332 HANDBOOK OF PHYSIOLOGY. 



pillary blood-vessels, ramifying and forming a network around the walls 

 and in the interstices of the ducts and acini. 



Process of Secretion. In secretion two distinct processes are con- 

 cerned which may be spoken of as I. Physical, and II. Chemical. 



1. Physical processes. These, already discussed in the last chapter, 

 are such as can be closely imitated in the laboratory, inasmuch as they 

 consist in the operation of well-known physical laws ; they are (a) Fil- 

 tration; (b) Dialysis. 



(a) Filtration is, as we have already mentioned, simply the passage 

 of a fluid through a porous membrane under the influence of pressure. 

 If two fluids be separated by a porous membrane, and the pressure on 

 one side is greater than on the other, it is evident that in the absence of 

 counteracting osmotic influences (see below), there will be a filtration 

 through the membrane until the pressure on the two sides is equalized. 

 Of course there may be fluid only on one side of the membrane, as in the 

 ordinary process of filtering through blotting-paper, and then the filtra- 

 tion will continue as long as the pressure (in this case, the weight of the 

 fluid) is sufficient to force it through the pores of the filter. The neces- 

 sary inequality of pressure may be obtained either by diminishing it on 

 one side, as in the case of cupping; or increasing it on the other, as in 

 the case of the increased blood-pressure, and consequent increased flow 

 of urine resulting from copious drinking. By filtration, not merely 

 water, but various salts in solution, and even colloids of all kinds, may 

 transude from the blood-vessels. The amount of a liquid which will 

 pass through a filter in a given time depends not only upon the amount 

 of pressure to which it is subjected, but also upon the natuye of the 

 fluid filtered, and upon the kind of membrane employed as the filter. 

 It seems probable that some fluids, such as the secretions of serous mem- 

 branes, are simply exudations or oozings (filtration) from the blood- 

 vessels, whose qualities are determined by those of the liquor sanguinis, 

 while the quantities are liable to variation, and are chiefly dependent 

 upon the blood-pressure. 



(b) Dialysis is the passage of fluids through a moist animal mem- 

 brane independent of pressure, and sometimes actually in opposition to it. 

 There must always be in this process two fluids differing in composition, 

 one or both possessing an affinity for the intervening membrane, and the 

 fluids must be capable of mixing one with the other ; the osmotic current 

 continuing in each direction (when both fluids have an affinity for the 

 membrane) until the chemical composition of the fluid on each side of 

 the septum becomes the same. 



2. Chemical processes. The chemical processes constitute the process 

 of secretion, properly so called, as distinguished from mere transudation 

 .spoken of above. In the chemical process of secretion various materials 

 "which do not exist as such in the blood are elaborated by the agency of 



