156 ON THE NATl UK VXD 1'lilNCIPLES ()T 



briefly to consider the general effects produced by 

 the obstruction of any freely-moving mass of fluid. 



I have, throughout these investigations, had con- 

 stant occasion to dwell upon and apply this general 

 hydraulic principle ; viz., that the degree of lateral 

 pressure which a fluid exercises while traversing a 

 1 1 ilte, or system of tubes, is altogether dependent 

 upon the relative facilities afforded for the influx and 

 efflux of that fluid. And, as a corollary from this 

 law, that any circumstance which tends to lessen the 

 rate of discharge, while the rate of influx continues 

 unaltered, will instantly, and in a very rapid ratio, 

 increase the lateral pressure of the column of fluid 

 till it becomes equal to the whole impelling force. 



Now in the case of the fluid traversing the vessels 

 of the living body, that impelling force is clearly 

 derived from, and proportionate to the amount of, 

 pressure of the general mass of aortic blood ; which 

 pressure is known to fluctuate very considerably, 

 and sometimes very suddenly, from various circum- 

 stances, particularly those affecting the vigour and 

 frequency of the heart's action. It follows, there- 

 fore, that the actual amount of lateral pressure with 

 which each minute column of detained blood acts 

 against the walls of its containing vessel, will, in 

 every case of obstructed capillary circulation, be 

 wholly regulated by two essential circumstances; 

 viz., the degree of completeness of the local obstruc- 

 tion on the one hand, and the actual amount of im- 

 pelling pressure then resident in the mass of arterial 

 blood, on the other. The hitter is, in all cases, evi- 

 dently (he in-th:e cause ol ;i morbidly increased lateral 



