THE TEMPERATURE OF THE MONKEY AND ITS DIURNAL VARIATION. 75 



Diurnal Variation of Body Temperature. 



So far no consideration had been given to the question of the presence or absence 

 of a diurnal variation in the temperature. By far the greater number of the observa- 

 tions had been made in the afternoon, usually between 5 and 6 o'clock, but one week, 

 having occasion to be working in the laboratory until late in the evening, we made 

 observations at 9 a.m. and at 12.30 p.m., 4.30 p.m., 9 p.m. and 11.45 p.m. 

 respectively on three consecutive days on three monkeys, and found distinct evidence 

 of a diurnal rhythm in each case. The highest temperatures were recorded between 

 4 and 5 p.m., and the lowest between 11 and 12 p.m. ; the variation was very regular 

 and the range a considerable one, as may be seen by an inspection of the tabulated 

 records of monkeys, XXVII., XXVIII. , and XXIX. (p. 72). 



To investigate this diurnal variation more fully we selected a very quiet animal — 

 monkey XXX. — and took readings at short intervals throughout the twenty-four hours 

 for eight days consecutively. During this time the monkey was confined in a cage in 

 a warm room, the temperature of which varied only from about 18° C. to 22° C. 

 throughout the whole period. It always had a plentiful supply of fruit, etc., but was 

 not fed at regular intervals. In the monkey the food is not always ingested at the 

 time the animal is supplied with it, for it usually fills its cheek pouches and empties 

 them at its leisure, it may be some considerable time later. At that season of the year 

 (May) daylight appeared between 3 a.m. and 4 a.m., but the shutters were not removed 

 from the windows till about 8 a.m. The cage in which it was confined was large 

 enough to admit of its moving about freely, but did not allow it to take very active 

 muscular exercise, yet notwithstanding this the daily rhythm was very regular and 

 distinct, and the range wide. The minimal period was between 3 a.m. and 5 a.m., and 

 the maximal period between 6 p.m. and 8 p.m. When plotted out, the curves of the 

 axillary, rectal, and room temperatures show that the axillary temperature is in most 

 cases higher than the rectal, but that the two vary together, and that the curve of 

 body temperature varies independently of that of the room, for very frequently when 

 the one is falling the other is rising. 



Subsequently another experiment of the same kind was made with another monkey 

 under the same conditions as obtained in the last case, and the result was similar (see 

 table — monkey XXXI., p. 73). The variation was not so regular, and there were 

 more secondary waves in the primary, but it was of the same type, and the minimal 

 and maximal periods occurred in the early morning and late in the afternoon 

 respectively. 



On glancing at the temperature curves of monkeys XXX. and XXX I., their close 

 similarity to that seen in the clinical chart of a hectic patient was at once evident, and 

 the possibility suggested itself that these animals, although apparently healthy, might 

 be suffering from some form of tubercular disease, a condition not very uncommon in 

 monkeys kept in confinement. The five monkeys (XXVII. , XX VIII.," XXIX., XXX., 



