134 ON THE NATURE AND PRINCIPLES OF 



In a former communication to this Journal (June 

 and July 1844), I endeavoured, by numerous phy- 

 sical and physiological facts and arguments, to 

 establish the following law, viz. that " all those 

 functions which involve either the discharge through 

 the coats of the minute blood-vessels of certain 

 matters previously present in the blood, or the en- 

 trance of various external substances into the mass 

 of circulating fluid, are dependent upon a prepon- 

 derance, in one or other direction, of the forces 

 acting on either surface of those vessels." It fol- 

 lows, therefore, as immediate consequences of this 

 law, that effusion, or the outward passage of fluids 

 through the capillary membrane is promoted, 1st, 

 directly by whatever increases the amount of pressure 

 acting on the internal surface of the blood-vessels ; 

 and 2ndly, indirectly by whatever diminishes the 

 pressure acting on their exterior. While absorption, 

 or the inward passage of fluids through the same 

 porous structure is rendered more active, 1st, by 

 whatever directly increases the amount of external 

 pressure ; and 2ndly, by whatever increases the ra- 

 pidity of the currents of blood, and thus diminishes 

 the pressure acting on the internal surface of those 

 blood-vessels which are subservient to this function. 

 In assigning to the minute arterial twigs and con- 

 tiguous capillaries, a certain, constant, but moderate, 

 amount of effusion as their natural and specific use 

 in the animal economy*, and in referring to the 



* Though compelled to speak of the, function of the arterial, as 

 clistiiiguMinl Ironi that of tiic venous capillaries, I am anxious to 

 have it distinctly understood as my opinion, that the >.//- </' these 



