536 TRANSUDATION. [BOOK n. 



pressure be brought to bear upon it. It is a filter of such a 

 kind that irrespective of pressure the things which it allows to 

 pass are not always of the same kind. It is a filter which while a 

 current is passing from the inside of a capillary to the lymph-space 

 allows of the coincident passage of material against pressure from 

 the lymph-space to the capillary. It is a filter which appears to 

 allow filtration to go on hand in hand with diffusion ; for there 

 are many facts which seem to point to the laws of diffusion holding 

 good for the capillary wall as regards diffusible substances; it acts 

 apparently in the same way as if one were making experiments 

 on filtration and on diffusion at one and the same time with the 

 same piece of filter-paper. Obviously the passage of lymph through 

 the capillary wall is a very complex process, so complex that, even 

 recognizing the influence of pressure, we may fairly ask, is any- 

 thing gained, and may not error be courted, by speaking of it at 

 all as a process of filtration ? It is a complex process the factors 

 of which are continually changing, a process which includes factors 

 resembling at least those both of filtration and of diffusion, but a 

 process in which both these factors are modified by the continual 

 changes taking place in the nature of the capillary wall. It is 

 much more like the still more complex process of secretion, of which 

 as we shall see there are more kinds than one, than any simple 

 physical process of which we have knowledge. 



Concerning the passage of the lymph from the confined 

 lymph-spaces into the open gangways of the lymph-capillaries 

 we know very little. If, as some think, the cavity of the 

 lymph-capillary is shut off on all sides and completely by a 

 continuous lining of sinuous epithelioid plates, then the passage 

 from the lymph-space into it must be regarded, as a sort of 

 repetition of the passage from the blood-capillary into the lymph- 

 space, as a second transudation. But if, as others think, and as on 

 the whole seems more probable, the lymph-spaces open, at places, 

 directly into the lymph-capillaries the passage is a simply 

 mechanical affair determined by the freedom of these openings. 



303. Under the influence of all the several actions above 

 discussed the lymph in the various lymph-spaces of the body 

 varies in amount from time to time, but under normal circum- 

 stances never exceeds certain limits. Under pathological con- 

 ditions those limits may be exceeded, and the result is what we 

 have already spoken of as oedema or dropsy. Similar excessive 

 accumulations of lymph may occur not in the ordinary lymph- 

 spaces, but in those larger lymph-spaces, the serous cavities, 

 any large excess of fluid in the peritoneal cavity being known as 

 ascites. 



We have seen that oedema is almost always, if not always, due, 

 not to an obstruction to the flow of lymph from the lymph-spaces, 

 but to an excessive transudation, the lymph gathering in the 

 lymph-spaces faster than it can be carried away by a normal 



