242 



VETERINARY PHYSIOLOGY 



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 mercury 



Lel't Right Right 



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. 



7. Action of the Valves of the Heart. 



A. Auricula-ventricular (fig. 117). These valves have been 



FIG. 117. State of the various parts of the Heart throughout the Cardiac Cycle. 

 1, auricular systole ; 2, beginning of ventricular systole (latent period) ; 

 3, period of outflow from the ventricle ; 4, period of residual contraction ; 

 5, beginning of ventric-ular diastole. 



already described as funnel-like prolongations of the auricles into 

 the ventricles. They are firmly held down in the ventricular 

 cavity by the chordae tendineae. When the ventricle contracts 

 the papillary muscles pull the cusps of the valves together and 

 thus occlude the opening between auricles and ventricles. The 

 cusps are further pressed face to face by the increasing pressure 

 in the ventricles, and they may become convex towards the 

 auricles. They thus form a central core around and upon 

 which the ventricles contract. 



On the left side of the heart the strong anterior cusp of 

 the mitral valve does not materially shift its position. It 



