1170 PHYSIOLOGY 



THE DIURNAL VARIATIONS IN THE BODY TEMPERATURE 



In any animal the seat of heat production, e.g. a contracting muscle, 

 must be warmer than an inactive tissue, and this again than a tissue from 

 which heat is being rapidly abstracted, such as the skin. Owing, however, 

 to the rapidity of the circulation of the blood, the temperature of the internal 

 organs can be regarded as approximately uniform. The temperature of man 

 is usually taken in the mouth, rectum, or axilla. In the case of the mouth 

 the temperature is liable to fluctuation with the rate of breathing, the mouth 

 being cooled by the passage of the air through the nasal cavities. There is 

 also probably loss of heat through the cheeks. In order to determine the 

 temperature in this situation the mouth should be kept closed for a few 

 minutes and then the bulb of the thermometer inserted under the tongue, 

 and the lips kept closed on the stem of the thermometer for five minutes. 

 Except in cases where the cutaneous vessels are much dilated, the tempera- 

 ture of a thermometer in the axilla takes a considerable time to rise to that 

 of a thermometer in the mouth ; it should never be left less than ten minutes 

 in this situation. The following Table represents the temperature of 

 different parts of the body : 



SURFACE 



Skin covered with clothes 33 to 35 C. 



Naked skin in bath at 5 C. . . . . . 17 C. 



25 C. . . . 26-5 C. 



MUSCLES 12 MM. BELOW THE SURFACE 



In bath at 5 C 36-3 C. 



25 C 36-9 C. 



The body temperature of man shows variations of several tenths of 

 a degree according to the time of day at which the temperature is taken. 

 The highest temperature is obtained about six or seven in the evening, and 

 the lowest at about four or five o'clock in the morning. With these diurnal 

 changes in temperature are associated parallel oscillations in the rate of 

 metabolism as shown by the elimination of carbon dioxide. They are 

 probably determined by the changes in the movement and tension of the 

 muscles occurring during the waking hours. If the habits of a man or 

 animal be reversed, so that he sleeps in the daytime and performs his normal 

 vocation by night, it is possible to reverse also the direction of the diurnal 

 variations in temperature. The temperature may be also affected tem- 

 porarily by various acts, such as the taking of food, or muscular work ; the 

 influence of the latter factor is often very marked. In many individuals a 

 hard game at tennis suffices to raise the temperature to 39 C. (102 F.), and 

 even the healthiest individual will show some change in his temperature 

 as the result of exercise. Pembrey found that the act of marching led to a 

 considerable rise in temperature, a rise which is apparently responsible for 

 the discomfort and fatigue observed under such circumstances. Such a 

 change in the body temperature is merely temporary in its effects, and 

 insignificant in comparison with the wonderful uniformity of temperature 



