FACTORS CONCERNED IN LYMPH FORMATION 469 



tion rendered necessary by functional activity is always associated 

 with an accelerated flow of lymph, which carries off the surplus 

 liquid, including a portion of the waste products. It is probable 

 that an important factor in the production of oedema may be the 

 derangement of the mechanism, whatever it is, through which the 

 adjustment of the rate of formation of tissue liquid to that of 

 lymphatic lymph is achieved. But it must be remembered that in 

 all the organs the blood capillaries not only supply materials to the 

 tissue spaces, but take up materials from them. Indeed, there are 

 facts which indicate that in general water and dissolved substances 

 pass more easily and in greater amount back from the tissues into 

 the blood than into the lymphatics. So that, while the lymphatics 

 constitute an accessory drainage system, the bloodvessels irrigate 

 the tissues and drain them as well. A consequence of this, as well 

 as of the great difference in the capacity of the different tissues for 

 storing water, is that the amount of tissue lymph formed can never 

 be estimated from the amount of lymphatic lymph leaving an organ. 

 Thus the flow of lymph from the limbs at rest is very small in com- 

 parison with the flow from the abdominal viscera, which constitutes 

 the great bulk of the lymph passing along the thoracic duct. This, 

 however, does not prove that very little liquid passes out of the 

 limb capillaries, for"the chief tissue of the limbs, the muscles, pos- 

 sesses a far greater storage capacity for water than the intestines and 

 digestive glands. In the one case we have a field whose soil takes 

 up a great deal of water and is not easily saturated; in the other a 

 field whose soil soon becomes water-logged and refuses to take up 

 any more. With the same supply from the irrigating ditch, little 

 or no water may drain off at the foot of the first field, a great deal 

 at the foot of the second. 



Where the main lymphatics are themselves blocked by mechanical 

 pressure or by inclusion in a ligature, the balance is, of course, 

 grossly upset by the failure of the outflow of lymph to keep pace 

 with its formation. Where an obstruction on the course of the 

 veins is responsible for the oedema, the lymphatic outflow, to be 

 sure, is not directly interfered with. But the nutrition and the 

 respiration of the vascular walls themselves, including the endo- 

 thelium of the capillaries, necessarily suffers from the insufficiency 

 of the blood- flow, and the crippled capillaries may very well become 

 abnormally permeable for water, salts, and the other constituents 

 of lymph. And while ordinary mechanical filtration may not be a 

 factor, or a very unimportant one, in the passage of liquid through 

 the normal capillary wall, it may become far more effective when 

 it acts on a damaged wall. The tissue cells also suffer from lack 

 of oxygen and nutritive material, and from the accumulation of 

 waste products, including acid substances, which cause them to 

 take up and to hold more water than normal. Even in the absence 



