MECHANICS OF THE CIRCULATION IN THE VESSELS 133 



position of the vein, since the hydrostatic factor (p. igo) in the venous 

 pressure is so important. Thus it is obvious that the pressure in the 

 veins of the hand will be greater when it is hanging down than when 

 it is raised to level of the heart or above it. Accordingly, the actual 

 readings of the manometer must always be corrected for the vertical 

 distance between the vein and the heart, the' height of a column of 

 blood equal to this distance being deducted from or added to the 

 manometer reading, according to whether the vein is below or above 

 the heart level. For practical purposes the heart level is supposed to 

 correspond to the lower end of the sternum (costal angle). 



For the measurement of the pressure in the right auricle, the follow- 

 ing simple and elegant method has been given by Gaertner, following 

 a suggestion of Frey: He raises the arm of the sitting patient, and 

 observes a small vein on the back of the hand. At the moment when the 

 vein collapses the elevation of the arm is stopped, and the vertical 

 distance between the vein and the heart measured. This expressed in 

 millimetres of blood (i.e., approximately of water) is the pressure in 

 the auricle, since the veins of the arm constitute manometer tubes 

 connected with the auricle. 



The venous pressure being so low, or, in other words, the potential 

 energy which the systole of the heart imparts to the blood being so 

 greatly exhausted /t 



before it reaches 

 the veins, other in- 

 fluences begin here 

 appreciably to 

 affect the blood- 

 stream : 



1. Contraction of 

 the Muscles. This 

 compresess the 

 neighbouring veins, 



and since the blood is compelled by the valves, if it moves at 

 all, to move towards the heart, the venous circulation is in this 

 way helped. 



2. Aspiration of the Thorax. In inspiration the intrathoracic 

 pressure, and therefore the pressure in the great thoracic veins, is 

 diminished, and blood is drawn from the more peripheral parts of 

 the venous system into the right heart (p. 226). 



3. Aspiration of the Heart. When the heart after its contraction 

 suddenly relaxes, the endocardiac pressure becomes negative, and 

 blood is sucked into it, just as when the indiarubber ball of a syringe 

 is compressed and then allowed to expand. But we cannot attribute 

 any great importance to this; and, of course, it is only the relaxa- 

 tion of the right ventricle which could directly affect the venous 

 circulation. 



4. Every change of position of the limbs, as in walking, aids the 

 venous circulation (Braune), and this independently of the muscular 

 contraction. When the thigh of a dead body is rotated outwards, and 



Fig. 60. Diagram of Measurement of Venous Pressure 

 (v. Recklinghausen). H, back of hand, with V, a vein; 

 B, the rubber bag with central opening; T, tube leading 

 from bag to manometer and pump ; G, glass plate. 



