VELOCITY AND PRESSURE OF BLOOD-FLOW. 503 



hydrostatic pressure equal to the height of the column of liquid 

 between the feet and the heart, which adds itself to the pressure 

 resulting from the circulation as caused by the heart. When the 

 animal is in a recumbent position the hydrostatic factor practi- 

 cally disappears. (See p. 518.) 



The Method of Determining Blood-pressure in the Largd 

 Arteries of Man. It is a matter of interest and practical impor- 

 tance to ascertain even approximately the arterial pressure in mail 

 and its variations in health and disease. The first practical method 

 for determining this point upon man was suggested by von Basch 

 (1887), who devised an instrument for this purpose, the sphygmo- 

 manometer. Since that time a number of different instruments 

 have been described, but attention may be called to two only, which 

 illustrate sufficiently well the principles involved. In the first 

 place, it must be clearly recognized that the arterial pressure in 

 the large arteries of man shows marked variations with the heart 

 beat; the pressure during the beat of the heart rises suddenly 

 to a much higher level than during the diastole. The relation of 

 the systolic (or maximum) and diastolic (or minimum) pressures 

 is indicated by the diagram in Fig. 195. The instruments that 

 have been invented for determining human blood-pressure are in 

 reality adapted, more or less accurately, to determine one or the 

 other or both of these pressures. No instrument has been devised 

 for determining the mean pressure, and as a matter of fact such 

 a thing as mean pressure does not exist in the large arteries, it is 

 simply an abstraction. What really occurs in these arteries is a 

 rapid swing of pressure with each heart-beat from the diastolic 

 to the systolic level, and to interpret fully our records it is impor- 

 tant to determine each of these values. The methods that have 

 been proposed for this purpose have undergone a gradual evolu- 

 tion and improvement. The determinations of systolic pressure 

 were first made, according to the principle suggested by von Basch, 

 by ascertaining the pressure that is just sufficient to occlude an 

 artery, so that a pulse wave cannot pass through. The simplest 

 form of apparatus for this purpose is the one proposed by Riva- 

 Rocci and represented in Fig. 201. Instruments of this kind are 

 known as sphygmomanometers. 



The leather or canvas band, a, is buckled snugly around the arm. On 

 the inner surface of this band there is a rubber bag which communicates with 

 the mercury manometer, d, and the pressure bulb, c. When the band is in 

 place rhythmical compressions of c will force air into the rubber bag surround- 

 ing the arm. This bag is blown up and exerts pressure upon the arm and 

 through the arm tissue upon the brachial artery. The amount of pressure 

 that is being exerted upon the arm is indicated at any moment by the mer- 

 cury manometer. The moment of obliteration of the artery is determined 

 by feeling (or recording) the pulse in the radial artery. The moment that 

 this pulse disappears, as the pressure upon the brachial is raised, indicates the 

 maximum or systolic pressure in the brachial artery. As the pressure is low- 



