502 ANIMAL HEAT 



the fall during the evening and early morning hours. A study of the 

 literature indicates considerable differences in the incidence of the maxi- 

 mum and minimum temperature, depending undoubtedly upon a variety 

 of causes. Despite differences as regards dietetic habits, work, age, 

 etc., the time of maximum temperature may be limited broadly between 

 4 and 8 P.M., and that of the minimum between 2 A.M. and 7 A. M., with an 

 average range of variation of over i C. in rectal observations. While a 

 hot or cold climate may determine minimal variations in the peripheral tem- 

 perature of the body, the internal temperature, as measured in the rectum, 

 appears to suffer only very slight modifications from such causes. The rise 

 in temperature incidental to muscular exertion is transitory. The follow- 

 ing quotations serve to indicate prevalent views regarding the diurnal 

 rhythm. Pembrey writes: 'As regards the causes of the daily varia- 

 tion in temperature, muscular activity and food appear to be the most impor- 

 tant factors. In ordinary life man is most active and takes food during the 

 day, and is least active during the night. . . . We may conclude that the 

 daily variation in temperature is one of the features of a corresponding varia- 

 tion in the activity of the tissues of the body, as shown by the rate of the con- 

 traction of the heart, the frequency of respiration, the intake of oxygen, the 

 output of carbon dioxide, the discharge of urea, and the capacity for muscu- 

 lar work.' Jiirgensen's observations on fasting subjects in which a more 

 or less "normal" curve of temperature variations was maintained exclude 

 the diet factor from a preponderating role. In estimating the relative im- 

 portance of the bodily activities as distinguished from other environmental 

 conditions, studies on the influence of the inversion of the daily routine have 

 been made. Those of Benedict claim special interest, because the observa- 

 tions were practically continuous over long periods of time. Such an ar- 

 rangement was made possible by the use of a specially devised electric 

 resistance thermometer reading to 0.01 C., which can be inserted 10 cm. 

 to 15 cm. in the rectum and retained there without inconvenience during 

 both waking and sleeping hours. Two subjects were carefully observed by 

 Benedict: one a person usually working during the day, but made to work 

 at night and sleep during the day for a series of consecutive days; the other, 

 an individual long accustomed to night-work in the capacity of a night 

 watchman. From the results of these observations it has been assumed that 

 the general form of the night curve remains practically intact, even when 

 the daily routine is inverted, indicating a fixity of rhythm that is difficult to 

 explain. As Benedict says: 'Why the temperature of the human body 

 reaches a minimum at 2 A.M. to 6 A.M., independent of whether the subject is 

 sleeping soundly in the recumbent position or whether he is awake and 

 sitting, or even standing and walking, is a problem that calls for extended 

 research.' However, the transposition of the daily routine through a period 

 of practically half a day, experienced as the result of the time changes 



