Chap, i.] TISSUES AND MECHANISMS OF DIGESTION. 409 



substances, such as leech extract or extract of crabs' muscles. The 

 presence of these substances in the blood appears to produce such 

 a change in the vascular walls, that without any change of pres- 

 sure, a larger amount of lymph and that richer in solids, espe- 

 cially in proteids, makes its way into the lymph-spaces, and so 

 into the thoracic duct. The result has an obvious resemblance 

 to the act of secretion, and the substances in question have 

 been called lymphagogues. We may therefore conclude that two 

 things chiefly determine the amount of transudation : the pres- 

 sure of the blood in the blood vessels, and the condition of 

 the vascular walls in relation to the blood, the latter being at 

 least as important as the former. 



We said just now that we may take the flow of lymph along 

 the thoracic duct as a measure of the transudation from the 

 blood vessels. But this is not strictly true. The lymph in 

 the lymph-spaces, which is the source of the lymph in the 

 thoracic duct, is not simply the result of the transudation 

 from the blood vessels, but the result of the combined action 

 of the blood vessels on the one hand and the tissues on the 

 other. The lymph in the lymph-space is the middleman ; the 

 tissues take from and give to it in some such manner as the blood 

 gives to and, we may add, takes from it. 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 quan- 

 tity of water at one time than at another. The water thus taken 

 up or given out, and other substances carried 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, 

 and thus the flow of lymph along the thoracic duct. For 

 instance, a certain quantity of sugar introduced into the blood 

 gives rise to a very rapid flow of somewhat dilute lymph 

 along the thoracic duct ; and similar results are produced by 

 much smaller quantities of sodium chloride and other sub- 

 stances. Since the blood is found to be, at the same time, 

 more watery, in spite of a copious secretion of urine, we may 

 conclude that the excess of lymph in the lymph-spaces is drawn 

 from the tissues. 



§ 245. 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 ex- 

 ceeds 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 cav- 

 ity being known as ascites. 



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



