THE CIRCULATION OF THE BLOOD 317 



sole means : it is aided by the contractions of the elastic 

 walls of the arteries themselves, the movements of the 

 chest in respiration, and the attraction of the tissues for 

 the arterial blood in the capillaries. In the chick, the 

 blood mores before the heart begins to beat ; and if the 

 heart of 'an animal be suddenly taken out, the motion in 

 the capillaries will continue as before. It has been 

 estimated that the force which the human heart expends 

 in twenty-four hours is about equivalent to lifting 217 

 tons one foot. 



