302 PROPERTIES CONFERRED BY COLLOIDAL CONSTITUENTS 



emulsion of lipoids suspended in a solution or jelly of protein must be 

 very complex. Essentially it is an emulsion enclosed within another 

 emulsion and many diversities of structure and arrangement may 

 evidently exist. One would anticipate that the architecture of such a 

 complex system would be profoundly affected by any factors affecting 

 the solubility of the proteins, and therefore their affinity for water. 

 A relative alteration of volume of the water-poor and water-rich phases 

 of the protein emulsion must necessarily disturb all the space-relations 

 of the enclosed fat-emulsion, and these displacements acting at the 

 Surface of the cell would be equivalent to opening or shutting so many 

 doors for the entry of water-soluble substances into the cell. The 

 striking effects of various inorganic substances upon the Permeability 

 of cells upon which we shall dwell in the succeeding chapter, probably 

 originate in changes of the affinity of the cell-proteins for water with 

 consequent dilatations or contraction of the constituent phases of the 

 protein jelly and enlargement or constriction of the interstitial spaces 

 between the lipoidal elements of the superficies of the cell. 



THE OSMOTIC PRESSURE OF PROTEIN SOLUTIONS. 



It was formerly believed that proteins in solution exerted, in common 

 with other colloids, either no osmotic pressure at all, or a pressure of 

 immeasurably small extent. More recent investigations have shown, 

 however, that the difference in this as in other respects between the 

 colloids and the "typical" crystalloids is merely a quantitative differ- 

 ence which is directly attributable to and deducible from the relatively 

 enormous size of their molecules. Thus a one per cent, solution of 

 Glucose contains y-g- gram-molecules of glucose per liter and exerts 

 an osmotic pressure of nearly one and a quarter atmospheres, but a 

 one per cent, solution of Hemoglobin, which has a molecular weight of 

 sixteen thousand, only contains y^ro gram-molecules of protein per 

 liter and, therefore, may be expected only to exert an osmotic pressure 

 of 0.014 of an atmosphere. 



The direct determination of the osmotic pressure of protein solutions 

 is a task fraught with immense difficulties, on account of the difficulty 

 of preparing ideally pure proteins. The investigations of Graham, 

 the originator of the distinction between crystalloids and colloids, 

 appeared to indicate that colloids in general exert a high osmotic 

 pressure. Subsequent investigators, however, attributed these results 

 to an admixture of crystalloids, which, as the above numerical compari- 

 son shows, might be expected to exert a disproportionate effect upon 

 the pressures exhibited. Starling endeavored to measure directly the 

 osmotic pressure of the proteins in blood-serum by using for his Osmom- 

 eter a membrane permeable to salts but impermeable to proteins, and 

 this method has, since then, been employed in all accurate work upon 

 the subject, since, as Reid has pointed out, it is the only method of 

 procedure which is applicable to the problem. Such a membrane is 

 to the colloids what an ideally semipermeable membrane is to all 



