186 



APPLIED PHYSIOLOGY 



red, but in a few moments they become white and shriveled from the 

 contraction of the arteries. 



When cold has acted upon the arteries for some time it paralyzes 

 them so that they dilate. When a boy begins to snowball, his hands 

 are cold, but after a while his hands glow with redness and warmth 

 because the paralyzed tubes admit more warm blood. 



319. Effects of injury upon the arteries. When injured 

 in any way, the injured part becomes red and warmer. 

 This is because the same cause which produces the injuries 

 also partly paralyzes the smaller arteries, so that they dilate 

 and bring an extra quantity of blood for the repair of the 

 wounded part. Here, as elsewhere, nature wonderfully 

 adapts the body to its surroundings. 



320. Nature's arrest of hemorrhage. Cut capillaries 

 cause only an oozing of blood which collects like drops of 



dew over the whole cut 

 a surface. Blood does not 

 spurt from a cut vein, but 

 wells out in a slow stream. 

 When an artery is cut, the 

 blood flows in a strong jet. 

 Bleeding from either of the 

 vessels usually stops in a 

 few moments. The mus- 

 cles of the blood tube 

 contract and lessen the 

 size of the tube, or even 

 entirely shut it up ; the blood also clots in the cut, and a 

 small plug of clot extends into the end of the blood tube. 

 In these two ways bleeding from small cuts is soon stopped 

 naturally. But in a large artery the blood pressure is so 

 great that it forces away the clot as fast as it is formed, 

 so that bleeding may continue until death occurs. 



Diagram of a bleeding cut. 



a upper edge of a cut. 

 b a cut blood tube. 



