CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 501 



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 ( 286) in the lymphatic vessels causes every 

 pressure exerted on the tissues in which they lie to assist in the 

 propulsion 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 movements, 

 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, at all events according to 

 one view, 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 

 the lymph in the lymphatic spaces outside the capillaries and 

 minute vessels undoubtedly stands at a lower pressure than the 

 blood inside the capillaries ; otherwise the transudation from the 

 blood into the tissues would be checked ; but the difference is 

 probably much less than the difference between the pressure in 

 the capillaries and that in the large venous trunks. So that the 

 lymph in the lymph-spaces of the tissues may be considered a^ 

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

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

 vessels 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 dealing with 

 respiration, negative, and thus lymph will be sucked in from the 

 thoracic duct, while the increase of pressure in the great veins 

 during expiration is warded off from the duct by the valve at its 

 opening. In the third place, the flow may be increased by 

 rhythmical contractions of the walls of the lymphatics themselves, 

 which, as we have seen, are remarkably muscular ; and the 



