138 NATURE AND LIFE. 
transformed into motion. Itis partly wasted by transpira- 
tion through the skin, by touch, and especially by radiation ; 
it is used in keeping up to a constant point the temperature 
of the animal, subjected to many causes of refrigeration. 
The mechanical labor performed by an animal is very 
complex. Independently of visible muscular motions, there 
ure all the changes of place in the interior organs, the con- 
tinual passage of the blood, the contractions and dilata- 
tions of a great gumber of parts. Now, these actions are 
only possible in so far as the phenomena of breathing are 
taking place in the active region. Prevent arterial blood 
from coming to the muscle, that is to say, prevent combus- 
tion taking place, and consequent heat evolving in it, 
and, although the structure of the organ suffers no harm, 
it loses its contractile power. Mere compression of the 
supplying artery of the muscle, so as to check the flow of 
blood in it, causes the organ to grow cool and lose its power. 
The labors of Hirn and Béclard have clearly established 
the relations between heat and muscular motion. Later ex- 
periments by Onimus have fixed, with equal precision, the 
efficiency of heat through the movements of circulation. 
We have said that the heat-producing power of ali- 
ments will be the more considerable in proportion as they 
. contain a greater quantity of elements that need a large 
supply of oxygen for their combustion. Therefore, meat 
and fats repair the losses of the system much more speedi- 
ly than vegetable substances. The latter are suitable for 
the inhabitants of warm countries who do not require to 
produce heat, which the atmosphere supplies them with 
abundantly. The inhabitants of cold regions, on the con- 
trary, whose accessions of heat ought to be as continual as 
energetic, are urged by instinct to use meats and fats, which 
throw out great heat in their combustion. For instance, 
it is a physiological necessity that the Lapps should feed 
on the oil of cetacea, as it is a necessity for men of the 
