VELOCITY AND PRESSURE OF BLOOD-FLOW. 509 



therefore, be corrected for the heart-level by determining the 

 vertical distance between the vein and the heart (costal angle), 

 and subtracting this distance, expressed in centimeters, from 

 the pressure, also expressed in centimeters, which was found 

 necessary to obliterate the vein. Measurements made by this 

 method and corrected for the heart-level show that in the normal 

 person the pressure within the small veins of the hand or arm 

 may vary between 10 and 20 cm. of water. The average pressure 

 during the day is about 15 <tm., but at night, when asleep, it 

 may fall as low as 7 to 8 cm. Unusual or pathological conditions 

 which cause a congestion in the venous side of the heart will raise 

 the venous pressure correspondingly.* 



When the venous pressure is measured in the small veins of the feet in a 

 person while standing we should suppose that after a reduction to the heart 

 level it would be about the same as that noted for the veins of the hands, 

 since the vessels are of about the same order with reference to their distance- 

 from the capillary bed. In a series of observations of this kind, reported by 

 von Recklinghausen, it was found, on the contrary, that after subtracting the 

 distance between the foot and the heart, the pressure within the veins was 

 negative by as much as 40 cm. The author explains this unexpected result 

 by supposing that the flow through the foot got up only enough pressure in 

 the veins to lift the blood to the level of the pelvis, and that the complete 

 closure of the venous valves at this level protected the veins from the full 

 pressure of the column of blood. Eventually, no doubt, the pressure in the veins 

 would have risen sufficiently to lift the blood to the heart-level, but it seems 

 probable that under the ordinary conditions of life this result is effected by 

 the cooperation of the muscles of the legs and the respiratory movements of the 

 thorax (see p. 517). The contractions of these muscles, aided by the venous 

 valves, squeeze the blood upward to the heart. The fact that in standing 

 quietly the flow through the feet may be suspended or impeded, for a time 

 at least, throws some light, as von Recklinghausen suggests, upon the fact 

 that it is so difficult to stand for any length of time without moving. 



The apparatus described above may be used for determining 

 capillary as well as venous pressures, according to the principle 

 described on p. 499. For this purpose the pressure box is laid 

 upon a given skin area and the pressure is raised until the skin 

 beneath is blanched. The pressure is then lowered slowly until 

 the skin again reddens, showing the reestablishment of the capil- 

 lary flow. The pressure thus obtained is corrected as described 

 for the level of the heart, f 



* For a description of some pathological cases, see Eyster and Hooker, 

 loc. cit. 



t For some technical details, see von Recklinghausen, loc. cit. 



