156 PHYSIOLOGY FOR DENTAL STUDENTS. 



glands through which the lymph vessels pass on their way to the 

 subclavian vein. 



Lymph Formation. Many physiologists have attempted to 

 discover the precise mechanism by which the plasma passes 

 through the capillary walls into the lymph spaces, but the com- 

 plete knowledge of the process is not yet at hand. The relatively 

 high blood pressure within the capillaries provides filtration 

 pressure by which a fluid might be filtered through the capillary 

 walls, and there is no doubt that such a process does occur, as, for 

 example, after the capillary pressure has been increased by con- 

 striction of the veins by a bandage, etc. Filtration, however, 

 cannot explain all the known phenomena of lymph formation. 

 Osmosis (p. 27) also plays a part as follows: The tissues use up 

 the nutritional elements brought to them by the lymph. The 

 diffusion pressure of the substances in the lymph is now reduced 

 so that it becomes less than that present in the blood. Therefore 

 substances within the blood must pass out through the capillary 

 walls into the lymph, thus keeping the concentration of the fluid 

 more or less constant. The waste products of the tissue pass into 

 the lymph and, by increasing the molecular concentration of the 

 lymph, draw water from the blood. Again, the breaking down of 

 the large protein molecules into smaller ones, in the processes of 

 tissue metabolism, will cause the molecular concentration of the 

 tissues to rise, increasing the osmotic pressure. This causes water 

 to be abstracted from the lymph, which in turn draws on the 

 blood for water. 



Lymphagogues. There are certain substances which affect 

 the rate of lymph formation in a very peculiar way. These are 

 called lymphagogues, and include extracts from many shell fish, 

 leech extract, peptones, etc. When such substances are injected 

 into the blood of an animal, there follows a great increase in the 

 rate of lymph formation and lymph flow. Indeed some people 

 are very susceptible to this action, and eating shell fish, oysters, 

 and some fruits will cause their tissues to become swollen be- 

 cause of an increased lymph formation. How these substances 

 can effect the change by altering the physico-chemical constitu- 

 tion of the blood plasma is not clear. Some investigators believe 



