CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 505 



tends to cause changes in them ; and those changes may materially 

 affect in one direction or another their action as filters. In an 

 ordinary hlter increase of pressure necessarily entails increase of 

 filtration ; in a living filter it may or may not, and the same 

 increase of pressure may according to circumstances produce very 

 different results as regards the transudation of lymph. 



Thus it seems reasonable to suppose as we have suggested 

 ( 227) that, other things being the same, an increase of blood- 

 pressure should necessarily increase the transudation of lymph. 

 Hence when a small artery dilates, since the pressure in the 

 still smaller branches and capillaries of that artery is, as we have 

 more than once pointed out, increased, more lymph appears in the 

 lymph-spaces; indeed it is one of the main purposes of the 

 widening of small arteries to supply the elements of the tissue 

 with more lymph, that is, with more food. But it does not 

 therefore follow that under all circumstances widening of the 

 artery should increase the passage of lymph ; something may 

 occur to counteract the natural effect of the increased pressure in 

 the blood vessels. An instance of this seems to be afforded by 

 the case of the submaxillary gland, when the chorda nerve is 

 stimulated while the gland is under the influence of atropin. As 

 we have seen, though the arteries dilate, no secretion takes place; 

 and we cannot explain the absence of a flow into the alveoli by 

 supposing that the extra amount of lymph which would 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 

 lymphatics 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 con- 

 nective tissue of the gland. Apparently, for some reason or other, 

 in spite of the increased pressure in the blood vessels 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 blood vessels produced by obstruction 

 to the venous outflow 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 

 pressure 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 shewn by the fact that the same amount 

 of obstruction may or may not give rise to excessive transudation 

 according to the condition of the blood or other circumstances. 

 For instance, though the obstruction produced by ligaturing a 

 vein frequently causes excessive transudation, it does not always 



