400 VETERINARY PHYSIOLOGY 



which consists of a tube or cannula with a sharp pointed 

 trocar, which can be thrust out of the end of the tube to 

 perforate the chest and ventricular wall, and then retracted 

 through a tap, which can be closed. On the tube is a 

 membrane carrying a small mirror, from which a beam of 

 light may be reflected on to a sensitive paper covering a 

 moving surface so that the variations of pressure are 

 photographed. 



(2) Results. — A. Pressure in the Great Veins (small 

 dotted line in fig. 173). — The pressure in these is so low 

 and undergoes such small variations that it may be 

 investigated by a water manometer. 



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 



EF 



D 



Fig. 172. — Piper's Stilette Manometer. A., trocar for puncturing (with- 

 drawn) ; C, tap to close cannula; E., rubber membrane with 

 mirror, F. 



pressure falls, but, as the auricles fill up, it again rises. 

 When the ventricles relax 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. 173). — 

 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, but the fall is 

 interrupted by a rise due to the upward pressure from the 

 closed auriculo-ventricular valves. It reaches its lowest 

 level early in ventricular systole. 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 again rises slightly and remains 



