THE VASOMOTOR MECHANISM 385 



in common phrase, is "sucked," into the thoracic duct. During 

 the succeeding expiration the pressure on the thoracic duct be- 

 comes greater again, and some of its contents are pressed out; but 

 on account of the valves of the vessels which unite to form the 

 duct, they can only go towards the veins of the neck. 



During digestion, moreover, contractions of the villi and of 

 the intestinal walls press on the lymph or chyle within them and 

 force it on; and in certain parts of the Body gravity, of course, 

 aids the flow, though it will impede it in others. 



The Action of Lymphagogues. Any substance that causes a 

 pronounced increase in the rate of lymph formation is known 

 as a lymphagogue. The source of lymph, we have already seen 

 (p. 294), is in the main by filtration through the capillary walls. 

 Evidently lymphagogues act by increasing this filtration. There 

 are two ways in which this might be brought about, and lympha- 

 gogues are assigned to one of two classes according to which of the 

 ways they use. The first is by making the capillary walls more 

 permeable, and so increasing the outpouring of lymph. Sub- 

 stances which have this effect are shell fish, strawberries, some meat 

 extracts, egg-white and related organic compounds. Not all 

 people are affected by these lymphagogues. Nor are those that 

 are susceptible to one necessarily susceptible to all. Where the 

 capillaries whose permeability is increased are superficial the out- 

 pouring lymph forms blotches on the skin. The condition is known 

 as urticaria or hives. Mechanical injury to the capillaries may 

 cause a similar outpouring, as seen in the swelling from a bruise. 



The second method of increasing the flow of lymph is by pro- 

 ducing an engorgement of the capillaries, a condition known as 

 hydremic plethora. This can be brought about by raising the 

 osmotic pressure of the blood, as by injecting into it a strong 

 sugar solution. The effect is to cause a rush of lymph into the 

 blood through the capillaries. The lymph thus withdrawn is 

 made good by an outpouring of tissue fluids into the lymph spaces. 

 It has been shown that in this situation the plethora is relieved 

 chiefly by an increased filtration through the capillaries of the liver. 

 The conclusion is drawn that these are the most permeable in the 

 Body. The lymph thus formed passes to the thoracic duct and back 

 to the blood, so that evidently no permanent advantage is gained. 

 The excess of fluid is finally discharged through the kidneys. 



