GENERAL FEATURES OF THE CIRCULATION 983 



considerable increase in volume, but from this point the increments 

 of volume with rising pressure rapidly diminish. Whereas the artery is 

 most distensible at about 100 mm. Hg, the vein has its limits of 

 optimum distensibility between and 10 mm. Hg. 



As the arteries branch, although each branch is smaller than the 

 parent vessel, the total area of the two branches into which the vessel 

 divides is greater. Thus there is a continual increase in the cross 

 area of the bed of the blood-stream as we pass from the heart 

 towards the periphery. This increase is especially marked at the 

 junction between the capillaries and the arterioles at one side and the 

 venules on the other, so that the total area of the bed in the region 

 of the capillaries can be taken as about 800 times that of the area of 

 the aorta where the blood leaves the heart. 



On cutting through an artery, blood escapes from the central end, 

 i.e. that nearest the heart, with great force and in a series of jerks, 

 each of which corresponds to a contraction of the ventricles. This 

 manner of escape shows that in the arteries the blood is at a high 

 pressure, and that the flow from the heart to the periphery is a pul- 

 satory one. The same lesson may be learnt by connecting a long 

 tube with the central end of a divided artery. This experiment, 

 which was first performed by the Rev. Stephen Hales, may be described 

 in his own words : 



" In December I caused a mare to be tied down alive on her back ; she was 

 fourteen hands high, and about fourteen years of age, had a Fistula on her 

 Withers, was neither very lean, nor yet lusty : Having laid open the left crural 

 Artery about three inches from her belly, I inserted into it a brass Pipe, whose 

 bore was one sixth of an inch in diameter ; and to that, by means of another 

 brass Pipe which was fitly adapted to it, I fixed a glass Tube, of nearly the same 

 diameter, which was nine feet in length : Then untying the Ligature on the 

 Artery, the blood rose in the Tube eight feet three inches perpendicular above 

 the level of the left Ventricle of the heart : But it did not attain to its full height 

 at once ; it rushed up about half way in an instant, and afterwards gradually 

 at each Pulse twelve, eight, six, four, two, and sometimes one inch : When it 

 was at its full height, it would rise and fall at and after each Pulse two, three, 

 or four inches ; and sometimes it would fall twelve or fourteen inches, and have 

 there for a time the same Vibrations up and down at and after each Pulse, as it 

 had, when it was at its full height ; to which it would rise again, after forty or 

 fifty Pulses." 



The method adopted by Hales of measuring the lateral pressure of 

 blood in the vessels offers very considerable drawbacks. The manipu- 

 lation of such long tubes is awkward, and the blood which escapes into 

 the tubes very soon clots and renders further observation impossible. 

 It is therefore customary when we desire to gain an idea of the average 

 pressure in any blood-vessel, especially in an artery, to use a mercurial 

 manometer for this purpose. This instrument, which was first 

 applied to physiological purposes by Ludwig, consists of a U -tube with 



