340 PEACTICAL PHYSIOLOGY 



so long as the latter is effective. For the first few beats each contraction 

 makes the heart more excitable. The same phenomenon is observed in 

 the muscle of curarised frogs with intact circulation, and also in the 



FIG. 219. Stanniused heart. Summation of stimuli. A, ineffective, and B, effective 

 stimuli. The time is marked in seconds. (L.H.) 



galvanometric records of the action current of nerve. Waller attributes 

 the staircase effect to the influence of C0 2 formed by the katabolism of 

 the active tissue. 



Summation of Stimuli. Pull out the secondary coil until the break 

 shock is just ineffective, and rhythmically stimulate the Stannius pre- 

 paration with this inadequate stimulus. The heart will respond to the 

 repeated excitation, and the first few beats will show the staircase 

 phenomenon. 



CHAPTER XXII. 

 THE HEART CONTINUED. THE ACTION OF DRUGS. 



The Suspension Method of Investigating the Action of Drugs on the 

 Frog's Heart. Pass a ligature under the vena cava inferior, where it 

 is joined by the hepatic veins and enters the sinus. Make a V-shaped 

 incision, and tie in a fine glass cannula. The cannula must be provided 

 with a rubber tube ending in a syphon tube. The tube is provided 

 with a clip, and the whole is filled with Ringer's solution, which is 

 contained in a flask. Attach a hook to the ventricle apex, and record 

 the heart by the suspension method. A slit is made into the aorta. 

 Open the clip, circulate the Ringer's fluid, and record a series of con- 

 tractions. Now replace the flask of Ringer's solution with one contain- 

 ing distilled water. The contractions will soon become less frequent, 

 less forcible, and more prolonged. Finally the heart will cease to beat. 

 It may be restored by Ringer's solution. 



