72 THE HUMAN HODY. 



heart, and diminishes as the vessels grow smaller. By 

 the time the capillaries are reached, there is no stretching, 

 hence no pulse. The veins have no pulse. 

 3. The pulse tells us, 



1. How fast the heart is beating. 



2. How strong its beat is. 



5. How firm the coats of the arteries are. The last fact 

 is important. The arterial walls are sometimes flabby, 

 and sometimes hard and resisting. Their condition indi- 

 cates the state of the system. 



VARIATIONS IN BLOOD-SUPPLY. 



SECTION VII. 1, You might suppose, from what has 

 been said of the circulatory apparatus, that the same 

 blood-vessels are always of the same size, and that the 

 blood-supply to the same part is at all times of the same 

 amount. This is not the case. Although there are no 

 stopcocks in the blood-vessels, there is a contrivance of 

 Nature by which the size of the blood-vessels and the 

 amount of blood in any part is increased or diminished 

 according to the need of the moment. 



2. In this way the blood is partly shut off from the 

 brain when we need sleep; it is turned on to the stomach 

 during digestion, and to the muscles in active exercise; it 

 is shut off from the skin when it is cold, and sent there 

 to be cooled when we are warm. 



3, The walls of the arteries consist partly of fibers of 

 involuntary muscle running crosswise. AVhen these mus- 

 cle-fibers contract, they make the artery smaller: when 

 they are relaxed, it is larger. They are not under the 

 control of the will, but are influenced by special causes. 

 For example, the arteries in the coats of the stomach will 



