THE RED BLOOD-CORPUSCLES 925 



or being extruded so as to lie just outside the stroma. Briicke, who 

 first observed this appearance, gave the name of ' zooid ' to the mass 

 of haemoglobin and of ' cecoid ' to the stroma. 



OSMOTIC RELATIONSHIPS OF THE RED CORPUSCLE. If 

 the blood-plasma be concentrated by evaporation or by the addition 

 of neutral salts its osmotic pressure rises and water diffuses from, the 

 corpuscles into the plasma in order to equalise the osmotic pressure 

 within and without the corpuscle. The latter therefore becomes 

 wrinkled or crenated. On the other hand, dilution of the plasma 

 diminishes its osmotic pressure below that of the corpuscle, and water 

 therefore passes into the latter, which swell up and become spherical, 

 and if the plasma be made sufficiently dilute the corpuscles burst with 

 the liberation of the haemoglobin they contain. The corpuscles of 

 mammalian blood neither gain nor lose volume in a solution containing 

 O9 per cent, sodium chloride. The osmotic pressure, as determined 

 by the freezing-point, of such a solution is identical with that of the 

 blood. For frogs' blood such a solution would be too concentrated 

 and bring about crenation. The salt solution which is normal for 

 frogs' blood only contains 0'65 per cent, sodium chloride. 



Although the average molecular concentration of blood-plasma in mammals 

 is equivalent to that of a 0-9 per cent, sodium chloride solution, it may vary even 

 in one animal within fairly wide limits, as is shown by the following deter- 

 minations of the freezing-point of blood-serum taken from animals under various 

 circumstances : 



Man (healthy) . . . -0-56 to -0-600 



Dog .... . . -0-55 to -0-645 



Ox -0-55 to -0-662 



Rabbit . . . -0-55 to -0-620 



The behaviour of the red corpuscles when immersed in solutions 

 of sodium chloride of different concentrations shows that its limiting 

 membrane or most external layer is impermeable to sodium chloride. 

 If this salt be added to defribinated blood and the crenated corpuscles 

 separated by the centrifuge, practically the whole of the added sodium 

 chloride remains in the plasma or serum. The red corpuscle is 

 impermeable to most neutral salts as well as to cane sugar and glucose. 

 We may therefore make ' normal ' solutions with sodium chloride, 

 sodium sulphate, potassium nitrate, or cane sugar, taking care that 

 each of the solutions shall be isotonic with a 0-9 per cent, solution of 

 sodium chloride. On the other hand, a solution of urea behaves 

 towards the corpuscles like distilled water. If some red corpuscles 

 be added to a 1 per cent, solution of urea in normal salt solution, they 

 neither shrink nor swell, and if the mixture be centrifuged and the 

 corpuscles and supernatant fluid examined separately, the percentage 

 of urea in the two cases will be found identical, though there woTild 



