322 THE TISSUES AND MECHANISMS OF DIGESTION. 



lymph, including the proteids, may at times be reabsorbed from the tissue 

 into the bloodvessels, it is distinctly contradicted. Finally, the introduction 

 of certain substances (termed lytnphagogues) into the blood, is followed by an 

 increase in the quantity of lymph without any related change in the blood- 

 pressure. We shall have to return to this question when we come to deal 

 with the secretion of urine ; but meanwhile we may adopt the conclusion, 

 which is especially supported by the phenomena of disease, that while dif- 

 fusion and filtration play their respective parts, diffusible substances pass- 

 ing in and out of the blood more readily than indiffusible substances and an 

 increase of pressure tending to promote transudation, the condition of the 

 vascular wall so profoundly influences the transit of material as to render 

 the process very complex. We may probably regard it as too complex to 

 be compared even with filtration through a filter capable of widely changing 

 in texture from time to time, and as more nearly resembling the process of 

 secretion. 



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. 



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. 



256. 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 patho- 

 logical conditions those limits may be exceeded, and the result is known as 

 cedema 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 obstruction 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 con- 

 ditions of the vascular system, and is the result not of hindered outflow, but 

 of excessive transudation. 



Owing to the numerous anastomoses of the lymph-vessels and the conse- 



