THE SECRETORY TISSUES AND ORGANS. 287 



the passage of albumen and fibrinogen. , Under healthy con- 

 ditions, for example, the urine contains no albumen, but any- 

 thing considerably increasing the capillary pressure in the kid- 

 neys will cause it to appear. Dialysis or osmosis has already 

 been considered (p. 42); by it substances pass through the in- 

 termolecular pores of a membrane independently of the press- 

 ure on either side, and for its occurrence two liquids of dif- 

 ferent chemical constitution are required, one on each side of 

 the membrane. At least if diffusion takes place, as is proba- 

 ble, between two exactly similar solutions, the amount and 

 character of the substances passing opposite ways in a given 

 time are exactly equal, so that no change is produced by the 

 dialysis; which practically amounts to the same thing as if 

 none occurred. When a solution is placed on one side of a 

 membrane allowing of dialysis, and pure water on the other, 

 it is found that for every molecule of the dissolved body that 

 passes one way a definite amount of water, called the en- 

 dosmotic equivalent of that body, passes in the opposite 

 direction. Crystalline bodies as a rule (haemoglobin is an 

 exception) have a low endosmotic equivalent or are readily 

 dialyzable; while colloids, such as gum and proteids, have a 

 very high one, so that to get, by dialysis, a small amount of 

 albumen through a membrane, a practically infinite amount 

 of water must pass the other way. Accordingly, if we find 

 such bodies in a secretion we cannot suppose that they have 

 been derived from the blood by mere osmosis. 



The Chemical Processes of Secretion. As above pointed 

 out certain secretions, called transudata, seem to be products 

 of filtration and dialysis alone, containing only such sub- 

 stances as those which are found in the blood-plasma, more 

 or less altered in relative quantity by the ease or difficulty 

 with which they severally passed through the layers met 

 with on their way to the surface. But in many cases the 

 composition of a secretion cannot be accounted for in this 

 way ; it contains some specific element, either a substance 

 which does not exist in the blood at all and must therefore 

 have been added by the secreting membrane, or some body 

 which, although existing in the blood, does so in such minute 

 proportion, compared with that in which it is found in the 

 secretion, that some special activity of the secreting cells is 

 indicated : some affinity in them for these bodies by which 

 they actively pick them up. 



