402 VETERINARY PHYSIOLOGY 



about constant from this point until the next auricular 

 contraction. 



C. Pressure in the Ventricles (continuous line in fig. 

 173). — The intra-ventricular pressure rises slightly during 

 auricular systole. It rises suddenly at the moment of 

 ventricular systole to reach its maximum, but on the trace 

 there is sometimes a shoulder due to the opening of the semi- 

 lunar valves. It then falls, but the fall is gradual, and is 

 interrupted by a more or less well-marked period during 

 which the pressure remains constant. When the ventricles 

 relax, the pressure suddenly falls to zero, then rises a 

 little, and is maintained until the next ventricular systole. 

 The diastolic expansion of the ventricle is due chiefly to 

 the inflow of blood from the auricles and veins, possibly in 

 part to the elasticity of the muscular wall, and to the filling 

 of the coronary arteries which takes place in diastole. 



D. Pressure in the Arteries (dot-dash line in fig. 173). — 

 This, since it is always high and undergoes no great and 

 sudden variations, may be measured by means of a mercury 

 manometer. The aortic pressure is high throughout. 

 There is a sudden rise soon after the beginning of ventricular 

 systole, as the blood rushes out of the ventricles. The 

 pressure then falls, but the fall is not steady. Often it is 

 interrupted by a more or less marked increase corresponding 

 to the later part of the ventricular contraction. At the 

 moment of ventricular diastole the fall is very sharp and is 

 interrupted by a well-marked and sharp rise. Following 

 this, the fall is continuous till the next svstolic eleva- 

 tion. 



These changes in the pressure in the different chambers 

 are due to— 



\st. The alternate systole and diastole of the chambers, 

 the first raising, the second lowering,, the pressure in the 

 chambers. 



^2nd. The action of the valves. 



7. Action of the Valves of the Heart. 



A. Auriculo-ventricular (fig. 174). — These valves have 

 already been described as funnel-like prolongations of the 



