BLOOD PRESSURE 359 



in the arterial blood pressure must become the greater, the longer 

 the interval between two successive ventricular discharges. This 

 rule, however, is not infallible, because in many cases a fall in pressure 

 resulting from an undue slowness of the heart, may be effectively 

 counteracted by an increase in the peripheral resistance. A com- 

 pensation of this kind takes place very frequently, but naturally, it 

 cannot overcome the dynamical disturbances produced by an exces- 

 sively infrequent heart. 



The elastic power of the vascular system lessens the work of the 

 cardiac musculature very materially, because it insures a constancy of 

 flow without necessitating an extra expenditure of energy on the part 

 of the heart. As each cardiac output is accommodated in the arteries, 

 their walls are forced outward. In this way, a large part of the work 

 of the heart is converted into potential energy in the form of elastic 

 tension which is utilized later on during the diastolic interim, and hence, 

 the work of this organ is actually distributed over more than twice 

 the time actually consumed in its muscular contraction. This enables 

 the heart to obtain the rest required for its anabolism. The importance 

 of the elasticity is also elucidated by the fact that a rigid vascular 

 system immediately converts the otherwise constant flow into one 

 possessing remittent and intermittent qualities. Each systole then 

 gives rise to a quick onrush of blood which is soon followed by a slowing 

 and a cessation of the flow. Very high and very low pressures are 

 then obtained alternately. 



The property of elasticity is possessed in a slight measure by all 

 types of cells and not only by those composing the elastic tissues. For 

 this reason, it cannot be said to be wholly lacking in other segments of the 

 vascular system, although we have just seen that it becomes of greatest 

 dynamical importance in the central arteries. The structure of the 

 capillaries is such that varying quantities of arterial blood can readily 

 be accommodated in them by simply changing the size of their lumen. 

 These perfectly passive changes are made possible by the fact that 

 they are distensible, although their elastic power is insignificant.. In 

 this connection, mention should also be made of the claim of Strieker 

 and others,^ that the capillary lining cells possess contractile 

 qualities which betray themselves in active variations of their 

 thickness at the sites of the different nuclei. The evidence so far 

 presented in favor of this view, does not seem sufficiently conclu- 

 sive to warrant further discussion of this subject. Somewhat dif- 

 ferent conditions are met with in the veins. Here the elasticity again 

 plays a more important part, because these channels are large and 

 are structurally in a position to oppose the pressure by a very moderate 

 recoil. It is to be noted especially, however, that the size of the venous 

 bloodbed is very largely dependent upon the quantity of the blood 

 transferred to them by the arteries. They themselves cannot vary 

 their caliber in an active way by vasomotor activity. 

 ^ Berichte, Akad. der Wissensch., Wien, 1865. 



