No. 599] OSMOTIC PRESSURE OF THE BLOOD 



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marine teleosts. The range is very much more restricted 

 in fresh-water teleosts and higher forms. Protoplasm is 

 an ever-changing substance. There is a constant ebb and 

 flow. Protoplasm of the higher forms has evolved through 

 long ages to a condition wherein it is associated with the 

 same salts it was entirely surrounded by when it first 

 began to be. The amounts have changed, but the propor- 

 tions have remained quite constant and the kinds have 

 remained the same as those in the sea. And that is why 

 the surgeon must inject a 0.9 per cent, saline solution into 

 the veins of his patient suffering from hemorrhage. And 

 that is why human blood has a certain osmotic pressure. 

 Macallum ascribes the first great reduction in salts which 

 took place in the elasmobranch to be due to the kidney, 

 whose primary function was not the elimination of the 

 wastes of metabolism, but the regulation of the concentra- 

 tion and composition of the salts of the blood. The elas- 

 mobranch kidney is very inert and sluggish in the matter 

 of the elimination of the organic wastes. The teleosts 

 acquired the habit of still further keeping down the saline 

 content while at the same time they eliminated the urea 

 readily. However, I do not see that the process is neces- 

 sarily limited to the kidneys alone. A thorough study of 

 the elasmobranchs and teleosts is needed to throw light 

 on this puzzle. I can see why the migratory habits of 

 teleosts or teleost ancestors (ganoids) would account for 

 reduction in salt content of the blood, but this throws no 

 light on the reduction of salts in elasmobranch blood as 

 compared with invertebrate blood. Nor does Maca am 

 indicate any use the large amount of urea might serve. 

 Balgioni ( '06) found that salt solution alone led to stop- 

 page of the elasmobranch ventricle in diastole. It in- 

 creased diastolic tonus, while urea increased systolic 

 tonus. The presence of the two in about equal amounts 

 mutually neutralized each other and made the continuous 

 rhythm of the heart possible. All we can say is that tor 

 the kind of protoplasm of which the elasmobranch heart 

 is composed, the urea is a necessary constituent of the 

 blood. Furthermore it does not appear to be necessary 



