CH. XXI.] THE EXCISED MAMMALIAN HEART 261 



innocuous, and this is specially the case if that blood has been pre- 

 viously whipped, and the fibrin removed. Physiologists therefore 

 owe Dr Einger a deep debt of gratitude for his discovery of the 

 solution now known as Ringer's solution. This is physiological salt- 

 solution to which minute quantities of calcium and potassium salts 

 have been added. In other words, the inorganic salts in the propor- 

 tion occurring in the blood will maintain cardiac activity for a long 

 time without the addition of any organic material. These salts are 

 not nutritive in the strict sense, but they constitute the stimulus for 

 the heart's action. Howell of Baltimore 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 thus to be sought in 

 the mineral constituents of the blood. These mineral compounds in 

 solution are broken up into their constituent ions; and of these, 

 sodium ions are the most potent in maintaining the osmotic con- 

 ditions that lead to irritability and contractility. A solution of pure 

 sodium chloride, however, finally throws the heart into a condition of 

 relaxation; hence it is necessary to mix with it small amounts of 

 calcium ions to restrain this effect. Potassium is not absolutely 

 necessary, but it also favours relaxation during diastole. Calcium, on 

 the other hand, is the element which produces contraction, and if 

 present alone or in excess, will produce an intense condition of tonic 

 contraction known as calcium rigor. 



Some physiologists have manifested a hesitation in accepting the simple view 

 that the various kations mentioned actually originate the heart-beat, and have 

 advanced the hypothesis that they influence a mysterious factor they have named 

 the inner stimulus. What this inner stimulus is, is entirely unknown, and whether 

 or not it is connected with one or more of Langley's receptive substances is equally 

 a matter of speculation. If it exists it is not able to originate cardiac rhythm in 

 the absence of the appropriate inorganic salts. 



The Excised Mammalian Heart. During the past few years it 

 has been shown that the mammalian heart can also be kept alive 

 and active after it has been excised. Valuable as the results have 

 been from a study of the frog's heart, one can hardly doubt that 

 those which one hopes to obtain in the future from a similar study 

 of the mammal's heart will be still more important, and still more 

 trustworthy for the drawing of deductions useful to man. Already 

 the new method has shown its usefulness not only in reference to 

 the metabolism occurring during normal cardiac activity, but also 

 from the pharmacological point of view. 



In order to maintain the action of the excised mammalian heart 

 certain precautions must be taken 



1. The perfusion fluid must be maintained at or about body 

 temperature (37 C.). 



