440 VETERINARY PHYSIOLOGY 



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

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

 wave (fig. 184, c). 



Between the chief crest and this secondary crest, a 

 smaller crest is often manifest (fig. 184, A., h). From its 

 position, it may be called the predicrotic wave. If 

 the wave has only one crest the pulse is called a one- 

 crested or monocrotic wave. If the dicrotic crest is well 

 marked it is called dicrotic. 



That the wave actually has the characters disclosed by a 

 sphygmographic tracing may be demonstrated by letting the 

 blood from a cut artery play upon a moving surface when a 

 Hcemautograph showing the waves is produced. 



To understand the various parts of the pulse wave, it is 

 necessary to compare it with the changes in the intra- 

 ventricular pressure throughout the cardiac cycle. This 

 may be done by taking synchronously tracings of the intra- 

 ventricular pressure and of the aortic pressure (fig. 173, 

 p. 401). 



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 

 pressure 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. In a sphygmographic tracing this crest is 

 exaggerated by the inertia of the instrument (fig. 184). 

 After the ventricle has emptied itself, the intra-ventricular 

 pressure tends somewhat to fall, and, at the same time, a 

 fall in the intra-aortic pressure begins, and goes on till 

 ventricular diastole, while the elastic wall of the artery 

 recovers and reduces the size of the vessel. With diastole, 

 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 the pressure falls sharply. This fall 



