310 PHYSIOLOGY. 



Venous blood-pressure is increased by all conditions which tend to 

 decrease the difference of pressure between the arterial system and 

 itself. The reverse will produce diminution in its tension. General 

 plethora increases it; anemia diminishes it. As one proceeds from 

 the heart there is found the development of a positive pressure. 



The blood-pressure in the veins near the heart is negative because 

 of the aspiration exercised by the thorax. Thus, in the superior vena 

 cava of the dog it is 3. millimeters of mercury, in the right brachial 

 + 3.9 millimeters of mercury, in the left femoral + 5.4, and in the 

 left saphena + 7.4. As these facts show, the venous pressure is higher 

 the more distant the vein is from the heart. 



In order that the blood stream shall continue in a uniform manner, 

 the quantity of blood received by the heart from the veins in the same 

 period of time must be equal to that ejected by the heart into the 

 arteries. 



Veno-motor Nerves. The veins are contractile. If you irritate 

 the peripheral end of a splanchnic there will be a narrowing of the 

 portal vein and its branches within the liver. Irritation of the 

 peripheral end of a sciatic causes a contraction of the veins of the 

 posterior extremity. Hence the veins, like the arteries, are in a state 

 of tonus similar to that which exists in the arteries and under control 

 of the central nervous system. 



Venous Pulse. This pulse in the jugular is a visible sign of a 

 circulation in the veins. The pulse is due to the auricular systole, 

 which produces not a reflux but a temporary arrest in the flow of 

 venous blood. The venous pulse seen in disease differs from the 

 preceding as regards cause. It is due to insufficiency of the tricuspid 

 valve. Here the reflux of blood, when the ventricle contracts, pro- 

 duces the venous pulse. 



In the figure is represented a schema of a jugular pulse tracing 

 with a tracing of cardiac pulsation. The first wave, the most 

 important, forms the venous pulse, and is due to contraction of the 

 auricle. It is not caused by a reflux, but by a temporary stoppage of 

 the flow of blood, a positive wave running backward in the veins. 

 The second wave according to Mackenzie is caused by the carotid beat- 

 :ing near by the vein. Bard, however, holds that the second wave is due 

 >to the rapid tightening and doming of the auriculo-ventricular valves. 



The third wave, according to Mackenzie is produced by the sud- 

 den checking of the venous flow at the period of great distention of the 

 auricles. This wave occurs toward the end of the systole of the 

 ventricle. Bard believes the third wave is caused by the base of the 



