THE NATURE AND MOVEMENTS OF LYMPH. 321 



in normal circumstances form part of the secretion, and in the case of a 

 fairly copious secretion would be considerable, now passes away by the lym- 

 phatics without reaching the cells of the alveoli, for in such cases no extra 

 flow in the lymphatics leading from the gland has been observed, and there 

 is no accumulation of lymph in the connective tissue of the gland. Ap- 

 parently, for some reason or other, in spite of the increased pressure in the 

 bloodvessels, more lymph than usual does not pass into the lymph-spaces. 



Then again, as we shall presently have occasion to point out, an increase 

 of pressure in the bloodvessels produced by obstruction to the venous out- 

 flow is much more efficient in promoting an increase of transudation^ at all 

 events an abnormal increase, than is an increase of arterial pressure ; and 

 the difference between the two cases appears to be too great to be accounted 

 for on the ground that an obstruction to the venous outflow raises the pres- 

 sure within the capillaries and small vessels more readily and to a higher 

 degree than does the widening of the arteries. Moreover, that obstruction 

 to venous outflow does not produce its effects in the way of transudation 

 simply and merely by raising the capillary pressure is shown by the fact that 

 the same amount of obstruction may or may not give rise to excessive trans- 

 udation, according to the condition of the blood or other circumstances. 

 For instance, though the obstruction produced by ligaturing a vein fre- 

 quently causes excessive traiisudation, it does not always cause it, and the 

 femoral vein of a dog may be ligatured without any excessive transudatiou 

 taking place ; yet if, after the ligature, certain changes be induced in the 

 blood, excessive transudation occurs in the leg the vein of which has been 

 ligatured, but not elsewhere. Pointing toward the same conclusion is the 

 fact that excessive transudation more readily occurs when a vein is plugged 

 by a thrombus arising from abnormal conditions of the vascular system than 

 when a vein is simply ligatured. And in general we may say, and this is a 

 point to which we shall return, that two things chiefly determine the amount 

 of transudation : the pressure of the blood in the bloodvessels, and the con- 

 dition of the vascular walls in relation to the blood, the latter being at least 

 as important as the former. 



Another aspect of the matter, moreover, deserves attention. In filtration 

 the movement takes place through the filter in one direction only, whereas 

 in the living body, the passage of material through the capillary wall takes 

 place in two opposite directions. In all the tissues, though more perhaps in 

 certain tissues than in others, the passage from the bloodvessel into the 

 lymph-space is accompanied by a passage from the lymph-space into the 

 blood ; while food for the tissue passes in one direction, waste products pass 

 in the other. In a secreting gland the greater part of the lymph coming 

 from the bloodvessels, the water and other matters, pass away into the lumen 

 of the alveolus after undergoing changes in the cell ; but even in such a case 

 there is some return from the cells into the bloodvessels, carbonic acid, for 

 instance, if nothing else, is given up by the cells to the blood ; and in such 

 organs as a muscle or the liver, the backward stream of material from the 

 tissue to the blood is extensive and important. Moreover, this backward 

 stream works against pressure ; indeed, as may be seen in a muscle, it is 

 when the bloodvessels are dilated and the pressure in the capillaries and 

 small vessels highest, as during and after the contraction of the muscle, that 

 the passage from the tissue into the blood is most energetic. Many of the 

 waste products of the tissue are, it is true, diffusible, and we might be tempted 

 to say that while the lymph which feeds the tissue traverses the vascular 

 wall by filtration in the direction of pressure, the waste products return to 

 the blood against pressure by diffusion ; but such a view cannot at present 

 be regarded as proved ; and if it be true, as is maintained by some, that 

 21 



