200 CIRCULATION OF THE BLOOD 



ing diastole the elastic recoil of the wall drives the blood forward into the 

 capillaries where its flow becomes constant. 



The elasticity of the arteries spares the heart a considerable quantity 

 of work. If they were rigid tubes, it would be necessary for the heart to 

 drive the entire quantity of blood contained -in them forward at once. But 

 since they are elastic, the volume of blood discharged at each systole is accom- 

 modated by the temporary enlargement of the larger arteries and is then 



PIG. 74. An artificial schema for illustration of some points in the mechanics of the circulation, 

 after Porter. The schema consists of an " auricle " in the shape of a small cylindrical reser- 

 voir, shown at the left; a "ventricle" in the shape of a small rubber pump, the pressure 

 within which is varied by means of a piston operated through an eccentric wheel which is 

 rotated by a crank; a valve between the auricle and ventricle representing the atrio- ven- 

 tricular valves; another beyond the ventricle representing the semilunar valves; tubes 

 representing blood vessels; a set of "capillaries" in the shape of a section of porous cane 

 where the " peripheral resistance " is high, and a side tube provided with a clamp by which 

 the peripheral resistance can be lowered; and mercury manometers which exhibit the 

 relative arterial and venous pressures. 



driven forward by the force stored in their walls, so that only a part of the 

 column must be moved at the time of systole (E. H. Weber). 



The rhythmical feeding of the vessels has still another advantage. The 

 "blood corpuscles are given a kind of to-and-fro motion, which, as experi- 

 ment has shown, materially facilitates the flow through the capillaries 

 (Hamel). 



3. THE FLOW OF BLOOD IN THE ARTERIES 

 A. ELASTICITY OF THE ARTERIAL WALL 



If a strip cut from an artery be stretched by adding to its load equal 

 increments of weight, the amount of lengthening produced by each successive 



