502 .MOVEMENT OF LYMPH. [BOOK n. 



peculiar interlacing of the muscular fibres above eaeh 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 eolleets at the spot. We have however no 

 experimental proof of this; for, though rhythmic variations ha\r 

 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 intestine, each contraction of the intestine filling the 

 lymph-channels more fully, and are not due to contractions of 

 the walls of the lacteal vessels themselves. In some of the lower 

 animals, for instance in the frog, the muscular walls of the vessels 

 are developed at places into distinctly contractile propulsive-organs, 

 spoken of as lymph-hearts, of which we shall have something to 

 say presently. Lastly, it is at least open for us, on the strength 

 of the analogy that osmosis may give rise to increased pressure on 

 one side of a diffusion septum, to suppose that the very processes 

 which give rise to the- appearance of lymph in the lymph-spaces 

 of the tissues, tend themselves to promote the flow of lymph. We 

 have at least, under all circumstances, one or other of these causes 

 at work, promoting a continual flow from the lymphatic roots to 

 the great veins. They are together sufficient to drive, in man, the 

 lymph from the lower limbs and trunk, against the effects of gravity, 

 into the veins of the neck. In the upper limb, the influences of 

 gravity owing to the varied movements of the limb, are as often 

 favourable to, as opposed to, the natural flow of the lymph; but as 

 we have already said, a long-continued unfavourable action of 

 gravity, especially in the absence of the aid of movements in the 

 skeletal muscles, as when the arm hangs down motionless for some 

 time, leads to accumulation of lymph at its origin in the lymph - 

 spaces. The strength of the causes combining to drive on the 

 lymph is strikingly shewn in animals when the thoracic duct is 

 ligatured ; in such cases a very great distension of the lymphatic 

 vessels below the ligature is observed. 



301. Although the phenomena of disease and, perhaps, 

 general considerations render it probable, that the nervous system 

 governs in some way the stream of lymph, regulating it may be 

 not only the flow along the definite lymph-canals but also the 

 transit of plasma into the lymph-spaces and the escape of lymph 

 thence into the definite canals, our knowledge on these points is 

 very imperfect. We have no proof that the muscular fibres in the 

 w r alls of the lymphatic vessels are governed by nerves, or that the 

 lymph spaces are influenced directly by nervous action ; and most 

 of the attempts to demonstrate any direct action of the nervous 

 system on the lymphatics have hitherto failed. 



It is very difficult to dissociate any such direct action from an 

 indirect influence through vaso-motor changes; for the condition 

 of the vascular system largely affects the formation and hence 

 the flow of lymph. Thus if, in a dog, cannulse having been placed 



