404 MOVEMENT OF LYMPH. [BOOK n. 



the lymph must flow with a not inconsiderable rapidity (if we 

 take about half the above estimate, the rate will be about 1 or 2 

 c.c. per minute) through the thoracic duct, and therefore must 

 also be continually streaming into that duct, along the various 

 lymphatic channels from the manifold lymph-spaces of the body. 

 This onward progress of the lymph is determined by a variety 

 of circumstances. In the first place, the remarkably wide-spread 

 presence of valves in the lymphatic vessels causes every pressure 

 exerted on the tissues in which they lie to assist in the propul- 

 sion forward of the lymph. Hence all muscular movements 

 increase the flow. If a cannula be inserted in one of the larger 

 lymphatic trunks of the limb of a dog, the discharge of lymph 

 from the cannula will be more distinctly increased by move- 

 ments, even passive movements, of the limb than by anything 

 else. When we come to speak of the entrance of chyle into the 

 lacteal radicles of the villi, we shall see that the muscular fibres 

 of the villus act as a kind of muscular pump, driving the chyle 

 past the valved end of the lacteal radicle into the lymphatic 

 canals below. In addition to the presence of valves along the 

 course of the vessels, the opening of the thoracic duct into the 

 venous system is guarded by a valve, so that every escape of 

 lymph or chyle from the duct into the veins becomes itself a 

 help to the flow. In the second place, we have already seen 

 that the blood-pressure in the capillaries and minute vessels is 

 considerably greater than that in the large veins, such as the 

 jugular ; in fact this difference of pressure is the cause of the 

 flow of blood from the capillaries to the heart. Now even as- 

 suming that the lymph in the lymphatic spaces outside the cap- 

 illaries and minute vessels necessarily stands at a lower pressure 

 than the blood inside the capillaries, on the ground that other- 

 wise the transudation from the blood into the tissues would be 

 checked, we must still admit that the difference is less, probably 

 much less, than the difference between the pressure in the cap- 

 illaries and that in the large venous trunks. So that the lymph 

 in the lymph-spaces of the tissues may be considered as standing 

 at a higher pressure than the blood in the venous trunks, for 

 instance in the jugular vein. That is to say, the lymphatic ves- 

 sels as a whole form a system of channels leading from a region 

 of higher pressure, viz. the lymph-spaces of the tissues, to a 

 region of lower pressure, viz. the interior of the jugular and 

 subclavian veins. This difference of pressure will, as in the case 

 of the blood vessels, cause the lymph to flow onward in a con- 

 tinuous stream. Further, this flow, caused by the lowness of 

 the mean venous pressure at the subclavian vein, will be assisted 

 at every respiratory movement, since at every inspiration the 

 pressure in the venous trunks becomes, as we shall see in deal- 

 ing with respiration, negative, and thus lymph will be sucked 

 in from the thoracic duct, while the increase of pressure in the 



