256 PHYSIOLOGY OF THE HEART [CH. XX. 



stimulus strong enough to produce a contraction at all brings out as 

 big a beat as the strongest. The second is, that the heart muscle 

 has a long refractory period ; that is to say, after the application of 

 a stimulus, a second stimulus will not cause a second contraction 

 until after the lapse of a certain interval called the refractory period. 

 The refractory period lasts as long as the contraction period. The 

 third difference depends on the second, and consists in the fact that 

 the heart muscle can never be thrown into complete tetanus by a rapid 

 series of stimulations ; with a strong current there is a partial fusion 

 of the beats, but this is entirely independent of the rate of faradisa- 

 tion. Indeed, as a rule, the heart responds by fewer beats to a rapid 

 than to a slow rate of stimulation. 



In spite of these differences there are many and important re- 

 semblances between heart muscle and voluntary muscle. 



The thermal and chemical changes are similar ; there is a using- 

 up of oxygen and a production of carbonic acid and sarco-lactic acid. 

 The using-up of oxygen was well illustrated by an experiment of 

 Yeo's. He passed a weak solution of oxy-hsemoglobin through an 

 excised beating frog's heart, and found that after it had passed 

 through the heart, the solution became less oxygenated and venous 

 in colour. 



The electrical changes are also similar, and have already been 

 dwelt upon in Chapters XII. and XIV. 



Instruments for Studying the Excised 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 continue 

 to beat for many hours. A very good nutritive fluid is defibrinated 

 blood diluted with twice its volume of physiological saline solution. 

 Kinger has, however, shown that nearly as good results are obtained 

 with physiological saline solution to which minule quantities of 

 calcium and potassium salts have been added ; in other words, the 

 inorganic salts of the blood will maintain cardiac activity for a time 

 without the addition of any organic material. Howell has shown 

 that such an inorganic mixture is especially efficacious in throwing 

 the sinus or venous end of the heart into rhythmical action. The 

 normal stimulus for the starting of the heart-beat is therefore to be 

 sought in the mineral constituents of the blood. These mineral 

 constituents in solutions are broken up into their constituent ions ; 

 and of these the sodium ions are the most potent in producing 

 rhythmical activity. (Loeb.) 



The fluid is passed through the heart by means of a perfusion 

 cannula (fig. 263). The heart is tied on to the end of the cannula ; 



