376 THE MECHANICS OF THE CIKCULATION, HEMODYNAMICS 



aid of these indirect methods closely agree with those given previously. 

 Thus, it has been found that the pressure in the small veins of the arm 

 and hand amounts to 100-200 cm. H2O. 



The Capillary Blood Pressure. — Obviously, the pressure prevailing 

 in the capillaries cannot be measured by the direct method; in fact, 

 even the indirect procedures so far devised have given only approxi- 

 mate values. Thus, v. Kries^ has made use of a thin plate of glass 

 which he placed upon the skin and gradually weighted until the skin 

 underneath it became pale. This method is based upon the proba- 

 bility that the first indication of the paling of the surface corresponds 

 to the moment when the pressure in the capillaries is balanced by the 

 pressure without. The latter may be expressed in centimeters of 

 water by dividing the weight which has been placed upon the glass 

 slide by the size of the area under compression. Roy 

 and Graham-Brown 2 have attempted to determine the 

 moment of compression of the capillaries by exposing 

 them, while under microscopic observation, to a pres- 

 sure brought to bear upon them by means of elastic 

 capsules connected with a manometer. 



When the arm was held at the level of the heart, 

 the pressure in the capillaries of the fingers amounted 

 to 24 mm. Hg. With the hand pendant at the side 

 of the body, the pressure rose to 62 mm. Hg. In the 

 capillaries of the ear, the pressure amounted to 20 mm. 

 F 196 — -^^ ^^^ ^^ those of the gums of a rabbit, to 33 mm. 

 Apparatus OF Hg. The determinations of von Recklinghausen^ have 

 VON Kries for given a value of 55 mm. Hg for the small arterioles 

 i^^ryTlo^oT- supplying the capillaries of the tips of the fingers. 

 PRESSURE. While the capillary pressure must vary in different 



organs and tissues, it seems that its average value 

 must lie somewhere between 40 and 50 mm. Hg. To illustrate: If the 

 intraventricular pressure is 125 mm. Hg, it will be found that the 

 peripheral arterial pressure amounts to about 105 mm. Hg. About 

 3 or 4 mm. Hg of the initial driving force are lost between the heart 

 and the aorta and the remainder between this blood-vessel and the 

 arterioles. Distally to these, the original driving force is used up very 

 rapidly, the greatest reduction occurring in the capillaries proper. 

 This cannot cause surprise, because the resistance in these channels 

 is very great. As we have seen, the blood arrives in the distal veins 

 under a pressure of only about 10 to 15 mm. Hg and hence, almost 

 100 mm. Hg of the original pressure have been used up in forcing the 

 capillary passage. As the blood approaches the heart, the pressure 

 becomes less and less, amounting at the cardiac vestibule to only —5 

 to —10 mm. Hg. Naturally, these negative values which are de- 



1 Verh. sachs. Gesellsch. der Wissensch., 1875. 



2 Jour, of Physiol., ii, 1879, 323. 



2 Archiv fiir Exp. Path, und Pharmak., Iv, 1907. 



