78 PHYSIOLOGY OF THE CIRCULATION. 



exerted by a moving fluid is wholly regulated by 

 the relative facilities afforded to its passage into, and 

 out of, the containing tube, so the various obstacles 

 opposing the free course and discharge of the arterial 

 blood will increase the lateral pressure of that fluid 

 as effectually as the diminution of the external orifice 

 of the pipe did in the foregoing experiments. The 

 truth of this latter statement may also be proved by 

 actual experiment. 



Exp. 4. Thus, having connected with the same 

 reservoir a leaden pipe, a yard long, perfectly straight, 

 of uniform calibre, and maintained in the horizontal 

 position, I bored a small hole on its upper surface, at, 

 the distance of a few inches from its commencement. 

 A full stream being then allowed to flow through it, 

 there was no escape of water from this aperture, 

 showing that the lateral pressure exerted by the 

 moving fluid was at a minimum. I now bent, laterally, 

 the external half of the pipe, so as to cause it to 

 describe a gradual curve, the horizontal position being 

 apparently unaffected. But when the stream was 

 again permitted to traverse it, the lateral pressure of 

 the fluid detained behind the curve had increased so 

 much as now to cause a small jet to escape through 

 the aperture. 



It is therefore evident that, in addition to contrac- 

 tion, any of those other peculiarities in the form and 

 arrangement of tubes, which are known to diminish 

 their power of discharge, will also increase the lateral 

 pressure of the fluid contained behind the impeding 

 portions of the tube ; and as it has, moreover, been 

 shown that the amount of lateral pressure incre;; 



