408 TRANSUDATION. [BOOK 11. 



increase the passage of lymph ; something may occur to coun- 

 teract 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 sup- 

 posing 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 connective 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. 



In the second place, increase of pressure does not always 

 produce the same amount of transudation. For instance, as we 

 shall presently have occasion to point out, an increase of pres- 

 sure in the blood vessels produced by obstruction to the venous 

 outflow is much more efficient in promoting an increase of tran- 

 sudation, 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. Further the 

 same amount of venous obstruction giving rise to the same 

 amount of capillary pressure may or may not give rise to exces- 

 sive transudation according to the condition of the blood or 

 other circumstances. For instance, though the obstruction 

 produced by ligaturing a vein frequently causes excessive tran- 

 sudation, it does not always cause it, and the femoral vein of a 

 dog may be ligatured without any excessive transudation tak- 

 ing 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 towards 

 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. 



In the third place if we measure the flow of lymph along the 

 duct (by introducing a cannula into the thoracic duct at its 

 end near the great veins), and we may take this as a measure 

 of the transudation going on, we find that this flow may be very 

 greatly increased, without any change of blood-pressure neces- 

 sarily taking place, by the introduction into the blood of certain 



