258 PHYSIOLOGY OF THE HEART. PCH. xx. 



as a rule, the heart responds by fewer beats to a rapid than to u 

 slow rate of stimulation. 



In spite of these differences there are many and important 

 resemblances 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 Prof. Yco'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 Frog's Heart. 



If a frog's heart is simply excised and allowed to remain with- 

 out 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. Dr. Ringer has, however, shown 

 that nearly as good results are obtained with physiological saline 

 solution to which minute 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. Prof. Howell has recently shown that 

 the sinus or venous end of the heart is especially susceptible 

 of being thrown into rhythmical action by such an inorganic 

 mixture. The normal stimulus for the starting of the heart-beat 

 is therefore to be sought in the mineral constituents of the blood. 

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

 cannula (fig. 255). The heart is tied on to the end of the 

 cannula ; the fluid enters by one and leaves by the other tube. 



Numerous instruments have been devised for obtaining graphic 

 records of the heart's movements under these circumstances, but 

 we shall be content with describing three of the best. They 

 have been much used in the investigation of the effects of drugs 

 on the heart, and the results obtained have been of much service 

 to physicians. 



(i) The heart having been securely tied on to the perfusion cannula. the 

 circulating fluid is passed through it. One stem of the cannula is then 

 attached by the small side branch on the left in fig. 255 by a tube containing 

 salT solution to a small mercurial manometer, provided with a float, on 



