CHAP, iv.] THE VASCULAR MECHANISM. 



243 



By means of a short cannula introduced through a large vessel, or 

 directly, as a trocar, through the walls of the ventricle (or auricle), the 

 blood in the cavity is brought to bear on an easily moving piston. The 

 movements of the piston are recorded by a lever, and the evils of 

 inertia are met by making the piston and lever work against the 

 torsion of a steel ribbon, the length of which, and consequently 

 the resistance offered by which, and hence the excursions of the piston 

 can be varied at pleasure. 



The curves obtained by this method vary according to circum- 

 stances. We may take as fair examples two curves from the left 

 ventricle, one (Fig. 39 A) of a rapidly beating, and the other 

 (Fig. 39 B) of a slowly beating heart. 



a. 



t> b' a.' 



FIG. 39. CORVES OF ENDOCABDIAC PRESSURE. FROM LEFT VENTRICLE OF DOG. 



A. a quickly beating, B. a more slowly beating heart. 

 The letters in this, and the succeeding Figs. 40, 41 are explained in the text. 



133. In attempting to interpret these curves with the view 

 of learning the changes of pressure taking place in the heart, it is 

 desirable to study them in connection with the tracing of which 

 we have already spoken, Fig. 40, taken by means of a light lever 

 placed on the exposed ventricle and which as we have seen is a 

 curve of the changes taking place in the front-to-back diameter 



162 



