86 PHYSIOLOGY OF THE CIRCULATION. 



bouring veins, gravity, or any other cause which im- 

 pedes the return of venous blood, must, at the same 

 time, induce a corresponding increase in the amount 

 of lateral pressure of the blood contained in the 

 capillaries supplying those veins. But it is chiefly 

 in secreting structures that a disposition of parts so 

 unfavourable to the free circulation of the blood 

 through the capillaries is observed to occur. And 

 on applying to these cases the law frequently referred 

 to, viz. that any impediment to the passage of a fluid 

 increases the amount of its lateral pressure, how are 

 we to avoid the conclusion that this peculiar physical 

 condition of the blood contained in the capillaries of 

 secreting organs is a provision of nature for the 

 accomplishment of the necessary exudation of certain 

 portions of that compressed fluid ? The blood con- 

 tained in the capillaries of different parts of the body 

 must therefore exert different degrees of lateral 

 pressure according as the impediments to its passage 

 into the veins are more or less considerable. An 

 opinion formerly prevailed, supported by Keil, Hales, 

 and other physiologists, that the extraordinary im- 

 pediments to the free return of the venous blood, 

 which are almost invariably observed in organs 

 whose natural secretion is highly albuminous, are, in 

 some way or other, connected with that peculiarity 

 in the secreted fluid. Our knowledge of the minute 

 anatomy of some of these organs is perhaps too 

 imperfect to justify a positive conclusion on the 

 subject. But the relative viscidity of an cftuscd 

 fluid being, by the facts before referred to, shown to 

 depend upon the degree of compression endured by 



