THE INTEGRATIVE ACTION OF THE VASCULAR SYSTEM 23 1 



and lose moisture from the surfaces of the respiratory 

 passages. The highly uncomfortable experience which we 

 have on a day which is not only hot but muggy is due to the 

 interference with free evaporation by the moisture already 

 in the air. 



If the body temperature tends to fall, an interesting series 

 of adjustments occurs, all directed towards preservation 

 of the steady state. First, heat which is being lost is con- 

 served: perspiration is reduced to a minimum, the surface 

 vessels are contracted and thus the warm blood from the 

 interior is not exposed to the cold surroundings, and in 

 animals provided with hair or feathers these appendages 

 of the skin are elevated so that a thicker layer than usual 

 of insulating air is enclosed in their meshes. In us only 

 futile "gooseflesh" remains of this last protective reaction, 

 and we resort therefore to extra clothing to prevent too 

 great heat loss. 



If the check on the outflow of heat is not sufficient to 

 protect against a fall of body temperature, more heat must 

 be produced. The first step in that direction is a discharge 

 of adrenin from the adrenal medulla. This remarkable 

 substance not only collaborates with the sympathetic 

 impulses which are constricting the surface vessels, but it 

 has the power to accelerate the processes of combustion in 

 all parts of the organism. Its discharge is like opening the 

 dampers of a furnace: burning goes on more rapidly and the 

 heat production quickly mounts. But even this extra heat 

 may not be enough to match the losses. In that event 

 muscular activity is automatically started, i.e., shivering 

 occurs. The greater production of heat with which we are 

 familiar when we run or play vigorous games then results; 

 indeed, shivering may more than double the rate of heat 

 development in the body. And if shivering does not suffice 

 to keep up the normal temperature we are hkely to be 

 impelled to engage in such strenuous physical exertion that 

 heat flows to the blood from many large muscle masses 

 engaged in the eff"ort and is dehvered promptly to all parts 

 of the body by the streaming blood. 



It is noteworthy that not only are there arrangements 

 which check a shift of body temperature in one direction 



