LYMPH AND TISSUE FLUIDS 1137 



of these substances in the layer of lymph immediately surrounding 

 the cell. There is at once a disturbance of the equilibrium, and a flow 

 of these substances from blood to lymph is set up. In consequence of 

 the wonderful arrangements in the tissues for ensuring the intimate 

 contact of blood and lymph without intermingling, these changes can 

 occur with great rapidity. We find, for instance, that if a very large 

 amount (40 grm.) of dextrose be injected into the circulation, osmotic 

 equilibrium between blood and lymph is established within half a 

 minute of the termination of the injection. In this case the rise of 

 osmotic pressure caused by the injection of the sugar attracts water 

 from the tissue-fluid, and this in its turn from the tissue-cells, until the 

 osmotic pressure inside and outside the vessels is the same. By this 

 means the volume of the circulating blood is increased at the expense of 

 the tissues. A process of this character may, however, work under 

 normal circumstances in the reverse direction, and lead to a passage of 

 fluid from blood to tissues and tissue spaces. Every active contrac- 

 tion of a muscle, for instance, is attended by the breaking down of a 

 few large molecules into a number of smaller ones, and this increase 

 in the number of molecules causes a rise of osmotic pressure in the 

 muscle fibre and surrounding lymph, and therefore a passage of fluid 

 from blood to lymph. In the same way a cell of the submaxillary 

 gland, when stimulated by means of its nerve, pours out a quantity of 

 fluid into the gland-duct, and so into the mouth. This fluid comes 

 in the first instance from the cell itself, but the cell recoups itself 

 from the surrounding lymph, raising the concentration of this fluid, 

 and the difference in concentration thus caused at once induces a 

 passage of water from blood to lymph. Hence salivary secretion 

 is associated with a large flow of fluid through the capillary walls 

 of the gland. In this passage the endothelial cells of the capillaries 

 play no part, the whole process being conditioned by changes in the 

 extravascular gland-cell. We have only to paralyse the gland-cell 

 by means of atropin in order to see that the active flushing of the 

 gland which accompanies activity produces merely a minimal increase 

 in the lymph-flow from the gland. 



The influence of tissue activity in the production of lymph is 

 still better shown in the case of a large gland, such as the liver. Stimu- 

 lation of this organ by the injection of bile salts into the blood-stream 

 causes a large increase in the lymph-flow from the organ, and there- 

 fore in the lymph-flow from the thoracic duct. 



It is important to remember that the relative insusceptibility of the 

 limb capillaries to pressure holds only for the absolutely normal 

 capillary.- Any factor which leads to impaired nutrition of the 

 vascular wall, such as deficiency of supply of blood or oxygen, the 

 presence of poisons in the blood or in the surrounding tissues, scalding 



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