2/2 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. XXI. 



In our study of endo-cardiac pressure, we saw that the systolic 

 plateau sometimes has an ascending, sometimes a descending, slope 

 (see p. 244) ; we now come to the explanation of this fact. If 



after the first sudden rise of 

 pressure in the aorta the 

 peripheral resistance is low 

 and the blood can escape more 

 rapidly than it is ohrown in, 



Fig. 266,-Dicrotic pulse, the plateau will sink. If, 



on the other hand, the peri- 

 pheral resistance is high, the aortic pressure will rise as long 

 as the blood is flowing in, and we get an ascending systolic 

 plateau and an anacrotic pulse. Thus an anacrotic pulse is 

 seen in Bright's disease, where the peripheral resistance is 

 very high. 



The production of the dicrotic wave is favoured by relaxation 

 of the arterioles when the heart is beating forcibly as in fever, 

 and to a certain extent after taking alcohol. Such a pulse is 

 called a dicrotic pulse (fig. 266), and the second beat can be 

 easily felt by the finger on the radial artery. 



The main waves of a pulse tracing can 

 be demonstrated without the use of any instru- 

 ments at all by allowing the blood to spurt from 

 a cut artery on to the surface of a piece of white 

 paper travelling past it. We thus obtain what 

 is very appropriately called a hasm-autograph 

 (fig. 267), 



If a long pulse-tracing' is taken, the effect 

 of the respiration can be seen causing an in- 

 crease of pressure, and a slight acceleration of 

 the heart's beats during inspiration. This we 

 shall study at greater length in connection 

 with blood-pressure. 



The Rate of Propagation of the Pulse- Wave. The 

 Fig. 267. Hsem-auto- method of ascertaining this may be illustrated by 

 graph, to be read the use of a long elastic tube into which fluid is forced 

 from right to left, by the sudden stroke of a pump. If a series of levers 

 are placed along the tube at measured distances those 

 nearest the pump will rise first, those farthest from it last. If these are 

 arranged to write on a revolving cylinder under one another, this will be 

 shown graphically, and the time interval between their movements can be 

 measured by a time tracing. The same principle is applied to the arteries of 

 the body ; a series of Marey's tambours are applied to the heart and to 

 various arteries at known distances from the heart ; then levers are arranged 

 to write immediately under one another, as in fig. 241. The difference in 

 the time of their up-strokes is measured by a time tracing in the usual \\ ay. 



