1016 



PHYSIOLOGY 



(4) A third positive wave (second systolic wave) which may present 

 secondary undulations. This rise of pressure is due to the gradual 

 filling of the auricles while the auriculo-ventricular valves are still 

 shut. 



(5) A negative wave fg which corresponds to the ' post-systolic 

 vacuum ' of Chauveau and Marey. At / the ventricle is entirely 

 relaxed and the auriculo-ventricular valves open, so allowing the 

 blood to flow freely from the auricle into the ventricle.* 



For comparison with this curve a tracing of the auricular pressure 

 taken by the more perfect method of Straub is given (Fig. 399). It will 

 be seen that in their general features the curves resemble one another ; 

 the more delicate manometer of Straub shows, however, greater and 



FIG. 399. Curve of pressures in left auricle of cat. (STRAUB.) 

 I, II, III Ton = 1st, 2nd, and 3rd heart sounds. 



more sudden variations than appear in Fredericq's tracing. Straub 

 draws attention to the fact that the pressure rises in the left auricle to 

 a greater extent than in the right auricle. In the latter case the big 

 veins act as a supplementary reservoir to the auricle, so that in no 

 period of the cardiac cycle need the pressure rise to any extent in the 

 latter chamber. 



NEGATIVE PRESSURE 



It will be noticed in the curve of endocardiac pressure (Fig. 396) that the 

 line drawn by the lever descends slightly below the base line at the end of 

 systole. This is the period at which a negative pressure may occur. Several 

 explanations have been suggested for the production of this negative pres- 

 sure. When the flow of fluid through a tube is suddenly interrupted, 

 the column of fluid, which has a certain degree of inertia, tends to go on, so 

 that a negative pressure is produced in its rear. If, however, the negative 



* It is evident that Fredericq's curve of the endocardiac pressure cannot be 

 quite correct at this point ; since the blood is flowing from auricle into ventricle 

 the pressure in the ventricle must be lower than that in the auricle. 



