318 THE TISSUES AND MECHANISMS OF DIGESTION. 



with a not inconsiderable rapidity (if we take about half the above estimate, 

 the rate will be about 5 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 widespread presence of valves 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 canula be inserted in one of the larger 

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

 canula will be more distinctly increased by movements, even passive move- 

 ments, 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 transuda- 

 tion 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 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 bloodvessels, cause the lymph 

 to flow onward in a continuous 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 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 valve below collects at the spot. We have, how- 

 ever, no experimental proof of this; for, though rhythmic variations have 

 been observed in the lacteals of the mesentery, it is maintained that these 

 are simply passive, i. e., caused by the rhythmic peristaltic action of the in- 

 testine, each contraction of the intestine filling the lymph-channels more 

 fully, and are not due to contractions of the walls of the lacteal vessels them- 

 selves. In some of the lower animals, for instance in the frog, the muscular 

 walls of the vessels are developed at places into distinctly contractile pro- 

 pulsive organs, spoken of as lymph-hearts. Lastly, it is at least open for 

 us, on the strength of the analogy that osmosis may give rise to increased 



