VELOCITY AND PRESSURE OF BLOOD-FLOW. 455 



the fact that the important part of the circulation, as regards the 

 nutritive activity of the blood, is the capillary path. It is while 

 flowing through the capillaries that the chief exchange of gases 

 and food material takes place. The average length of a capillary 

 is estimated at 0.5 mm.; so that with a velocity of 0.5 mm. per 

 second the average duration of the flow of any particle of blood 

 through the capillary area is only about 1 sec. 



The Pressure Relations in the Vascular System. That the 

 blood is under different pressures in the several parts of the vascu- 

 lar system has long been known and is easily demonstrated. When 

 an artery is cut the blood flows out in a forcible stream and with 

 spurts corresponding to the heart beats. When a large vein is 

 wounded, on the contrary, although the blood flows out rapidly, 

 the stream has little force. Exact measurements of the hydrostatic 

 pressure under which the blood exists in the large arteries and veins 

 were first published by Rev. Dr. Stephen Hales, an English clergy- 

 man, in his famous book entitled "Statical Essays, containing 

 Haemostaticks," 1733.* This observer measured the static pressure 

 of the blood in the arteries and veins by the simplest direct method 

 possible. After tying the femoral artery in a horse he connected 

 it to a glass tube 9 feet in length. On opening the vessel the blood 

 mounted in the tube to a height of 8 feet 3 inches, showing that 

 normally in the closed artery the blood is under a tension or pressure 

 sufficient to support the weight of a column of blood of this height. 

 A similar experiment made upon the vein showed a rise of only 12 

 inches. 



Methods of Recording Blood-pressure. Since Hales's work 

 the chief improvements in method which have marked and caused 

 the development of this part of the subject have been the application 

 of the mercury manometer by Poiseuillef (1828), the invention of 

 the recording manometer and kymographion by LudwigJ (1847), 

 and the later numerous improvements by many physiologists, and 

 latterly the development of methods for measuring blood-pressures 

 directly in man. The Hales method of measuring arterial pressure 

 directly in terms of a column of blood is inconvenient on account 

 of the great height, large fluctuations, and rapid clotting. The 

 two former disadvantages are overcome by using a column of mer- 

 cury. Since this metal is 13.5 times as heavy as blood, the column 

 which will be supported by the blood will be correspondingly shorter 

 and all the fluctuations will be similarly reduced. Poiseuille 

 placed the mercury in a U tube of the general form shown in Fig. 

 181, M. One leg was connected with the interior of an artery by 



