192 THE HUMAN MECHANISM 



warm-blooded animals is not, however, to be taken too literally. 

 No animal has an absolutely constant temperature. In the first 

 place, there are slight variations from time to time under the 

 changing conditions of life. The temperature is higher by 

 from one to four degrees during muscular activity than 

 during rest; it varies during the day, being highest in the 

 afternoon and lowest in the small hours of the morning; it 

 is often raised half a degree or more by taking food, and 

 marked changes of surrounding temperature may cause a 

 change of one degree or even more in that of the body. 

 These changes between 97.5 and 99.5 F. are of everyday 

 occurrence and are entirely normal; so that when we speak 

 of the temperature of the body being constant we mean that 

 it varies only within narrow limits or that it is constant in 

 comparison with that observed in cold-blooded animals. 



6. The temperature of different organs. Nor is this all; 

 some parts of the body have a higher temperature than 

 others. Thus the temperature of the liver is often as high 

 as 107 F. ; that of the muscles varies between 99 and 

 105 F. ; that of the blood in the right side of the heart is 

 usually a degree or so higher than that of the blood in the 

 left side. But it is in the skin that we meet with the widest 

 variations from the general average. Everyone knows that 

 on a very cold day the temperature of the skin may be far 

 below 98.6 F. ; indeed, the experience of " frosted " ears or 

 feet shows that at times cutaneous temperature may descend 

 to, or even below, the freezing point itself; and it is very 

 exceptional indeed when the skin temperature is above 92 

 or 93 F., even on very hot summer days. These variations 

 are due to the fact that the skin is the organ which is 

 immediately exposed to the changing environment and 

 hence peculiarly subject to cooling influences. It is there- 

 fore customary to distinguish between an outer body zone 

 of variable temperature and the more constant temperature 

 of internal organs. 



