1GO ON THE NATl'lIE AM) PRINCIPLES OF 



other animals. We must, therefore, in the absence 

 of any other principle, either consent to adopt this 

 law, or be satisfied to continue in a state of ignorance 

 which is not the less complete from its being disguised 

 beneath a mass of pompous but unmeaning words. 

 This, it must be confessed, is only a negative argu- 

 ment in support of the proposition ; but- 



2ndly. It is an indisputable fact that in all ob- 

 structed blood-vessels there exists, behind the impe- 

 diment, a mass of detained blood, the lateral pressure 

 of which is unnaturally great. The immediate cause, 

 therefore, to which I have assigned effusion is de- 

 monstrably true, so far as regards the fact of its ex- 

 istence, as an adequate expelling force, at that parti- 

 cular portion of the blood-vessels whence the exuded 

 or extravasated matters are manifestly derived. 



And, 3rdly, as it can be shown that the general 

 force of the blood, both in different individuals and 

 in the same individual at different times, is liable to 

 very great changes in its amount, so also will the 

 lateral pressure of the blood contained in any ob- 

 structed vessel be necessarily subjected to an equal 

 range of variation. This fluctuation in the amount 

 of lateral pressure of the minute blood-columns in 

 different individuals will therefore explain the variety 

 in the substances (constituents of the liquor san- 

 guinis) expelled, through the porous membrane of 

 the capillary vessels, in a number of cases of ob- 

 structed circulation. We may, consequently (re- 

 serving for future consideration some modifying 

 circumstances), claim for the above-mentioned law a 

 firmer foundation than mere negative evidence, and 



