228 THE HUMAN BODY. 



blood were sent into them in sudden jets at each beat of 

 the heart, they would run much risk of being torn. 



The Muscles of the Arteries. The arteries have rings of 

 plain muscular fibre in their walls ; when these contract 

 they narrow the artery, and when they relax they allow it 

 to widen under the pressure of the blood in its interior. The 

 vessel then carries more blood to the capillaries of the organ 

 which it supplies. Blushing is due to a relaxation of the 

 muscular layer of the arteries of the face and neck, allow- 

 ing more blood to flow to the skin. 



Why the Arteries have Muscles. The amount of blood 

 in the body is not sufficient to allow of a full stream of 

 blood through all its organs at one time: the muscular fibres 

 controlling the diameter of the arteries are used to regulate 

 the blood-flow in such a manner that parts hard at work 

 shall get an abundant supply, and parts at rest shall only 

 get just enough to keep them nourished. Usually when 

 one set of organs is at work and its arteries dilated, others 

 are at rest and their arteries contracted. Few persons, for 

 example, feel inclined to do brain-work after a heavy meal; 

 for then a great part of the blood of the whole body is led 

 off into the dilated vessels of the digestive organs, and the 

 brain gets but a small supply. On the other hand, when the 

 brain is at work its vessels are dilated, and often the whole 

 head flushed; and when the muscles are exercised, a great 

 portion of the blood of the body is carried off to them ; there- 



What would be apt to happen if blood were sent into them in 

 sudden jets? 



How are the muscles of the arteries arranged? What results from 

 their contraction? From their relaxation? To what is blushing 

 due? 



Why cannot all the organs have a full blood stream through them 

 at the same time? For what purpose are the muscular fibres in the 

 walls of arteries used? What is the usual condition of the arteries 

 of a resting organ? 



