SOURCES OF BODILY HEAT. 451 



not, at least indirectly through the blood, which, flowing 

 from part to part, carries heat from warmer to colder 

 regions. Thus, although at one time one group of muscles 

 may especially work, liberating heat, and at other times 

 another, or the muscles may be at rest and the glands the 

 seat of active oxidation, the temperature of the whole Body 

 is kept pretty much the same. The skin, however, which 

 is in direct contact with external bodies, usually colder than 

 itself, is cooler than the internal organs; its temperature in 

 health is from 36 to 37 C. (96.8-98.5 F.), being warmer 

 in more protected parts, as the hollow of the armpit. In 

 internal organs, as the liver and brain, the temperature is 

 higher; about 43 C. (107 F.) in health. In the lungs 

 there is a certain quantity of heat liberated when oxygen 

 combines with haemoglobin, but this is more than counter- 

 balanced by loss of the heat carried out by the expired air 

 and that used up in evaporating the water carried out in 

 the breath, so the blood returned to the heart by the pul- 

 monary veins is slightly colder than that carried from the 

 right side of the heart to the lungs. 



The Sources of Animal Heat. These are two-fold; 

 direct and indirect. Heat is directly produced whenever 

 oxidation is taking place; so that all the living tissues at 

 rest produce heat as one result of the chemical changes sup- 

 plying them with energy for the maintenance of their 

 vitality; and whenever an organ is active and its chemical 

 metamorphoses are increased it becomes hotter: a secreting 

 gland or a contracting muscle is warmer than a resting one. 

 Indirectly, heat is developed within the Body by the trans- 

 formation of other forms of energy: mainly mechanical 

 work, but also of electricity. All movements of parts of the 

 Body which do not move it in space or move external ob- 

 jects, are transformed into heat within it; and the energy 

 they represent is lost in that form. Every cardiac contrac- 

 tion sets the blood in movement, and tin's motion is for the 

 most part turned into heat within the Body by friction with- 

 in the blood-vessels. The same transformation of energy oc- 

 curs with respect to the movements of the alimentary canal, 

 except in so far as they expel matters from the Body; and 



