THE CIRCULATION OF THE BLOOD AND LYMPH 99 



the blood itself can be directly registered, and these changes 

 may be spoken of as the blood-pressure pulse. At bottom, 

 as already pointed out, the phenomenon is exactly the same 

 as that we have been dealing with in our study of the 

 external pulse. We are only now to follow, by a more 

 direct, and in some respects a more perfect method, the 

 same wave of blood along the same channel. 



FIG. 28. ARRANGEMENT FOR TAKING A BLOOD-PRESSURE TRACING. 



M, manometer ; Hg, mercury ; F, float armed with writing-point ; A, thread 

 attached to a wire projecting from the drum and supporting a small weight the thread 



keeps the writing-point in contact with the smoked paper on the drum ; B is a strong 



rubber tube connecting 



tube, which is taken off when a tracing is to be obtained. 



the manometer with the artery ; C, a pinchcock on the rubber 



Measurement of the Blood-pressure. Hales was the first to 

 measure the blood-pressure. This he did by connecting a 

 tall glass tube with the crural artery of a horse. The height 

 to which the blood rose in the tube indicated the pressure 

 in the vessel. Poiseuille, nearly half a century later, applied 

 the mercury manometer, which had already been used in 



72 



