CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 411 



companied with a want of adequate renewal of the blood ; this 

 probably affects the epithelioid lining of the blood vessels in 

 such a way as to increase the transudation. And indeed, as is 

 seen in cases of heart disease with prolonged or repeated venous 

 obstruction, the oedema, as time goes on and the tissues become 

 impaired, is more easily excited and with greater difficulty 

 removed, though the actual amount of obstruction, the actual 

 increase of pressure in the small vessels, remains the same, or 

 at least is not proportionally increased. 



Still another kind of oedema is one due to changes taking 

 place in the blood, quite apart from variations of blood-pressure. 

 This kind of oedema is seen in some diseases of the kidney, in 

 " Bright's disease " for instance. In such cases the blood con- 

 tains less proteids, and indeed less solids, is more watery and 

 of lower specific gravity than is normal. But the oedema is not 

 in these cases to be explained on the view that the more watery 

 blood passes more readily through the capillary walls, for it may 

 be shewn experimentally that the mere thinning of the blood, 

 as by the injection of normal saline solution into the blood ves- 

 sels, will not at on.ce lead to oedema, at least in the limbs and 

 trunk, and it is these which in Bright's disease especially be- 

 come cedematous. In all probability the oedema of Bright's 

 disease if it be really due to the abnormal character of the blood, 

 is produced by the abnormal blood so acting on the blood vessels 

 that these allow a transudation greater than the normal. Lastly, 

 calling to mind what we said just now as to the relations of 

 the tissue to the lymph, we must remember that the cause of 

 oedema may also lie in changes in the tissue itself. 



But these are pathological questions into which we must not 

 enter here. We have touched upon them because they illus- 

 trate the important processes taking place in the lymph-spaces, 

 and as we have more than once insisted the lymph in the lymph- 

 spaces is the middleman of all the tissues, and hence facts illus- 

 trating the laws which govern the flow of lymph into and out 

 of the lymph-spaces are of fundamental physiological impor- 

 tance. 



