v THE BLOOD: PLASMA 143 



On introducing into the veins of a horse seven litres of a 

 hypertonic solution (solution of higher concentration than an 

 isotonic) of 5 per- cent sodic sulphate, he saw that the salt was 

 immediately eliminated by the excretory organs, the hyper tony of 

 the blood serum lasting only for a few moments after the injection, 

 although analysis of the same serum, when once more isotonic, 

 proved it still to contain a very abnormal quantity of sodium 

 sulphate. 



Again, he found that the serum recovered its normal osmotic 

 pressure in a very short time after the intravascular injection of 

 a hypotonic solution of 0'5 per cent sodic sulphate. 



He further found that the rise of osmotic pressure in the 

 serum, caused by the anhydraemia produced artificially by the 

 subcutaneous injection of pilocarpin and eserin (which cause 

 marked loss of water by exaggerated secretions of sweat and 

 saliva), lasts only a short time, as also the hydraemia occasioned 

 by copious bleeding. 



Hamburger concluded from these facts that the vascular 

 system has the property of maintaining constant the osmotic 

 pressure of the plasma, notwithstanding the most varying changes 

 in the chemical composition of the blood. 



He explained this fact on the hypothesis of a secretory 

 property of the vascular endotheliuni, which, when stimulated by 

 the increase or decrease of the osmotic pressure of the blood, reacts 

 by a rapid reinstatement of isotony. The secretory capacity of 

 the capillary endotheliuni was, as we shall see, experimentally 

 confirmed by Heidenhain. 



Starting from these results, Winter made other experiments 

 with the cryoscopic method. He found that the freezing-point 

 of blood serum in the mammalia which he investigated was 

 practically constant. Freezing nearly always took place at 

 - 0'55 C., which point corresponds to that of a solution of 0'91 per 

 cent NaCl in distilled water. According to him, therefore, the 

 osmotic pressure of the blood, being independent of species and 

 individual, must in all probability depend, like temperature, on 

 the general conditions of the mammalian environment. 



The 0'9 1 per cent NaCl is not hypertonic for erythrocytes as 

 some believe, but is much nearer their isotonic value than the 

 solution at 0'61 per cent, which rather represents the minimal 

 limit of concentration compatible with a rough anatomical in- 

 tegrity of the erythrocytes apart, that is, from the changes in 

 shape which they undergo. In fact, it is shown by the observa- 

 tions of Hamburger, Malassez and others that the lowest 

 concentration of a solution of NaCl at which the erythrocytes 

 resist diffusion of their haemoglobin is 0'61 per cent. Even the 

 solution of NaCl at 0'75 per cent which for a long time was 

 considered physiological, is hypotonic. For man we must take 



