406 



VETERINARY PHYSIOLOGY 



this case, the trace of the intra-ventricular pressure is as in 

 fig. 175, a, with a well-marked Period of Residual Con- 

 traction. It is not the absolute force of the cardiac 

 contraction or the absolute intra-arterial pressure which 

 governs this, but the relationship of the one to the other. 

 The heart may not be acting very forcibly, but still, if the 

 pressure in the arteries is low, its action may be relatively 

 strong. 



Fig. 175. — Diagram to show the Relationship of the Pulse Wave to the 

 Cardiac Cycle and the effect of altering the relationship between the 



activity of the heart and the arterial blood pressure. h is 



the curve of intra-ventricular pressure, and h^ is a pulse curve 



with an active heart and a relatively low arterial pressure. — a and a^ 

 are the same with a sluggish heart and a relatively high arterial 

 pressure. The period of outflow under each condition is also shown. 



The Coronary Arteries, unlike all the other arteries, are 

 filled during ventricular diastole. During systole they are 

 compressed by the contracting muscle of the heart, and it is 

 only when the compression is removed in diastole that blood 

 rushes into them. This helps to dilate the ventricles. 



The interpretation of the various details of the Cardiogram 

 (fig. 173) is now rendered more easy. The ventricles, still 

 full of blood, are suddenly pressed against the chest wall in 

 systole. As the blood escapes into the arteries they press 

 with less force, and hence the sudden slight downstroke. 

 But, so loijg as the ventricles are contracted, the apex 

 is kept tilted forward, and hence the horizontal plateau is 



