268 PHYSIOLOGICAL PHYSICS. [Chap. xxn. 



Dropsies are of this nature. The backward pressure 

 communicated through the veins on lungs, liver, and 

 other organs, owing to obstruction to the onward 

 current of blood from the heart, are very good 

 examples of the amount of filtration that can occur 

 through the walls of the blood-vessels by great 

 increase of the pressure within them. This, again, 

 has nothing to do with exosmosis. As soon as the 

 obstacle to the current of blood is removed, and the 

 normal flow re-established, absorption comes in, 

 endosmosis from the infiltrated tissues arises, and the 

 poured-out fluid (serum) is taken up again into the 

 circulating stream. Whether or not transudation 

 under pressure has anything to do with secretion, 

 with such a secretion as that of the kidney, is another 

 question. It is generally supposed that into the 

 capsule of the glomerulus of the kidney such a filtra- 

 tion takes place. At least the physical conditions are 

 present, and the current of blood is separated from 

 the cavity of the glomerulus by the fine walls of the 

 vessels, and the epithelium of the capsule ; the 

 afferent vessel is large in proportion to the efferent 

 vessel. Owing to the difference between the two, and 

 the difference in favour of the incoming blood, the 

 pressure must be considerably increased in the tuft, 

 and thus filtration under pressure is a natural result. 

 How such filtration should permit the escape of saline 

 matters and retain albumen, it is not easy to explain. 

 If the agent were osmosis, the retention of albuminous 

 substance by the colloidal septum (the walls of the 

 vessels) is easily enough understood. But the con- 

 ditions in the kidney are not such as to favour 

 osmosis, at least so far as the glomerules are con- 

 cerned. 



If we accept the process as one of filtration, and 

 take the results of Schmidt referred to, there seems 

 no rational ground for holding that in the kidney 



