80 PHYSIOLOGY OF THE CIRCULATION. 



before mentioned, it is equally evident that this 

 great lateral pressure of the fluid contained in the 

 arteries can only be occasioned by the existence 

 of a considerable difference between the relative 

 facilities afforded for the influx and efflux of the 

 arterial blood. As to the absolute force of the 

 outward pressure, it must necessarily be subject 

 to a still more extensive range of variation than 

 that of the onward impulse derived from the heart's 

 action ; for it is equally affected by a change in 

 the condition of the heart, or of the smaller vessels, 

 both which changes are continually occurring. It 

 may therefore, in the same individual, be at one 

 time so slight as to be incapable of accomplishing 

 its healthy uses, while, at another period, its dis- 

 tending power may be so considerable as to rupture 

 some of the containing vessels, and thus threaten 

 the destruction of life. 



Having, I trust, satisfactorily proved that the 

 whole mass of arterial blood exerts, during health, 

 a considerable amount of outward or distending 

 pressure, it now only remains for me to consider 

 the second point which I proposed for examination, 

 viz. whether any uses are served in the animal eco- 

 nomy by the existence of this peculiar physical 

 condition of the arterial blood ; and in studying the 

 physiological effects resulting from the operation of 

 this force, it will be necessary to view it as existing, 

 first, in the large arteries, and, secondly, in the 

 minute arterial ramifications ; for as the physical 

 and vital properties of these vessels are essentially 

 different, so will the application of the same dia- 



