LYMPH AND TISSUE FLUIDS 1065 



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 

 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 con- 

 ditioned by changes in the extra vascular gland cell. We have only to 

 paralyse the gland cell by means of atropine 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. Stimulation 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 therefore 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 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 off 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. 



LYMPHAGOGUES- Among the substances which have a direct action 

 on the vessel wall are a number of bodies which were described by Heiden- 

 hain 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 (Fig. 488). They may be extracted from the dried tissues 

 of crayfish, mussels, or leeches by simple boiling with water. Commercial pep- 

 tone has a similar effect. Heidenhain 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 capillaries, so 

 that the main increase in the thoracic duct lymph is derived from the liver, 



