THE STORY OF THE HEART 



59 



conveyance over the body. An artery is a vessel 

 possessing a fairly well developed coating of 

 muscular fibres in its walls. It is therefore a more 

 or less elastic tube, and offers little resistance to 

 the wave of blood propelled along its length at each 

 stroke of the heart. The muscular coating of the 

 artery, serving to produce the contraction of the 

 tube, assists the work of the heart, and it is the 

 pulsation which is set up by the wave of blood 

 propelled in part by the contraction of the blood- 

 vessels themselves which gives rise to the phenome- 

 non known as the 

 pulse. A "pulse" 

 can be found in 

 every artery of 

 the body, but it is 

 most definitely 

 felt at the wrist 

 about an inch or 

 so above the base 

 of the thumb on 

 that particular 

 side of the arm 

 where an artery 

 called the radial 

 artery lies near 



the surface of the body. To count the pulse is of 

 course a familiar and convenient fashion of ascer- 

 taining how fast the heart is beating. If we trace 

 any artery sufficiently far in its course we find it to 

 divide and sub-divide until we reach branches so fine 

 that they can only be seen by aid of the microscope. 

 A very perfect view of these finest divisions of 

 the arteries called capillaries (Fig. 15) can be ob- 

 tained by observing the web of a frog's foot under 



Fig. 15. CAPILLARIES OF THE SKIN. 



