THE CAUSATION OF THE HEART-BEAT 



1081 



The influence of the chemical composition of the medium on the contraction 

 of the heart may be investigated in the following ways : 



One of the simplest methods is that employed by Gotch, represented in the 

 diagram (Fig. 443). The apparatus consists of a small glass jar with inlet and 

 outlet tubes. A disc of cork is fixed on to a brass rod so that it can be let down 

 into the fluid. On the upper end of the brass rod is poised a light lever with a 

 paper point. To fix the heart in the apparatus, the top of the ventricle is trans- 



FIG. 443. Gotch's frog heart apparatus. 



fixed by a fine hook to which is attached a thread connected with the lever. 

 The heart is fastened to the cork by a pin through the bulbus aortse. The glass 

 jar is filled with the fluid whose action it is desired to investigate. It is usual to 

 start with Ringer's 'fluid in order to obtain a normal beat, and then to try in 

 turn the various constituents of tlu's fluid. 



Another method of investigating the action of the heart of cold-blooded 

 animals is by perfusing the heart cavities with the fluid under investigation. 

 Two forms of perfusion are made use of. In the method first introduced by 

 Williams a double cannula is tied into the ventricle, the rest of the heart being 

 cut away. The tubes leading to and away from the perfusion cannula are armed 

 with valves so as to allow the fluid only to pass in one direction. The contractions 

 of the ventricle may be recorded either by connecting the outgoing tube with a 

 manometer, which may be a mercurial or a membrane manometer, or by con- 

 necting some form of recording apparatus with the vessel in which the heart is 



