374 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



if the chest is opened, the pressure in the central veins rises immediately 

 to above zero with a corresponding elevation of the pressure from here 

 outward. Hence, it may be concluded that this negative venous pres- 

 sure is dependent upon the elastic recoil of the lungs. 



Under normal conditions, the area of negative venous pressure 

 begins at about the junction of the hepatic vein with the inferior 

 cava, and at the point where the external jugular vein passes deep 

 into the supraclavicular fossa. The so-called "danger line" of the 



FIG. 193. DIAGRAM TO PRESSURE THROUGHOUT THE VASCULAR SYSTEM. 

 Z, abscissa or zero-line; P, curve of pressure (A) in arteries (C) in capillaries and (V) 

 in veins. The greatest fall in pressure occurs in the capillaries in which the resistance is 

 greatest. 



surgeon corresponds to this line of zero pressure, because it has always 

 been thought that an injury to a vein centrally to this point, must 

 inevitably lead to an entrance of air into the vascular system and a 

 frothing of the blood by the cardiac valves. This danger, however, 

 is not so imminent as might be supposed, because the walls of the veins 

 yield easily and, as they are not firmly attached to the surrounding 

 tissues, collapse very readily, thereby preventing the ingress of air. 

 Moreover, while dangerous on account of the possible occurrence 



of emboli, small quantities of air are fre- 

 quently gotten rid of by absorption. 



The principle of the indirect method 

 of measuring venous blood pressure is 

 precisely the same as that made use of in 

 determining the arterial pressure. An 

 outside pressure which can be accurately 

 FIG. 194. SMALL RUBBER measured, is brought to bear upon a 



CAPSULE USED FOR OBLITERATION .c i x-i . i 



OF VEIN. superficial .vein until its central stump 



becomes empty. As the venous pressure 



is low, a water manometer is employed as the indicator in conjunc- 

 tion with an ordinary pressure bulb. In accordance with v. Reckling- 

 hausen, 1 the obliteration of the vein is accomplished by means of a 

 small capsule of thin rubber (Fig. 194) which communicates with a 

 manometer" and is held in place upon the skin by a flat box made of 

 glass or wood. Hooker 2 employs a small glass chamber which is 

 fastened to the skin in the region of the vein by a film of collodion 

 solution. 



1 Archiv fur Exp. Path, und Pharm., Iv, 1906. 



2 Am. Jour, of Physiol., Ixxiii, 1914, Proc. 27. 



