266 VETERINAEY PHYSIOLOGY 



2nd. That the rise of the wave is much more abrupt than 

 the fall. 



3rd. That upon the descent of the primary wave there are 

 one or more secondary waves. 



One of these is constant and is very often well marked. 

 It forms a second crest, and is hence called the dicrotic wave. 



Between the chief crest and this secondary crest, a smaller 

 crest is often manifest (fig. 123, 3). From its position, it is 

 called the predicrotic wave. Sometimes other crests appear. 

 If the wave has only one crest it is called a one-crested or 

 monocrotic wave. If only the dicrotic crest is well marked 

 it is called dicrotic. If three crests are present, tricrotic ; 

 if several crests, polycrotic. 



To understand the various parts of the pulse wave it is 

 necessary to compare it with the intra-ventricular pres- 

 sure changes. This may be done by taking synchronously 

 tracings of the intra-ventricular pressure, and of the aortic 

 pressure (fig. 124). 



Such a tracing shows that at the moment of ventricular 

 systole the pressure in the aorta is higher than that in the 

 left ventricle. 



As ventricular systole advances the intra-ventricular pres- 

 sure rises and becomes higher than the aortic. At that 

 moment the aortic valves are thrown open and a rush of 

 blood takes place into the aorta, raising the pressure and 

 expanding the artery, and causing the upstroke, and crest of 

 the pulse curve. As the ventricle empties itself the intra- 

 ventricular pressure tends somewhat to fall, and, at the same 

 time, a fall in the intra-aortic pressure also takes place. If 

 all the blood does not leave the ventricle in the first gush, 

 e.y. when the intra-aortic pressure is high as compared with 

 the force of the heart (fig. 124, continuous line), there is a 

 residual outflow which arrests the diminution in the aortic 

 pressure, or may actually raise it, causing the predicrotic 

 wave. As this residual outflow diminishes, the aortic pres- 

 sure again falls and continues to fall until the moment of 

 ventricular diastole. At this instant the intra-ventricular 

 pressure suddenly becomes less than the intra-aortic, and the 

 semilunar valves are forced downwards towards the ventricles, 

 and thus the capacity of the aorta is slightly increased and 



