120 PRINCIPLES OF PHYSIOLOGY 



that the blood must flow in the required 

 direction. 



62. The hydraulic principles of the circu- 

 lation are remarkable. Blood must flow from 

 situations of higher pressure to situations of 

 lower pressure. High pressure is kept up in 

 the great arteries by the contractile action of 

 the left ventricle of the heart acting like a 

 force pump and, with each stroke of contrac- 

 tion, throwing blood into them, so that, in a 

 sense, they are over-distended. During the 

 intervals between the heart beats, the walls 

 of the arteries recover themselves by the 

 resiliency of their elastic coats. This disten- 

 sion and elastic recoil constitutes the pulse, 

 which is a wave of motion along the walls of 

 the arteries, starting from the heart, travel- 

 ling onwards with a certain velocity, and 

 becoming smaller and smaller until there is 

 no pulse in the smallest vessels, the capil- 

 laries. In consequence also of the loss of 

 energy by friction, and by the distension of 

 the arterial coats, the movement of the blood 

 becomes slower and slower until, in the capil- 

 laries, the blood is slowly meandering onwards 



