594 EXPERIMENTS DEALING WITH 



be drawn between the crystalloids and colloids in solution. In the 

 filtration of crystalloids the concentration of the filtrate is 

 approximately that of the original solution. In the filtration of 

 colloids the concentration of the filtrate is always less than that 

 of the original solution, and varies directly as the permeability of 

 the membrane. The relation of the filtering force to the con- 

 centration of colloid in the filtrate is different. With a given 

 membrane, when the filtering force is increased the absolute 

 quantity of colloid passing through is also increased: but its 

 concentration is decreased because the rate of filtration of the 

 solvent rises even more rapidly. 



By diffusion (dialysis) is meant the interchange of fluid or 

 substance in solution between two fluids separated by a permeable 

 membrane and under the same hydrostatic pressure. 



The rate of diffusion depends upon the relation between the 

 permeability of the membrane and the size of the molecules of 

 the substance in solution. The molecules of blood proteids are 

 so large that their diffusibility through dead animal membranes is 

 insignificant and may be disregarded. 



Neither experimental filtration nor diffusion alone represents 

 the conditions in the body. To imitate these we must have a 

 fluid circulating in tubes under pressure and separated by a porous 

 membrane from a different fluid under less pressure. The inter- 

 change between these two fluids will be the sum of filtration 

 and diffusion, a process which has been called transudation. 



The amount of transudate formed experimentally under these 

 conditions depends on three factors : (a) the difference between the 

 hydrostatic pressure of the fluid inside and outside the mem- 

 brane i.e. the filtering force, (6) the permeability of the membrane, 

 and (c) the difference between the osmotic pressure of the two 

 fluids. 



Theory of Ludwig. In 1850 Ludwig published his celebrated 

 mechanical view of lymph formation, into which only two factors 

 entered, namely, filtration and diffusion. 



According to this view lymph is essentially the fluid part of 

 the blood filtered off by the capillary blood pressure through the 

 capillary wall and altered by the membrane introducing a great 

 resistance to the passage of proteids. In quantity it will vary 

 directly as the variations in the capillary pressure. Its composition 

 is the resultant of two variables : the composition of the blood, 



