206 ' CIKCULATION 



ing of the artery walls with the consequent increase and 

 decrease in blood must necessarily cause the capillaries, 

 whose walls are also elastic, to become stretched. Hence 

 variation in the tension of the artery walls is always 

 accompanied by similar variations in the blood supply 

 to those capillaries which the arteries feed. 



The conditions which may thus affect these vaso-motor 

 nerves are many. Heat to a certain limit, exercise, the 

 presence of food in the digestive tract, etc., all affect the 

 vaso-dilator nerves and cause the artery walls to relax. 

 Cold, on the contrary, affects the vaso-eonstrietors and 

 causes the contraction of the walls. Mental emotions in 

 some cases cause relaxation of the walls, and in other 

 cases, contraction. Thus, blushing is due to overcharg- 

 ing of the facial capillaries from relaxation of those artery 

 walls which supply these capillaries; while paleness is due 

 in the same manner to contraction of the supplying artery 

 walls. In both cases the mental emotions are responsible 

 for the nerve stimulus. 



Injury to tissue cells invariably results in a relaxation of 

 the walls of the arteries supplying the affected part, and 

 thus permits an increase in the blood supply to the injured 

 area. Such an increase is spoken of as congestion. 



Congestion and inflammation. — A simple case of con- 

 gestion is seen when we scratch the skin with a pin, the 

 scratched surface becoming reddened from the action of the 

 injured cells upon the artery supply. If the injury is 

 severe and the cells are killed, swelling often follows the 

 redness, and painful sensations may result. 



Such congestion, with its attendant swelling, is merely 

 an indication of the first steps in the repair of the injury. 

 The flow of blood to the injured part is soon followed, in 



