THE MECHANISM OF THE HEART PUMP 1029 



the simple current measurer described on p. 1001 can be inserted in the 

 arterial circuit at X, so as to give immediately the output of the left 

 ventricle. 



Other methods are based upon the application of the plethysmo- 

 graphic method to the heart in situ. We may either, as in Tigerstedt's 

 method, employ the pericardium itself filled with oil or air as the 

 oncometer, and register the changes in the volume of the heart by 

 connecting the cavity of the pericardium with some form of piston 



FIG. 406. Diagram of Roy's cardiometer. 



On the right of the figure are the two quarter-spheres which are clamped 

 on to the pericardium at the root of the heart. 



recorder, or we may make use of Roy's cardiometer. This is a brass 

 sphere in three segments. The two quarter-spheres (Fig. 406) are 

 applied round the base of the heart and clamped together, the cut 

 pericardium being attached to their constricted neck. The third 

 segment, a hemisphere, is then applied over the apex of the ventricles 

 and clamped to the parts already in situ. Attached to the centre of 

 this hemisphere is a modified piston-recorder containing a piston 

 working in oil, with which the whole of the apparatus is filled. At a 

 is a spring which can be adjusted so as to exercise a constant pull 

 upon the piston and reproduce to some extent the negative pressure 

 under which the heart normally works. The piston-rod carries a lever 



