THE NATURE AND MOVEMENTS OF LYMPH. 319 



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 tis- 

 sues, tend themselves to promote the flow of lymph. We have at least, un- 

 der all circumstances, one or other of these causes at work, promoting a 

 continual flow from the lymphatic roots to the great veins. They are to- 

 gether 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 favorable to as opposed to the natural flow of the lymph ; but, as 

 we have already said, a long-continued unfavorable action of gravity, espe- 

 cially 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 shown in animals when the thoracic duct 

 is ligatured ; in such cases a very great distention of the lymphatic vessels 

 below the ligature is observed. 



254. Although the phenomena of disease and, perhaps, general con- 

 siderations 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 walls 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. 



255. The passage of material, namely, of water containing certain sub- 

 stances in solution, from the interior of the bloodvessel, where they form part 

 of the plasma, into the lymph-capillary where they are called lymph, consists 

 of two steps : the passage from the bloodvessel into the lymph-space, and the 

 passage from the lymph-space into the lymph-capillary ; for, as we have seen, 

 it is only in particular places that the lymph-capillary immediately surrounds 

 the bloodvessel. Once arrived in the lymph-capillary the lymph finds an 

 open path along the rest of the lymphatic system, but the connection between 

 the lymph-space and the lymph-capillary is, as we have seen, peculiar and at 

 least not a free and open one. 



The passage of material from the bloodvessel into the lymph-space we 

 speak of as transudation. What can we say as to the nature of this process ? 

 There are two known physical processes with which we may compare it : 

 diffusion through a membranous or other porous partition, and filtration 

 through a similar partition. Diffusion, though influenced by fluid pressure, 

 is not the direct result of fluid pressure, but may, on the contrary, be the 

 cause of differences of pressure on the two sides of the partition, and may 

 work against fluid pressure. When a strong solution and a weak solution 

 of salt are separated by a diffusion septum, diffusion takes place whether the 

 columns of fluid be at the same level on the two sides of the septum or at 

 different levels ; and if the columns be at the same level to start with, that 

 of the stronger solution soon comes to exceed the other in height, on account 

 of the osmotic flow of water from the weaker into the stronger solution. 

 Filtration, on the other hand, is the direct result of pressure; without dif- 

 ference of pressure filtration does not take place ; and, the filter remaining 

 of the same nature and in the same condition, the amount of filtrate is 

 dependent on the amount of pressure. May we speak of the process of 

 transudation as a simple process of diffusion or a simple process of filtration ? 

 that is to say, can all the phenomena of transudation be explained as simply 



