CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 507 



vascular wall so profoundly influences the transit of material as to 

 render the process very complex. We may probably r^gapd-it as 

 too complex to be compared even with filtration thyottgh a filter 

 capable of widely changing in texture from time tof tifcfi^, and as 

 more nearly resembling the process of secretion. 



Concerning the passage of the lymph from 

 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. 



In either case the flow from the lymph-spaces will be 

 facilitated by all events which promote, and checked by those 

 which hinder the flow of lymph along the lymph-capillaries and 

 the other lymphatic channels. 



We may here remark as influencing the quantity of lymph in 

 the lymph-spaces and vessels, that the quantity of lymph taken 

 up from the lymph-spaces by the actual elements of the tissue 

 may vary considerably. We remarked in 30 on the peculiar 

 relations of living tissue to water, and there are reasons for 

 thinking that the very substance of a cell or a fibre (a muscular 

 fibre for instance) may hold in itself a larger quantity of water 

 at one time than at another. The water thus taken up or given 

 out, and the substances which may be carried in solution by that 

 water, come from and go to the lymph. The condition of the 

 tissue determines by itself the amount of lymph in the lymph - 

 spaces. 



303. Under the influence of all these several actions the 

 lymph in the various lymph-spaces of the body varies in amount 

 from time to time, but under normal circumstances never exceeds 

 certain limits. Under pathological conditions those limits may be 

 exceeded, and the result is known 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. 



The possible causes of oedema are on the one hand an obstruc- 

 tion to the flow of lymph from the lymph-spaces, and on the other 

 hand an excessive transudation, the lymph gathering in the lymph- 

 spaces faster than it can be carried away by a normal flow ; with 

 the former the lymphatic system itself, with the latter chiefly 

 the vascular system is concerned. As a matter of fact however 

 oedema is almost always, if not always, due to abnormal conditions 



