THE RED BLOOD-CORPUSCLES 819 



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 0-9 per cent, sodium chloride. The osmotic pressure, as deter- 

 mined 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 determinations 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 defibrinated 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 percent, 

 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 super- 

 natant fluid examined separately, the percentage of urea in the two cases 

 will be found identical, though there would be a great preponderance of 

 sodium chloride in the supernatant fluid. If a 1 or 2 per cent, solution of urea 

 in water be added to defibrinated blood, the corpuscles will swell up and 

 burst just as if distilled water had been added. There are a large number of 

 substances to which the corpuscles are permeable, e.g. .alcohol, chloroform, 

 ether, &c. In their permeability the corpuscles resemble most other vege- 

 table and animal cells in permitting the passage of all those substances which 

 are soluble in fats and the allied substances, cholesterin, lecithin, and 

 protagon, which are invariable constituents of all living cells. According 



