VELOCITY AND PRESSURE OF BLOOD-FLOW. 



497 



It should be added that in this curve and in all the figures 

 so far quoted in regard to the actual pressure within the different 



Fig. 201.— Figure of the Riva-Rocci apparatus (SaW)") : a. The leather collar with 

 inside rubber bag to go on the arm; c, the bulb for blowing up the rubber bag and thus 

 compressing the artery; d, the manometer dipping into the reservoir of mercury, b, to meaa- 

 sure the amount of pressure. 



arteries and veins, it is assumed that the animal is in a recumbent 

 posture. In an animal standing upon his feet, especially in an 

 upright animal like man, it is obvious that the effect of gravity 

 will modify greatly the actual figures of pressure. Upon the arte- 

 ries and veins of the feet, for example, there will be exerted a 

 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. 512.) 



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 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 



