328 PHYSIOLOGY CHAP. 



tion is completed at a mean velocity some five times greater than 

 that of the aortic circulation (Jolyet, 1880). 



(c) Law of Pressure. Seeing that pressure in the individual 

 parts of the system is determined by the sum of the resistances 

 which the blood has to overcome in order to reach the centre of the 

 circulation, i.e. the heart, it follows that it must diminish progres- 

 sively in both the aortic and the pulmonary circulations, from 

 arteries to capillaries, and from these to the veins, in which last, as 

 we have seen, it falls to zero. Since the sum of the resistances, 

 caeteris paribus, derives mainly from the friction surface, and this 

 increases slowly from large to small arteries, rapidly from the latter 

 to the capillaries, and then diminishes again slowly from capillaries 

 to veins, it follows that pressure must fall slowly in the arteries, 

 rapidly in the capillaries, and then slowly again in the veins. 



Broadly speaking, it may be assumed that the sum of the 

 resistances which the blood expelled from the right heart has to 

 overcome in order to pass through the pulmonary system, in com- 

 parison with that overcome by the left heart in traversing the 

 systemic system, is approximately proportional to the difference in 

 capacity of the two systems, so that the mean pressure in the 

 pulmonary circulation must be correspondingly less than that of 

 the aortic. The dissimilar thickness of wall in the two ventricles 

 is, as we have seen, an indicator of dissimilar work or force expended 

 by the two systems. 



III. The circulatory system differs from the artificial system of 

 rigid tubes in two important particulars the complete elasticity 

 of its walls and the intermittent character of the impulse, and 

 therefore of the output of blood from the heart. 



If the driving force exerted on the blood by the heart were 

 continuous and uniform, the elasticity of the system would have no 

 effect other than to produce a greater or less degree of vascular 

 dilatation, in proportion with the force of the prevailing pressure ; 

 but the blood-flow would remain stationary in every part, and be 

 governed by the same laws as in the system of rigid tubes. If in 

 the latter the driving force is not continuous but intermittent, the 

 current through the tubes and the outflow at the end of the system 

 are also intermittent. But when the impulsive force works inter- 

 mittently in a system of elastic tubes, then, during the impulse, a 

 portion only of that force will be employed in propelling the fluid 

 along the tubes ; while the other portion, by which the tubes are 

 dilated, will be stored up in the form of elastic tension, and given 

 back by the reaction at the close of the impulse. Owing 

 to this elasticity, the current which is intermittent at the 

 head of the system becomes remittent during its course, till at the 

 outflow or extremity of the system it is continuous and uniform. 



Marey's experimental schema is the best way of demonstrating 

 the effect of the elasticity of the vessel walls in regulating the 



