186 GEOEGE RALPH MINES. 



nature of the colloid itself in determining its relations to electro- 

 lytes. 



(1) Different surfaces in the same solution may take on different 



electrical charges. 



(2) Different surfaces possessing the same charge in some par- 



ticular solution may require different concentrations of the 

 same electrolyte to produce equal changes in their charges. 



The physiological expression of a difference in the heart membranes 

 of the kind indicated in the first of these conclusions would be of this 

 type : that one kind of heart should behave in a neutral solution 

 as another would behave in an acid or in an alkaline solution, other 

 conditions being equal. 



The heart of the mollusc Pecten maximus differs from all the vertebrate 

 hearts studied in this way. On perfusing it with " neutral saline " it 

 stops instantly in systole. It will beat, however, if we alter the P + of 



H 

 the solution in the direction of slightly increased acidity ; if the change 

 is carried a little further, diastolic arrest — the characteristic acid effect 

 — is then produced. Now the P+ of the solution which perfuses the 



H 

 heart of the living Pecten — the blood of the animal — is about 6'5 to 7, 

 that is to say, very slightly on the alkaline side of our " neutral saline." 

 How then can the heart beat in such a solution ? The answer "is 

 simple. The blood of Pecten contains a large concentration of mag- 

 nesium. The divalent Mg • • is present in sufficient concentration 

 to bring the charge of the membranes to the value required for their 

 proper differential permeability. 



The assertion that the blood of Pecten is practically the same as sea- 

 water overlooks one important fact about it. Though its osmotic 

 pressure and concentrations of Na, Mg, K, Ca, etc. are closely the same 

 as in sea-water, there is a distinct difference in the P + of the two 



H 

 fluids. The P+ of sea-water varies from about 7'9 to 8*3, while that 



H 

 of the Pecten blood is rather below 7. The difference may be demon- 

 strated by adding a drop of a solution of rosolic acid to a little sea- 

 water and to a similar quantity of the colourless blood of Pecten, The 

 colour given by the former is bright pink, that of the latter yellow or 

 yellowish pink. 



Thus we find that sea -water, despite its high concentration of 

 magnesium, on perfusion instantly stops the Pecten heart in systole. 



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