240 VETERINARY PHYSIOLOGY 



the mercury cannot respond to them on account of its 

 inertia. 



Various means of obviating these difficulties have been 

 devised. One of the best is to allow the changes of pressure 

 to act upon a small elastic membrane tested against known 

 pressures. A tube is thrust through the wall of the heart and 

 connected with a tambour covered by a membrane to which a 

 lever is attached. 



A. Pressure in the Great Veins (small dotted line in fig. 

 116).. When the auricles contract, the flow of blood from the 

 great veins into these chambers is arrested, and, as a result, 

 the pressure in the veins rises. As the auricles relax the 

 blood is sucked from the veins and the pressure falls, but, as 

 the auricles fill up, it again rises. When the ventricles relax 

 and suck blood from the auricles, blood again flows in from the 

 great veins and the pressure falls, again to rise as the auricles 

 and veins are both filled up, towards the end of the pause. 



B. Pressure in the Auricles (dash line in fig. 116). At the 

 moment of auricular contraction there is a marked rise in the 

 intra-auricular pressure. When the auricular systole stops, 

 the pressure falls rapidly, reaching its lowest level when 

 the ventricles are throwing their blood into the arteries. 

 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. 116). 

 The intra-ventricular pressure suddenly rises at the moment 

 of ventricular systole to reach its maximum. From this it falls, 

 but the 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 diastolic 

 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. 116). 

 The aortic pressure is high throughout. There is a sudden rise 

 as the blood rushes out of the ventricles. The pressure then 



