THE RELATION OF THE HEART-BEAT TO ELECTROLYTES. 187 



But if by the addition of a little dilute hydrochloric acid we change 

 the P+ of the sea-water to the same value as that found for the blood 



H 

 of Pecten (about 1'5 cc. of "l M HCl in 100 cc. of sea-water), we have at 

 once a solution which sustains the beat of the heart admirably. A solu- 

 tion of the same composition as this but without magnesium causes the 

 heart to stop in systole. Further, as is predicted theoretically, it is 

 possible to replace the Mg by a very small concentration of a simple 

 trivalent ion. Thus the Pecten heart which has stopped beating in 

 " neutral saline " can be made to beat again and be kept beating for a 

 considerable time by the addition to the solution of a concentration 

 of about -00003 M of Ce " ' • or Nd • • '. Here we have one tri- 

 valent ion doing the work of some 40,000 divalent ions, and it is 

 therefore not surprising that the solution with rare earth does not, as 

 a rule, keep the heart going so regularly as the solution with mag- 

 nesium. Higher concentrations of the simple trivalent ions (e.g. 

 •0001 M) stop the Pecten heart in diastole exactly in the same way as 

 a too acid solution. 



We may express the difference between the heart of Pecten and the 

 hearts of the vertebrates which have been studied, by saying that the 

 membranes have different iso-electric points. This kind of difference 

 is most probably related to a difference in the chemical composition of 

 the membranes ; it being a well-established fact that different protein 

 substances found in the animal kingdom do show differences of this 

 kind, according to the predominance of the " acidic " or " basic " amino- 

 acids included in their composition. 



It is by no means a new suggestion that the differences between 

 different species are at root differences in the chemical composition of 

 their tissues ; but as far as I am aware it has not before been pointed 

 out by what mechanism such differences can affect the physiological 

 behaviour of the living heart. 



The second conclusion formulated above as to the relations between 

 <lifferent surfaces and the same electrolytes finds biological application 

 in the differentiation of more closely allied species. 



The hearts of the elasmobranchs Paia and Scyllium are kept Ideat- 

 ing satisfactorily for many hours by perfusion with the same " neutral " 

 solution containing sodium, potassium, calcium, and magnesium 

 chlorides and urea, the solution being thoroughly aerated and of P + 



H 

 about 6 '5. The concentration of Mg in the solution generally used 

 was -005 M. Kemoval of the Mg from the solution only causes the 

 beats to cret rather faster. Increase of the concentration of magnesium 



