64 PHYSIOLOGY OF THE CIRCULATION. 



from the blood-vessels to the different tissues of 

 the body. 



2ndly. Secretion and excretion : and in the per- 

 formance of these functions we may presume that 

 the mechanism of effusion is similar, whether the 

 matters discharged by the secreting organs are sepa- 

 rated directly from the blood, or are maturated in 

 an intermediate series of cells. 



Srdly. Under the term vascular absorption, I 

 would comprise all those functions which consist 

 essentially in the entrance within the blood-vessels 

 of various external substances. 



Of the forces which produce this passage of fluids 

 into and out of the blood-vessels, and of the laws by 

 which these forces are regulated, we know very 

 little with certainty. The explanation at present 

 most generally received is that advanced by Ma- 

 gendie, Fodera, and other French physiologists, viz. 

 that all these functions are referable to the power of 

 capillarity inherent in the porous coats of the vessels, 

 and are thus analogous to the phenomena of eiidos- 

 mosis and exosmosis. 



To this doctrine it may be objected 



1st. That the majority of experiments adduced 

 in its support were performed with fluids stagnant, 

 and subjected to equal pressure conditions which 

 can seldom or never occur, in precisely an equal 

 degree, to the fluids situated on either surface of the 

 minute blood-vessels of the living body. 



2ndly. --This hypothesis does not explain how, in 

 particular organs, one or other action is permitted 

 wholly to preponderate ; and it is quite certain that 



