THE CIRCULATION 227 



apparently synchronous with the closure of the auriculo- 

 ventricular valves. From this point the pressure in the 

 auricles rises until the moment when the ventricles relax, 

 when another fall in the pressure is observed. The pressure 

 remains about constant from this point until the next 

 auricular contraction. 



C. Pressure in the Ventricles (continuous line in Fig. 114). 

 The intra-ventricular pressure suddenly rises at the moment 

 of ventricular systole to reach its maximum. From this it 

 falls, but 4, he fall is gradual, and is interrupted by a more or 

 less well-marked period during which the pressure remains 

 constant. As the ventricles relax the pressure suddenly falls 

 to below zero, and then rises to a little above zero, at which it 

 is maintained until the next ventricular systole. The dia- 

 stolic expansion of the ventricle is in part due to the elasticity 

 of the muscular wall, and in part to the filling of the coronary 

 arteries which takes place only as the muscular fibres relax. 



D. Pressure in the Arteries (dot-dash line in Fig. 114). 

 There is a sudden rise in the aortic pressure 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 systolic elevation. 



In the dog the extent of variation of the pressure in 

 auricles and ventricles is roughly as follows measured in 

 millimetres of Hg 



Left Right Eight 



Ventricle. Ventricle. Auricle. 



Maximum . +140 +60 +30 



Minimum . 30 15 7 



These changes in the pressure in the different chambers 

 are due 



1st To the alternate systole and diastole of the chambers, 

 the first raising, the second lowering the pressure in the 

 chambers. 



2nd. To the action of the valves. 



