VELOCITY AND PRESSURE OF BLOOD-FLOW. 453 



Therefore to blanch this area would require a column of water contain- 

 ing 1000 c.mms. with a cross-area of 4 sq.mms. The height of this column 

 would therefore be equal to ^-^ or 250 mms. of water, that is, 18.5 mms. 

 Hg. 



The results obtained by this method are not very constant and 

 can only be considered as approximate. It would appear, how- 

 ever, that the pressure lies somewhere between 20 and 40 mms. 

 of mercury. Thus, upon the gums of a rabbit von Kries found a 

 capillary pressure of 33 mms. Hg. 



The general relations of the pressures in arteries, veins, and 

 capillaries may be expressed in a curve such as is shown in Fig. 

 187. 



The Method of Determining Blood-pressure in the Large 

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

 tance to ascertain even approximately the arterial pressure in man 

 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 

 are among the most recent and convenient. 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. 181. 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 pres- 

 sures. No instrument has been devised for determining the mean 

 pressure, and, indeed, from a physiological standpoint such an 

 instrument would not be so valuable as one that gives us the 

 figures corresponding to the systolic and the diastolic pressures 

 and thus allows us to calculate an approximate mean. For it is 

 evident that in the latter case we should be in possession of more 

 data with which to analyze the causes for any given variation in 

 pressure. The principle of determining the systolic pressure alone 

 is very simple: it consists in determining the amount of pressure 

 necessary to completely obliterate the artery, that is, to prevent 

 a pulse from passing through the region under compression. This 

 principle was used originally by von Basch, but its application has 

 been made perhaps most successfully in the simple apparatus 

 suggested by Riva-Rocci, which is adapted especially for measure- 

 ments of pressure in the brachial artery. One form of this instru- 

 ment is represented in Fig. 188. 



