ABSORPTION 269 



if the cell could be proved to be the factor in lymph formation, 

 it would make a substantial addition to our knowledge of the 

 subject ; the difficulty of explaining the selective power of the 

 cell which secretes lymph is only the same difficulty which exists 

 in considering the cell which secretes urine, or bile, or sweat. 

 Neglecting entirely the nervous mechanism, of which there is so 

 little evidence, the two chief theories of lymph formation resolve 

 themselves into the physical and secretory, and each of these 

 must be examined separately. 



The Physical Theory is based upon a knowledge of the laws 

 governing filtration, diffusion, and osmosis, and were first em- 

 ployed by Ludwig. 



Two liquids miscible, but utterly unlike, if^brought into contact 

 will gradually form a homogeneous mixture as the result of diffusion. 

 If they be separated by a membrane permeable to their molecules, 

 diffusion will occur through this, and a mixture of uniform compo- 

 sition result. Diffusion through a membrane is known as osmosis. 

 Substances which are diffusible are known as crystalloids, those 

 which are non -diffusible are called colloids. Sugar or salt are good 

 examples of diffusible bodies, proteins and starch are examples of 

 colloids, the large size of the molecules of the latter preventing their 

 passage through an animal or other membrane. This difference in 

 the behaviour of these two classes of substance as regards their 

 osmotic properties, affords a useful and ready means, known as 

 dialysis, of separating the crystalloids from the colloids. 



If two masses of water be separated by a membrane the dififusi- 

 bility of each being equal, as many molecules will pass into one 

 chamber as enter into the opposite, though to all appearances no 

 change in the fluid is taking place. If one chamber contains salt 

 solution and the other plain water, it will be found that much more 

 water passes into the salt solution than salt solution into the water, 

 the rate of transference of the salt depending upon the concentration 

 of the salt solution ; the force which brings this about is known as 

 the osmotic pressure. It can be shown that the osmotic pressure is 

 proportional to the number of molecules of the crystalloid in 

 solution. 



Filtration is the passage of fluid through a membrane as 

 the result of pressure. If the pressure of blood in the capil- 

 laries could be shown to be higher than the pressure in the 

 lymph vessels or spaces, adequate ground would exist for regard- 

 ing filtration as an agent in lymph production. As a matter of 

 fact, the pressure in the lymph spaces is unknown, the pressure 

 in the lymph capillaries is unknown ; but there is the best of 

 reason for believing that the pressure falls from the blood capil- 

 laries to the lymph capillaries, and by increasing the pressure in 

 the former — say, by tying or compressing a vein — cedema results 

 in consequence of the filtration of fluid from blood capillary to 

 tissue space. The fluid in the tissue space may or may not be 

 lymph as we find it in a lymphatic vessel. The plasma has 



