74 ANIMAL MECHANICS 



But the coats of the artery which receive the rush 

 of blood from the heart, although thin, are limber 

 and elastic ; and by this elasticity or yielding they 

 take off or subdue the shock of the heart's action, 

 while no force is lost ; for as the elastic artery has 

 yielded to the sudden impulse of the heart, it con- 

 tracts by elasticity in the interval of the heart's 

 pulsation ; and th'e blood continues to be pro- 

 pelled onward in the course of the circulation, 

 without interval, though regularly accelerated by 

 the pulse of the heart. 



If a steam-engine were used to force water 

 along the water-pipes, without the intervention of 

 some, elastic body, the water would not flow con- 

 tinuously, but in jerks, and, therefore, a reservoir 

 is constructed containing air, into which the water 

 is forced, against the elasticity of the air. Thus, 

 each stroke of the piston is not perceptibly com- 

 municated to the conduit-pipe, because the inter- 

 vals are supplied by the push of the compressed 

 air. The office of the reservoir containing air 

 is performed in the animal body by the elasticity 

 of the coats of the arteries, by which means the 

 blood which flows interruptedly into the arteries 

 has a continuous and uninterrupted flow in the 

 veins beyond them. 



A muscle is fibrous, that is, it consists of minute 

 threads bundled together, the extremities of which 

 are connected with the tendons which have been 



