528 MOVEMENT OF LYMPH. [BOOK 11. 



valve below collects at the spot. We have however no experi- 

 mental 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 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, if the processes which give rise to the 

 appearance of lymph in the lymph-spaces of the tissues are, as we 

 shall see we have at least some reason to think, analogous to the 

 process of secretion, we may perhaps 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 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, with other causes, to accumulation of lymph at its 

 origin in the lymph-spaces. The strength of the causes com- 

 bining to drive on the lymph is strikingly shewn in animals when 

 the thoracic duct is ligatured; in such cases a very great dis- 

 tension of the lymphatic vessels below the ligature is observed. 



301. We might perhaps expect to find that this stream of 

 lymph is in some way governed by the central nervous system, as 

 we have seen the blood stream in the blood vessels to be. But we 

 have as yet at least no proof that the muscular fibres in the coats 

 of the lymphatic vessels are in any way governed by nerves 

 corresponding to vaso-motor nerves. Indeed the mere flow of 

 lymph along the lymph-channels from the lymph-capillaries to the 

 mouth of the thoracic duct is a relatively constant, and hence 

 unimportant matter, compared with the appearance of lymph in 

 the lymph-spaces and lymph-capillaries. When an unusual quantity 

 of lymph is gathered in the lymph-spaces and lymphatic vessels of 

 a tissue or organ, a condition known as oedema, the cause of the 

 accumulation is rarely, or to a slight extent only, any hindrance or 

 obstruction to the flow along the lymphatic vessels. Owing to 

 the numerous anastomoses of the lymph-vessels and the con- 

 sequent establishment of collateral streams, obstruction in the 

 lymph-passages themselves rarely if ever gives rise to a block; 

 and it may be here remarked that owing to the same free col- 



