464 ANIMAL PHYSIOLOGY. 
formation on this subject, the numbers denoting the heat- 
units produced: 
RESTING DAY. WORKING DAY. 
Rest, 16 hours. Sleep, 8 hours. Rest, 8 hrs. Work, 8 hrs. Sleep, 8 hrs. 
2470°4 820 1235°2 216°99 820 
Total, 2790°4. Total, 37248 
It appears from a multitude of considerations that the body 
is like a steam-engine, producing heat and doing work; but it 
is found that while a very good steam-engine, as a result of the 
. chemical processes going on within it, converts 4 of the poten- 
| tial energy of its supplies into mechanical work, the other } 
| appearing as heat, the body produces 4 as work and 4 as heat, 
from its income of food and oxygen. 
While it is perfectly clear that it is in the metabolic pro- 
cesses of the body that we must seek for the final cause of the 
heat produced, it is incumbent on the physiologist to explain 
the remarkable fact that the mammalian body maintains, 
under a changing external temperature and other climatic 
conditions, and with a varying diet, during rest and labor, a 
temperature varying within, usually, no more than a fraction 
of a degree centigrade. This we shall now endeavor to explain 
in part. 
The Regulation of Temperature.—It is manifest from the facts 
adduced that so long as life lasts heat is being of necessity con- 
stantly produced. If there were no provision for getting rid of 
a portion of this heat, it is plain that the body would soon be 
‘consumed as effectually as if it were placed in a furnace. We 
observe, however, that heat is being constantly lost by the 
‘breath, by perspiration (insensible), by conduction and radia- 
‘tion from the surface of the body, and periodically by the 
urine and feces. We have seen that, while heat is being pro- 
duced in all the tissues and organs of the body, some are es- 
pecially thermogenic, as the glands and muscles: The skin 
presents an extensive surface, abundantly supplied with blood- 
vessels, which when dilated may receive a large quantity of 
blood, and when contracted may necessitate a much larger in- 
ternal supply, in the splanchnic region especially. It is a mat- 
ter of common observation that, when an individual exercises, 
the skin becomes flushed, and so with the increased production 
of heat, especially in the muscles (see page 195), there is a pro- 
vision for unusual escape of the surplus; at the same time 
sweat breaks out visibly, or if not, the insensible perspiration 
