CIRCULATION. 439 



of mercury. The difference between one half of either of these pressures and 

 the pressure in the veins of the neck, which pressure is not far from zero, is 

 quite enough to produce a flow from the one point to the other. To this flow 

 a resistance is caused by the friction along the lymph-path, which resistance 

 causes the lymph to accumulate in the gaps of the tissues, and the pressure 

 there to rise, until the tension of the tissues resists further accumulation more 

 forcibly than friction resists the onward movement of the lymph. The little- 

 known forces which continually produce fresh lymph, and pour it into the 

 tissue-gaps against resistance, cannot be discussed here further than has been 

 done in treating of the origin of the lymph (p. 362). 



Thoracic Aspiration. The causes have already been stated fully of that 

 low, perhaps negative, pressure in the veins at the root of the neck which ren- 

 ders possible the continuous discharge of the lymph into the blood (p. 387). 

 It need only be noted here that when inspiration rhythmically produces, or 

 heightens, the suction of blood into the chest, it must also produce, or heighten, 

 the suction of lymph out of the mouths of the thoracic and right lymphatic 

 ducts. Moreover, as the thoracic duct lies with most of its length within the 

 chest, each expansion of the chest must tend to expand the main part of the 

 duct, and thus to suck into it lymph from the numerous lymphatics which 

 join the duct from without the chest ; while the numerous valves in the duct 

 must promptly check any tendency to regurgitatiou from the neck. 



The Bodily Movements and the Valves. Like the flow of the blood in 

 the veins, the flow of the lymph in its vessels is powerfully assisted by the 

 pressure exerted upon the thin-walled lymphatics by the contractions of the 

 skeletal muscles ; for the very numerous valves of the lymphatics render it 

 impossible for the lymph to be pressed along them by this means in any other 

 than the physiological direction toward the venous system. Experiment shows 

 that even passive bending and straightening of a limb in which the mus- 

 cles remain relaxed, increases to a very great extent the discharge of lymph 

 from a divided lymphatic vessel of that limb. It is probable, therefore, 

 that movement in any external or internal part of the body, however pro- 

 duced, tends to relieve the tension in the tissues by pressing the lymph along 

 its path. 



Conclusion. The movement of the lymph produced in these various ways 

 is doubtless irregular; but a substance in solution, injected into the blood, can 

 be identified in the lymph collected from an opening in the thoracic duct at 

 the neck in from four to seven minutes after the injection. 1 The physiological 

 importance of the lymph-movement is shown not only by the large amount 

 of matter which daily leaves the lymphatic system to join the blood, but also 

 by the evil effects which result from an undue accumulation of lymph, more 

 or less changed in character, in the gaps of the tissues: Such an accumulation 

 constitutes dropsy. It may occur in a serous cavity or in the subcutaneous 

 tissue ; in the latter case giving rise to a peculiar swelling which " pits on 



1 S. Tschirwinsky : " Zur Frage fiber die Schnelligkeit des Lymphstromes und der Lymph- 

 filtration," Centralblatt fur Physiologic, 1895, Band ix. p. 49. 



