260 PHYSIOLOGY OF THE HEART [CH. XXI. 



as t do occur are differences of degree and detail rather than of kind, 

 and Wooldridge, many years ago, succeeded in performing the 

 Stannius experiment on the heart of a mammal. 



The Isolated Heart. 



If a frog's heart is simply excised and allowed to remain without 

 being fed, it ceases to beat after a time varying from a few minutes 

 to an hour or so. But if it is fed with a nutritive fluid, it will con- 

 tinue to beat for many hours. Other substances such as drugs may 

 be added to the perfusion fluid, and their effects noted. The fluid 

 may be passed through the heart, and the apparatus employed may 

 be exemplified by the following diagram of Schdfer's heart -plethys- 

 mograph (fig. 234). A frog's heart is tied on to the end of a perfusion 



FIG. 234. Schafer's Heart-plethysmograph. 



caimula, one tube of which serves for the fluid to enter, the other for 

 it to leave. The end of the cannula projects into the ventricle ; the 

 frog's heart, it should be remembered, possesses no coronary vessels ; 

 the spongy texture of the cardiac tissue enables it to take up what it 

 requires from the blood in its interior. 



The cannula passes through the well-fitting stopper of an air-tight 

 vessel containing oil. On one side of the vessel is a tube, in which 

 a lightly moving piston is fitted ; to this a writing-point is attached. 

 The piston is moved backwards and forwards by the changes of 

 volume in the heart causing the oil to alternately recede from and 

 pass into this side tube. The corresponding tube on the other side 

 can be opened and the tube with the piston closed when one wishes 

 to cease recording the movements. It is with instruments of this 

 kind that a vast amount of valuable work was performed, and the 

 name of the late Dr Sydney Einger is specially connected with 

 investigations of drug action by means of this method. 



The best nutritive fluid to employ is undoubtedly the natural 

 fluid, the blood. But in order to use blood there are practical 

 difficulties ; it is difficult, for instance, to obtain much blood from a 

 frog; it is difficult to prevent it from clotting, and if agents are 

 added to check clotting, such agents usually act deleteriously in the 

 cardiac tissue. The blood of another animal may not be altogether 



