THE MOVEMENT OF THE BLOOD. 159 



difference in pressure and inversely proportional to the resistance en- 

 countered by the blood-current. 



The difference in pressure that produces the movement of the blood 

 is created by the heart. In the greater as well as in the lesser circulation 

 the point of highest pressure is at the root of the arterial system, and the 

 point of lowest pressure at the terminal portions of the veins. Hence, 

 the blood, constantly flows from the arteries through the capillaries 

 and into the large venous trunks. 



The heart maintains the difference in pressure necessary for the 

 circulation of the blood by throwing a certain quantity of blood into 

 the root of the aorta at each systole, after first withdrawing a like quan- 

 tity of blood from the terminations of the venous trunks by means of 

 the diastole of the auricles. 



To these laws relating to the causes of the movement of the blood- 

 mass, and which were formulated chiefly by E. H. Weber, must be 

 added an important one by Bonders. That investigator demonstrated 

 that the heart, by the work it performs, not only produces the difference 

 in pressure necessary for the movement of the blood, but it also increases 

 the mean pressure existing in the circulatory system. The terminal 

 portions of the large veins that empty into the heart are larger and 

 more elastic than the initial portions of the arteries; and if the heart 

 transfers the same mass of fluid from the veins into the beginnings of the 

 arteries, the arterial pressure must be increased in greater degree than 

 the venous pressure is diminished, and the pressure as a whole must be 

 raised. 



The movement of the blood-mass would be jerky or intermittent 

 ( i ) if the walls of the tube were rigid ; for pressure exerted on the fluid 

 contained in rigid tubes is propagated at once throughout the entire 

 length of the tubes, and the movement of the fluid ceases simultaneously 

 with the impact that causes the increase in the pressure. (2) The move- 

 ment would be intermittent also within elastic tubes if the interval 

 between two successive systoles were longer than the duration of the 

 movement of the column necessary to equalize the difference in pressure 

 produced by the systole. If, however, this interval is shorter than is 

 necessary for equalizing the pressure, the current becomes continuous. 

 The more rapidly systole follows upon systole, the greater will be the 

 difference in pressure, the elastic walls of the arterial tubes at the same 

 time undergoing greater distention. In the continuous current thus 

 produced the sudden increase in pressure caused by the systolic injec- 

 tion of a mass of blood corresponding to the size of the ventricular cavity 

 can always be recognized as an intermittent, jerky acceleration of the 

 current (pulse). 



This intermittent acceleration of the current is propagated along the 

 arterial pathway with the velocity of the pulse- wave, as both are due 

 to the same cause. Each pulse-beat is therefore attended with a tem- 

 porary, rapidly advancing acceleration of the fluid-particles. Just as 

 the form of the pulse-movement, however, is not simple, so also is this 

 pulsatory acceleration of the current not simple. The latter appears in 

 the complicated form of the current pulse-curve, which likewise exhibits 

 the primary elevation and the recoil-elevation like a (pressure-)sphygmo- 

 graphic curve. Every up-stroke in the limb of the curve corresponds 

 to an acceleration and every down-stroke to a retardation of the moving 

 particles of fluid. 



