98 PRACTICAL PHYSIOLOGY 



A fine pin is bent into the shape of a hook and passed through the tip 

 of the apex of the ventricle. A thread is attached to this hook and to 

 the lever. The tissues round the base of the heart are pinned down to 

 the cork plate on which the frog rests. 



FIG. 91. Contractions of the frog's neart. A = auricular, V= ventricular con- 

 traction. The time is marked in seconds. The curve should be read from left 

 to right. (L.H.) 



Fio. 92. Contraction of the frog's heart. The curve should be read from right to 

 left. The effect of rendering heart bloodless. Note the plateau on the top of the 

 normal ventricular curve, and the pointed top after the blood has escaped at the point 

 marked by the star. Time marked in fifths of seconds. (L.H.) 



The lever is provided with a long light straw. A fine wire spring is 

 attached to the lever, and the heart pulls against this. Adjust its 

 tension so that the lever is horizontal, 1 and record the heart-beats on a 



1 With the form of heart-lever (Fig. 90) the contraction is represented by the 

 down-stroke; with the lever (Fig. 93) the contraction is indicated by the up- 

 stroke. The curves obtained with the former lever can be best compared with 

 those made with the latter by turning the tracing upside down and reading from, 

 right to left. 



