THE BLOOD AND LYMPH. 129 



the endothelial cells are active in bringing about these differences. 

 Their character is not such as would be expected of cells carrying 

 on active processes. Since the lymph is a solution separated from 

 the blood by membranous capillary walls, and also coming into 

 most intimate relations to the cells and intercellular substances of the 

 tissues, the purely physical processes which might affect its composi- 

 tion are by no means simple. They may be grouped under three 

 heads : filtration, osmosis, and diffusion. When fluids containing 

 proteids are filtered under pressure, the filtrate contains a smaller 

 percentage of proteids than the original fluid. The element of filtra- 

 tion may, therefore, account for the smaller proteid content of the 

 lymph than that of the blood-plasma. Osmosis is the flow of water 

 through a membrane from a solution of lower molecular concen- 

 tration to one of higher molecular concentration, and diffusion is the 

 passage of dissolved solids from a solution of greater concentration with 

 respect to a given substance to a solution which contains that sub- 

 stance in less concentration. By the term " molecular concentration " 

 is meant the relative number of molecules, free atoms, or radicles 

 •contained in a given bulk of liquid. The molecular concentration 

 of the body fluids is usually measured by the depression of the 

 freezing-point, as compared with that of pure water. For example, 

 a solution of urea containing 60.12 grams in a litre and a solution 

 of glucose in which 180.12 grams of the sugar are dissolved in 

 enough water to make 1 litre of solution both freeze at a temperature 

 1.85 degrees C. below that of pure water — i. e., they depress the 

 freezing-point 1.85 degrees C. If these solutions were placed on 

 opposite sides of a membrane, there would be no osmosis — i. e., no 

 water would pass from one solution into the other ; or if water did 

 pass, equal quantities would pass in both directions, so that the 

 interchange would cause neither concentration nor dilution of either 

 solution. Both the urea and salt would diffuse through the mem- 

 brane, so that eventually both solutions would have the same com- 

 position ; each containing half of the urea and half of the sugar. 

 Since the molecular weights of urea and dextrose are, respectively, 

 00.12 and 180.12, these solutions are called equimolecular solutions, 

 and are said to have the same molecular concentration and to exert 

 the same osmotic pressure. The depression of the freezing-point is, 

 then, a measure of molecular concentration, and therefore of osmotic 



pressure, or of what may be considered as the attraction for water 

 9 



