506 CIRCULATION OF BLOOD AND LYMPH. 



venous side of the heart will raise the venous pressure correspond- 

 ingly.* 



Capillary Pressure. — Numerous devices have been proposed to 

 measure capillary pressure in the skin of man. The general 

 principle at first adopted was to determine in some way the load 

 or pressure necessary to blanch the skin. Much of the apparatus 

 used for this purpose has not been satisfactory in application, 

 and the underlying assumption that the color of the skin is due 

 mainly to the capillary circulation is probably erroneous. Hooker 

 gives good reasons for believing that the skin color is due chiefly 

 to the blood in the superficial venous plexuses. A pressure suffi- 

 cient to blanch a spot on the skin would measure therefore the 

 pressure in the small veins rather than in the capillaries. For 

 these reasons the figures reported by various observers for capillary 

 pressure have varied widely. Danzer and Hookerf have perfected 

 an apparatus, designated as a micro-capillary tonometer, in which 

 a different principle is employed. The capillaries in the skin are 

 made visible under a microscope by the use of oil on the skin and a 

 strong illumination, and the apparatus is so constructed that air 

 pressure can be applied to the skin while the flow of blood in the 

 capillaries is being observed. When the pressure is raised to a 

 certain level the capillary stream is blocked. As the pressure is 

 lowered again the flow begins. They take the pressure at this 

 latter point as a measure of the pressure within the capillaries. 

 Observations made with this instrument indicate that in man the 

 capillary pressure is equal on the average to 22 mms. of mercury 

 when in the sitting position. It is somewhat lower when lying 

 down and higher when standing. The extremes noted for a 

 series of individuals varying in age from eight to forty-seven years 

 were from 17.5 to 26.5 mms. 



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

 loc. cit., and Clark, "Archives of Internal Medicine," 16, 587, 1915. 



t Danzer and Hooker, "The American Journal of Physiology," 52, 136, 

 1920, and Hooker, ibid., 54, 30, 1920. For a general discussion consult 

 Hooker, "Physiological Reviews," 1, 112, 1921. 



