THE AUTONOMIC NERVOUS SYSTEM 121 



when there is a call for more oxygen and for the rapid 

 removal of wastes from the tissues. Muscular activity 

 is the occasion when this need is most urgent and when 

 the response of the heart is most striking. In a more 

 moderate degree the heart is required to maintain a more 

 active circulation after the taking of a full meal. The 

 combined effect of vigorous exercise and a recent dinner 

 probably constitutes a maximal demand upon the reserve 

 power of the heart. 



Vasomotor adaptations serve to satisfy local, as con- 

 trasted with general, needs. Activity in any limited 

 portion of tissue, as in a gland or in a small group of 

 muscles, can be supported without making an appeal to 

 the heart. It is only necessary that the blood-vessels 

 leading to the active region shall be dilated so as to con- 

 duct thither a somewhat larger share than usual of the 

 total stream. This circumscribed increase in blood- 

 supply will mean a certain subtraction from the flow in 

 other directions, but if a single small gland is to be com- 

 pared with all the rest of the body we must admit that 

 the loss shared by so many organs may easily be imper- 

 ceptible in any one of them. When a relatively large 

 mass of tissue has its vessels dilated, there is often a com- 

 pensatory construction somewhere else. The foremost 

 illustration of this principle is found in the reciprocal 

 relation between the surface of the body and the deeper 

 parts. We observe that heat applied to the skin increases 

 the flow of blood through the heated area; reason and 

 experience both show that when such an increase occurs 

 the internal vessels must be less dilated than they were 

 before. 



Some space may well be devoted to a discussion of the 

 effects of external temperature on the distribution of the 

 blood. When we have evidence of increased blood-content 

 in any part we say that a congestion exists. When the 

 skin is flushed as a result of being warmed, it may be said 

 to be congested, though the word is so often used of ab- 

 normal conditions that we may prefer the milder term 



