LYMPH FORMATION AND CIRCULATION 119 



but during activity the flow becomes marked. This can not be explained 

 by nitration, but may be accounted for by a physico-chemical process 

 namely, osmosis. The energy required for the activity of the tissue 

 cell is produced by chemical changes, whereby large molecules become 

 broken down into numerous smaller ones. These smaller molecules are 

 then discharged into the surrounding tissue fluids, the osmotic pressure 

 of which they increase, with the consequence that water is attracted 

 by osmosis from the plasma in the blood capillaries (see page 4). This 

 increases the volume of tissue fluid, which is then drained away by the 

 lymphatics. The increase in molar concentration will also affect the 

 tissue juices, tending to make the cell swell up by absorbing water. 

 In gland cells this extra water is immediately extruded to form the 

 water of the secretion (see page 421). 



An analogous method of lymph formation is not confined to situations 

 where the capillaries are relatively impermeable, for it also occurs in 

 the liver, the lymph flow from which is greatly increased by the injec- 

 tion of bile salts. A similar process no doubt results from muscular 

 activity, although in this case the tissue spaces must form a continuous 

 system of their own, there being, according to most authorities, no 

 lymphatics. 



Considerable interest has been taken in the stimulating effect which 

 certain chemical substances have on the secretion of lymph from the 

 thoracic duct. These so-called lymphagogues belong to two classes 

 crystalline and colloidal. Of the former, glucose, urea, and sodium 

 chloride in hypertonic solution, are the best known. Starling explains 

 their action as dependent upon an increase in the osmotic pressure of 

 the blood. This attracts water into the blood from the tissue juices, 

 and leads to an hydremic plethora, with a consequent increase in capil- 

 lary pressure. If the blood pressure is lowered by hemorrhage before 

 the hypertonic solution is injected, very little stimulation of lymph flow 

 occurs, because there is no available fluid in the tissue to produce the 

 plethora. This observation does not, however, very strongly support 

 the explanation, because so many other disturbances may result from 

 hemorrhage. 



The colloidal lymphagogues include watery extracts of the dried tis- 

 sues of leeches, crayfish, and mussels, as well as commercial peptone. 

 They probably act by damaging the endothelium of the capillaries, so 

 that filtration occurs more readily. Although their action is displayed 

 more particularly on the lymphatics of the liver and intestines, it is also 

 apparent on the skin capillaries, producing cutaneous edema and the 

 formation of blisters (nettle rash). 



