ii2 THE ENGINES OF THE HUMAN BODY 



presses the cushion of air which thus stores up part of 

 the strength of each stroke. The air in the chamber 

 becomes a compressed elastic cushion. The water in 

 the air chamber being thus maintained under a steady- 

 pressure flows continuously, instead of in jets. Nature 

 overcame the intermittent outflow from the heart, not 

 by using an air chamber, but by constructing a chamber 

 with elastic walls. The aorta stands for the air or com- 

 pression outflow chambers ; its walls are made of a 

 remarkably elastic material. Part of the strength of each 

 stroke of the heart is spent in expanding the aorta ; at 

 the end of the stroke the recoil of the aortic wall presses 

 the blood onwards and thus maintains a steady flow 

 between the strokes of the heart. In the arteries, par- 

 ticularly in the smaller arteries, the elastic coat is 

 strengthened by another containing muscular tissue. 

 The elastic tissue of which the wall of the outflow 

 or aorta chamber is composed is unfortunately a very 

 delicate material, and even by the age of 40 it has usually 

 lost its perfect elasticity. Hence after 40 the work of 

 the heart is harder ; its outflow box is less resilient, less 

 able to help the onflow between the pump strokes. 



In order to maintain a steady flow of blood through 

 the capillary fields which permeate the length and breadth 

 of the entire body, it is clear that a high head of pressure 

 must be maintained in the compression or aortic chamber 

 of the human pump. Harvey estimated the amount of 

 blood which the heart throws into the aorta in a given 

 time, but he made no attempt to find out the head of 

 pressure maintained in the aorta. That was measured 

 a century after Harvey's time by a country clergyman 

 named Stephen Hales — a natural philosopher of the most 

 lovable and ingenious kind. He tied a glass tube into 

 an artery which sprang from the aorta of an aged mare, 

 and holding it upright noticed the height to which the 

 blood rose. If this experiment could be made in a 

 young man in good health, the column of blood, as 

 we have learned in another and less painful way, would 

 rise 64 inches (160 ctm.) above the level of the heart. 



