1138 



PHYSIOLOGY 



or freezing, increases at the same time its permeability. Under such 

 conditions the limb capillary reacts to changes of pressure like a liver 

 capillary, the slightest increase of pressure causing an appreciable 

 increase in the lymph production. This increased lymph production 

 may be too great to be carried oft' by the lymphatic channels, so that 

 the exuded fluid stays in the tissue spaces, distending them and 

 causing the condition known as oedema or dropsy. 



LYMPH AGOGUES. Among the substances which have a direct 

 action on the vessel wall are a number of bodies which were described 

 by Heidenhain as lymphagogues of the first class. As their name 

 implies, these bodies on injection into the blood-stream cause an 

 increased flow of lymph from the thoracic duct. They may be 



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eOmmutes 70 



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 Jnj of mussel extract 



FIG. 477. Changes in lymph flow in portal, inferior cava, and arterial pressures, 

 resulting from injection of a member of the first class of lymphagogues (extract 

 of mussels). (STABLING.) 



extracted from the dried tissue sof crayfish, mussels, or leeches by simple 

 boiling with water. Commercial peptone has a similar effect. Heiden- 

 hain regarded these bodies as direct excitants of the secretory activities 

 of the endothelial cells. They are, however, general poisons, having 

 a special action on the vascular system, and their effect on lymph 

 production is probably due simply to their deleterious action on the 

 capillary wall. Although these bodies act chiefly on the liver capil- 

 laries, so that the main increase in the thoracic duct lymph is derived 

 from the liver, they can be shown also to have some effect in the 

 same direction on the intestinal and skin capillaries. In fact the 

 injection or ingestion of these bodies often gives rise to a copious erup 

 tion of nettle-rash, i.e. swellings of the skin due to an increased exuda- 

 tion of lymph into the meshes of the cutis. 



An increased lymph-flow from the thoracic duct may be produced 

 also by the injection of large amounts (10 to 40 grm.) of innocuous 



