380 STUDIES IN ADVANCED PHYSIOLOGY. 



question naturally arises, in what manner the production of 

 this heat is maintained and regulated so perfectly ? 



The agencies which figure in this regulation are of two 

 kinds : 



1. Artificial Agencies. Such artificial agencies are, 

 first, clothing and fuel; second, increased exercise. It is 

 the commonest experience to maintain the temperature of 

 the body on a cold day by increasing vigorously the amount 

 of exercise. Third, with the approach of cold weather we 

 change more or less consciously the kind and quantity of 

 foods, eating in winter as a rule not only more food, but 

 food of greater. heat-producing quality. Fourth, as pointed 

 out just above, we may regulate the temperature to some 

 extent by the administration of drugs, a regulation fre- 

 quently called in in the reduction of temperatures in feverish 

 conditions. 



2. Natural Agencies. The second group of agencies 

 are those which might be designated as the natural ones ; 

 that is, agencies which come into play without our con- 

 scious intervention. These are, first, an increased activity of 

 the heart and lungs in cold weather; second, contraction 

 or relaxation of the capillaries of the skin; third, the per- 

 spiration of the skin, in the evaporation of which much heat 

 may be eliminated from the body; fourth, a natural in- 

 creased hunger; fifth, involuntary movements, such as the 

 familiar shivering and chattering of teeth which follow un- 

 clue exposure to cold; sixth, the thermogenic nerves. 



THERMOGENIC NERVES. 



It has now been settled beyond a matter of question that 

 there are nerves in the body which are concerned directly 

 with the production of heat. The centers from which 

 these thermogenic nerves arise, lie probably in the spinal 

 cord. These centers in the spinal cord may, however, be 

 stimulated or inhibited by higher centers lying in the brain 

 and medulla. Repeated experiments in this direction seem 



