and that in proportion as its coagulation is slower, its heat is longer preserved. Now, it is a well known fact, that when death is the result of asphyxia, the vessels are found full of fluid blood; this is also a reason which contributes to explain the slowness of the cooling. And it should be remarked, that asphyxia is one of the most frequent causes of sudden death either accidental or voluntary.
BICHAT ON LIFE AND DEATH.