THE CIRCULATION OF THE BLOOD. 

 CHAPTER XX. 



THE HEART. 



Bernstein's Experiment. A ligature is drawn round the ventricle of 

 a frog's heart half-way between the base and apex, and tightened 

 sufficiently to destroy the physiological but not the physical continuity 

 of the tissue at this point. The apex ceases bo beat and remains red 

 and full of blood. The bulbus aorterosus is now compressed so as to 

 obstruct the outflow and raise the intra-ventricular pressure. The apex 

 begins to beat rhythmically. The snail's heart ceases to beat when 

 emptied of blood. 



Engelmann's Experiment. The ventricle is snipped on opposite sides 

 so as to produce several interdigitating cuts. The wave of contraction 

 still passes down the zigzag strip. All possible nervous channels of 

 conduction are supposed to be divided by these means, and the experi- 

 ment is taken as proving the conduction of the excitatory state from 

 muscle cell to muscle cell. 



The Action of the Valves and the Influence of the Diastolic and 

 Systolic Pressures on the Output and the Absolute Force of the 

 Heart. A large frog or toad is pithed, and the heart exposed. A 

 ligature is passed under each aorta, and the right aorta is tied. 

 Ligatures are placed under each vena cava superior, and both these 

 vessels are tied. The ligatures may be threaded through the eye of a 

 needle, and the point of the needle stuck into a wooden handle. A 

 ligature is also passed under the vena cava inferior. A V-slit is made 

 into this with sharp scissors and a cannula inserted. The cannula is 

 provided with a rubber tube on which a clip is placed. This tube is 

 connected by a T-piece to a reservoir and a vertical tube. The latter 

 is provided with a paper mm. scale (Fig. 214). The reservoir, tube, 

 and cannula, are filled with Ringer's solution before the cannula is intro- 

 duced. The left aorta is now slit and a fine cannula introduced. This 

 is connected by a tube to a T-piece. One branch of the T-piece is 

 closed by a clip and the other is joined to a vertical tube 1 metre in 

 height. This tube is provided with a mm. scale. The aortic cannula and 

 tube are filled with Ringer's solution before the cannula is introduced. 



