SECRETION. o25 



1. Physical processes. These 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) Filtration, (b) Diffusion. 



(a) Filtration is simply the passage of a fluid through a porous mem- 

 brane 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 influ- 

 ences (see below) there will be a filtration through the membrane until 

 the pressure on the two sides is equalized. Of course there may only 

 be fluid on one side of the membrane, as, in the ordinary process of filter- 

 ing through blotting-paper, and then the filtration* 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 necessary 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, may transude from the blood-vessels. It seems probable that 

 some fluids, such as the secretions of serous membranes, are simply exu- 

 dations 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) Diffusion 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 composi- 

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

 and the fluids capable of being mixed 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. These constitute the process of secretion prop- 

 erly 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 the gland-cells from 

 the blood, or, to speak more accurately, from the plasma which exudes 

 from the blood-vessels into the interstices of the gland-textures. 



The best evidence for this view is: 1st. That cells and nuclei are con- 

 stituents of all glands, however diverse their outer forms and other char- 

 acters, and are in all glands placed on the surface or in the cavity whence 

 the secretion is poured. 2nd. That many secretions which are visible 

 with the microscope may be seen in the cells of their glands before they 

 are discharged. Thus, bile may be often discerned by its yellow tinge in 

 the gland-cells of the liver; spermatozoids in the cells of the tubules of 



