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.