CHATONNET, J. 
difficulty may arise from the great potency of the mechanisms of 
struggle considered as a whole in some animal species. In the dog, 
for example, the ability to withstand the cold is very important; the 
physical regulation may spare the load on heat production and in the 
severe cold the heat production can increase to very high values 
without any inefficiency being detected. Only extreme cold can sur- 
pass the total capacity for defense in the normal animal. In mod- 
erate cold exposure, oneeffectormechanismmay easily be replaced 
by the others. It is therefore necessary to know the greatest capacity 
for thermogenesis and, if that requires excessive conditions of 
temperature, to remove part of these effector mechanisms and to 
determine then the maximum capacity for heat production. After- 
wards, every further reduction of that maximum can be ascribed 
to the subsequent operation. The comparison of these various levels 
of thermogenesis is only valuable if they are true maxima and if no 
substitutes are possible. 
CONTROL OF THE MUSCULAR 
ACTIVITY 
Voluntary muscular activity, shivering, and thermal muscle tone 
are effected by the same muscle but according to different kinds of 
stimulation. Of course some parts of the muscular apparatus are 
concerned more with one sort of activity, but not exclusively. 
In the spinal cord, there is some evidence of the existence of 
descending pathways especially controlling shivering. It is known 
that corticospinal tracts are not necessary for that control. More- 
over, it appears that different patterns of muscular activity can 
substitute for each other. Indeed, in the dog deprived of reactions 
of shivering by section of the anterior part of the cord, muscular 
activity may still increase in cold ambient temperatures, in the form 
of agitation. 
Authors such as Freund and Strasmann (1912) and Isenschmid 
(1926) tried to measure the importance of thermogenesis provided 
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