CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 527 



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 movements of the body 

 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 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 assuming that the lymph in the lymph- 

 spaces outside the capillaries and minute vessels necessarily 

 stands at a lower pressure than the blood inside the capillaries, on 

 the ground that the transudation from the blood into the tissues 

 would otherwise be checked, we must still admit that the differ- 

 ence is less, 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 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 

 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 distinctly muscular ; and the peculiar 

 interlacing of the muscular fibres above each valve suggests that 

 the walls here act after the fashion of a tiny heart and by a 

 rhythmical systole drive on the fluid, which by the action of the 



