LYMPH AND TISSUE FLUIDS 1015 



previously ligatured, the whole of the thoracic duct lymph in an animal at 

 rest is derived from the intestines. It will be found that lowering of the 

 capillary pressure in these organs by obstructing the thoracic aorta stops 

 the flow of lymph absolutely, whereas a rise of capillary pressure, such as that 

 produced by ligature of the portal vein, causes a four- or fivefold increase 

 of the lymph. 



The effect of rise of capillary pressure on the lymph-flow is still more 

 striking in the case of the liver. If the inferior vena cava be obstructed 

 just above the opening of the hepatic veins, there is a great fall of arterial 

 pressure, but, owing to the damming back of the blood, a rise of pressure 

 in the liver capillaries to three or four times the normal height. This rise 

 causes a large increase in the lymph-flow from the thoracic duct. The 

 lymph may be increased eight to ten times in amount, and it contains more 

 protein than before. If the portal lymphatics be previously ligatured, 

 obstruction of the inferior vena cava has no effect on the lymph-flow, 

 showing that the whole of this increase is derived from the one region of the 

 body where the capillary pressure is increased, viz. the liver. 



We must conclude that in those regions of the body where the capillaries 

 are fairly permeable the most important factor in lymph production is the 

 intracapillary pressure. 



In the case of the limbs and connective tissues generally, the pressure 

 factor is probably, under normal conditions, of less importance, so that the 

 second condition, the chemical, comes here more into prominence. The 

 capillary wall not only permits of filtration under certain pressures but also 

 allows the passage of water and dissolved substances by diffusion and 

 osmosis. These osmotic interchanges between blood and cell through the 

 intermediation of the lymph are constantly going on in the normal life of the 

 tissue, and are quite independent of the amount of lymph produced. Thus a 

 gland- cell may use up oxygen, calcium, or sugar, and create a vacuum 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 sub- 

 stances 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 contraction 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 



