916 PHYSIOLOGY 



distensibility, i. e. the increase in capacity of an artery and a vein under 

 gradually increasing internal pressure. It will be seen that an artery, which 

 has a certain capacity at zero pressure, gradually distends with increasing 

 pressure. The increase in capacity is small at first, and becomes most 

 rapid between 90 and 100 mm. Hg. After this point every increment of 

 pressure brings about a gradually diminishing increment of capacity. Thus 

 a change of internal pressure causes the greatest change in capacity when 

 the pressure in the artery corresponds, as we shall see, to the average arterial 

 pressure in the normal animal. In the vein, on the other hand, the capacity, 

 which is nothing at zero pressure, becomes considerable on raising the 

 pressure to 1 mm. Hg. A further rise of pressure to 10 mm. Hg. causes a 

 considerable increase in volume, but from this point the increments of volume 

 with rising pressure rapidly dimmish. 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 on 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 pulsatory 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 la}d open the left crural Artery about three inched 

 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 ouch 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 manipula- 



