MEASUREMENTS OF BODY-TEMPERATURE.
While it is possible to control arbitrarily the temperature of the calorimeter by increasing or decreasing the amount of heat brought away, and thus compensate exactly for the heat eliminated by the subject, the hydrothermal equivalent of the system itself being about 20 calories—on the other hand the body of the subject may undergo marked changes in temperature and thus influence the measurement of the heat production to a noticeable degree; for if heat is lost from the body by a fall of body-temperature or stored as indicated by a rise in temperature, obviously the heat produced during the given period will not equal that eliminated and measured by the water-current and by the latent heat of water vaporized. In order to make accurate measurements, therefore, of the heat-production as distinguished from the heat elimination, we should know with great accuracy the hydrothermal equivalent of the body and changes in body temperature. The most satisfactory method at present known of determining the hydrothermal equivalent of the body is to assume the specific heat of the body as 0.83.[14] This factor will of course vary considerably with the weight of body material and the proportion of fat, water, and muscular tissue present therein, but for general purposes nothing better can at present be employed. From the weight of the subject and this factor the hydrothermal equivalent of the body can be calculated. It remains to determine, then, with great exactness the body temperature.
Recognizing early the importance of securing accurate body-temperatures in researches of this kind, a number of investigations were made and published elsewhere[15] regarding the body-temperature in connection with the experiments with the respiration calorimeter. It was soon found that the ordinary mercurial clinical thermometer was not best suited for the most accurate observations of body-temperature and a special type of thermometer employing the electrical-resistance method was used. In many of the experiments, however, it is impracticable with new subjects to complicate the experiment by asking them to insert the electrical rectal thermometer, and hence we have been obliged to resort to the usual clinical thermometer with temperatures taken in the mouth, although in a few instances they have been taken in the axilla and the rectum. For the best results the electrical rectal thermometer is used. This apparatus permits a continuous measurement of body temperature, deep in the rectum, unknown to the subject and for an indefinite period of time, it being necessary to remove the thermometer only for defecation.
As a result of these observations it was soon found that the body temperature was not constant from hour to hour, but fluctuated considerably and underwent more or less regular rhythm with the minimum between 3 and 5 o'clock in the morning and the maximum about 5 o'clock in the afternoon. In a number of experiments where the mercurial thermometer was used under the tongue and observations thus taken compared with records with the resistance thermometer, it was found that with careful manipulation and avoiding muscular activity, mouth breathing, and the drinking of hot or cold liquid, a fairly uniform agreement between the two could be obtained. Such comparisons made on laboratory assistants can not be duplicated with the ordinary subject.
It is assumed that fluctuations in temperature measured by the rectal thermometer likewise hold true for the average temperature of the whole body, but evidence on this point is unfortunately not as complete as is desirable. In an earlier report of investigations of this nature, a few experiments on comparison of measurements of resistance thermometer deep in the rectum and in a well-closed axilla showed a distinct tendency for the curves to continue parallel. A research is very much needed at present on a topographical distribution of body temperature, and particularly on the course of the fluctuations in different parts of the body. A series of electric-resistance thermometers placed at different points in the colon, at different points in a stomach tube, in the well-closed axilla, possibly attached to the surface of the body, and in women in the vagina, should give a very accurate picture of the distribution of the body-temperature and likewise indicate the proportionality of the fluctuations in different parts of the body. Until such a research is completed, however, it is necessary to assume that fluctuations in body-temperature as measured by the electric rectal thermometer are a true measure of the average body-temperature of the whole body. Indeed it is upon this assumption that it is necessary for us to make corrections for heat lost from or stored in the body. It is our custom, therefore, to compute the hydrothermal equivalent by multiplying the body-weight by the specific heat of the body, commonly assumed as 0.83, and then to make allowance for fluctuations in body-temperature.
When it is considered that with a subject having a weight of 70 kilos a difference in temperature of 1° C. will make a difference in the measurement of heat of some 60 calories, it is readily seen that the importance of knowing the exact body-temperature can not be overestimated; indeed, the whole problem of the comparison of the direct and indirect calorimetry hinges more or less upon this very point, and it is strongly to be hoped that ere long the much-needed observations on body-temperature can be made.