CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 405 



great veins during expiration is warded off from the duct by the 

 valve at its opening. In the third place, the flow may be in- 

 creased by rhythmical contractions of the walls of the lymphatics 

 themselves, which 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 however no ex- 

 perimental 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 per- 

 istaltic action of the intestine, each contraction of the intestine 

 filling the lymph-channels more fully, and are not due to con- 

 tractions 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. [Lastly, if the 

 very processes wjiich give rise to the appearance of lymph in the 

 lymph-spaces of the tissues are, as we shall see we have some 

 reason to think, analogous to the process of secretion, then 

 remembering the pressure which is developed by the secretion 

 of a secreting gland, such as a salivary gland ( 189), we may 

 regard these very processes as tending 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 

 sutficient 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 strik- 

 ingly 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. 



243. 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 as yet at least no proof that the mus- 

 cular fibres in the walls of the lymphatic vessels are governed 

 by nerves, or that the lymph-spaces are influenced directly by 



