PHYSIOLOGY OF THE CIRCULATION. 77 



amount of lateral pressure exerted by the water tra- 

 versing the horizontal pipe, as measured by the 

 height of the column in the glass tube, was altogether 

 dependent on the degree of facility afforded to the 

 passage of the fluid through that pipe, by the relative 

 areas of its inner and outer orifices. 



While the conditions affecting the influx and 

 efflux were equal, the lateral pressure was very in- 

 considerable ; but when the discharge was diminished 

 by the application of an impediment to the outer 

 orifice alone, then the lateral pressure increased in a 

 very rapid ratio. Thus, in the second experiment, a 

 diminution in the diameter of the outer orifice, from 

 one-half to one-third of an inch, increased the lateral 

 pressure of the contained fluid from something less 

 than a column of water one inch in height to that of 

 one nine inches high, which was equal to three- 

 fourths of the whole impelling force ; and in the last 

 experiment, a diminution of the diameter of the ex- 

 ternal orifice to one-third, viz. from one-half to one- 

 sixth of an inch, caused the lateral pressure of the 

 fluid in the horizontal pipe to approach to within 

 half an inch of that of the impelling column. 



Before applying these principles to the fluid con- 

 tained in the arteries, I may be allowed to repeat a 

 former observation, viz. that whatever proportion 

 the area of the aorta may bear to that of the sum of 

 its terminal branches, the obstacles preventing the 

 free passage of blood through the latter will diminish 

 its rate of discharge from them as virtually as though 

 their joint calibre were actually less than that of the 

 main trunk ; and as the amount of lateral pressure 



