DIFFUSION AND OSMOSIS. 881 



connective tissues; the collagen (ossein) of the white, fibrous connective 

 tissues and bones, from which gelatin is made; and the reticulin of the retic- 

 ular connective tissues of the lymphatic tissues. They resemble the pro- 

 teids closely in general composition, reactions, and end-products of hydro- 

 lytic cleavage, but differ from them evidently in some structural feature, 

 since it has been found by nutritive experiments that they can not be used 

 by the body to replace its proteid tissues (see p. 791). Collagen (gelatin) 

 has the following percentage composition (Chittenden) : C, 49.38; H, 6.8;- 

 N, 17.97; S, 0.7; O, 25.13. 



DIFFUSION AND OSMOSIS. 



In recent years the physical conceptions of the nature of the processes 

 of diffusion and osmosis have changed considerably. As these newer concep- 

 tions have entered largely into current medical literature, it seems advis- 

 able to give a brief description of them for the use of those students of phys- 

 iology who may be unacquainted with the modern nomenclature. The 

 very limited space that can be devoted to the subject forbids anything more 

 than a condensed elementary presentation. For fuller information refer- 

 ence must be made to special treatises.* 



Diffusion, Dialysis, and Osmosis. When two gases are brought into 

 contact a homogeneous mixture of the two is soon obtained. This inter- 

 penetration of the gases is spoken of as diffusion, and it is due to the con- 

 tinual movements of the gaseous molecules to and fro within the limits of 

 the confining space. So also when two miscible liquids or solutions are 

 brought into contact a diffusion occurs for the same reason, the movements 

 of the molecules finally effecting a homogeneous mixture. If the two liquids 

 happen to be separated by a membrane diffusion will still occur, provided 

 the membrane is permeable to the liquid molecules, and in time the liquids 

 on the two sides will be mixtures having a uniform composition. Not only 

 water molecules, but the molecules of many substances in solution, such 

 as sugar, may pass to and fro through membranes, so that two liquids sepa- 

 rated from each other by an intervening membrane and originally unlike 

 in composition may finally, by the act of diffusion, come to have the same 

 composition. Diffusion of this kind through a membrane is frequently 

 spoken of as dialysis or osmosis. In the body we deal with aqueous solu- 

 tions of various substances that are separated from each other by living 

 membranes, such as the walls of the blood capillaries or of the alimentary 

 canal, and the laws of diffusion through membranes are of immediate im- 

 portance in explaining the passage of water and dissolved substances through 

 these living septa. In aqueous solutions such as we have in the body we must 

 take into account the movements of the molecules of the solvent, water, 

 as well as of the substances dissolved. These latter may have different de- 

 grees of diffusibility as compared with one another or with the water mole- 

 cules, and it frequently happens that a membrane that is permeable to 

 water molecules is less permeable or even impermeable to the molecules of 

 the substances in solution. For this reason the diffusion stream of water 

 and of the dissolved substances may be differentiated, as it were, to a greater 

 or less extent. In recent years it has become customary to limit the term 

 osmosis to the stream of water molecules passing through a membrane, 

 while the term dialysis, or diffusion, is applied to the passage of the mole- 

 cules of the substances in solution. The osmotic stream of water under vary- 

 ing conditions is especially important, and in connection with this process 

 it is necessary to define the term osmotic pressure as applied to solutions. 



Osmotic Pressure. If we imagine two masses of water separated by 

 a permeable membrane, we can readily understand that as many water mole- 



* Consult Cohen, "Physical Chemistry for Physicians and Biologists," 

 translated by Fischer, 1903. H. C. Jones,' " The Theory of Electrolytic Dis- 

 sociation," 1900; ''Diffusion, Osmosis, and Filtration," by E. W. Reid, 

 in Schafer's "Text-book of Physiology," 1898. 

 56 



