124 NUTRITIONAL PHYSIOLOGY 



branch and thence along the main pipe. The intermittent 

 delivery through the valve is transformed more or less 

 successfully into a continuous flow through the remote 

 outlet. 



The neutralizing of intermittency in the blood system is 

 referable to the same principle, but the elastic compensator 

 is not to be found as a single localized feature; it is discov- 

 ered in the universal capacity of the arteries to stretch and 

 to regain their former size. At the beginning of the aorta 

 or of the pulmonary artery we have complete intermittency, 

 the blood alternately forging ahead and halting. But each 

 portion of blood ejected from the heart finds room for 

 itself partly by distending the arteries and not altogether 

 by driving forward the blood which is before it. Hence, 

 when the ventricles relax and the outpouring of blood 

 ceases there is still an onward movement in the smaller and 

 more distant arteries, because the larger ones, which were 

 momentarily overdistended, are now contracting and send- 

 ing along part of their accumulation. The farther we go 

 from the heart the more largely the driving of the blood is 

 to be attributed to the reaction of the stretched arterial 

 walls and the more nearly uniform it becomes. This does 

 not mean that the whole power keeping up the circulation 

 is not to be sought in the heart-beat ; it merely means that 

 this energy may be stored temporarily by these elastic 

 structures and rendered back again. 



The facts we have been treating may be expressed in 

 another way. The arterial tree forms a reservoir of con- 

 siderable capacity. Within it is an amount of blood so 

 large that the single contribution of the ventricle makes 

 a rather small addition to it. The escape of the blood 

 through the terminal twigs cannot cease while there is 

 so much stored under a high pressure in the aorta and its 

 branches. The heart may omit or "drop" a beat without 

 noticeably diminishing the flow through the capillaries. 

 A standstill will be reached only when the arteries have 

 attained a degree of contraction such that the internal 

 pressure is no higher than that in the veins. The homely 



