304 MOVEMENTS OF CHYLE AND LYMPH, [BOOK n. 



ments the pressure within the intestine is never very great. Of 

 more obvious use is the contraction of the villus itself. The longi- 

 tudinal muscular fibre-cells, in contracting, pull down the villus on 

 itself; the contents of the lacteal chamber are thus forced into the 

 underlying lymphatic plexus. When the fibre-cells relax, the 

 empty lacteal chamber is expanded ; the chyle cannot flow back 

 from the lymphatic channels by reason of the valves present in 

 them, and in consequence the lacteal chamber is filled from the 

 substance of the villus, and thus the entrance into the villus of 

 material from the intestine is facilitated. The villus in fact acts as 

 a kind of muscular suction-pump. 



Movements of the Chyle. Having reached the lymphatic chan- 

 nels the onward progress of the chyle is determined by a variety of 

 circumstances. Putting aside the pumping action of the villi, the 

 same events which cause the movement of the lymph generally also 

 further the flow of the chyle ; and these are briefly as follows. In 

 the first place, the wide-spread 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 a 1 

 muscular movements increase the flow. If a cannula be inserted 

 in one of the larger lymphatic trunks of the limb of a dog, the dis- 

 charge of lymph from the cannula will be more distinctly increased 

 by movements, even passive movements, of the limb than by any- 

 thing else. In addition to the valves along the course of the 

 vessels, the embouchement 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 capil- 

 laries 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 differ- 

 ence is probably not great. So that the lymph in the lymphatic 

 spaces of the tissues may still 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 lymphatic 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 continuous stream. Further, this flow, caused by 

 the lowness of the mean venous pressure at the subclavian, will be 

 assisted at every respiratory movement, since at every inspiration 

 the pressure in the venous trunks becomes negative, and thus 



