446 VETERINARY PHYSIOLOGY 



(b) Pathological. — If the auriculo-ventricular valves are 

 incompetent (p. 410) blood is forced back into the auricles 

 and veins when the ventricles contract, and the crest during 

 ventricular S3^stole becomes more and more marked (fig, 187,6). 

 The height of this crest is a good index of the amount of 

 regurgitation. 



B. Changes in Pressure Synchronous with the 



Respiration. 



1. Arterial. — Not only do rhythmic changes in the 

 arterial pressure occur with each beat of the heart, but 



Fig. 188. — Tracing of the Arterial Blood Pressure to show large respiratory 

 variations, and small variations due to heart beats upon these, and 

 the sudden fall in the pressure produced by stimulating the inferior 

 cardiac branch of the vagus nerve. 



larger changes are caused by the respirations — the rise in 

 pressure in great measure corresponding to the phase of 

 inspiration, the fall in pressure to the phase of expiration. 

 This statement is not quite accurate, as will be seen when 

 considering the influence of respiration on circulation (see p. 

 535). These variations are easily seen in a tracing of the 

 arterial pressure taken with the mercurial manometer (fig. 

 188). 



