224 CIRCULATION OF THE BLOOD 



thoracic pressure, the pressure in all the veins rises considerably, so that a 

 negative pressure can no longer be demonstrated. 



In order to determine the pressure in a vein it is necessary to avoid stop- 

 page of the blood; a T-cannula is used and on account of the low pressure a 

 soda solution is substituted for mercury in the manometer. 



Just as in the arterial system, the pressure in the veins is conditioned upon 

 the quantity of blood flowing from the heart in unit time, and upon the 

 resistance. If the veins meet with great resistance in emptying their blood 

 to the heart, the pressure in them increases. This happens for example when 

 the heart is checked or brought to a standstill by stimulation of the vagus. 

 In this case the heart is unable to drive forward all the blood which collects 

 in the veins, and the consequent accumulation raises the venous pressure. 

 If in spite of the inhibition, the right heart still expels in unit time just as 

 much blood as it did before, the venous pressure suffers no change. The 

 pressure in the veins is increased likewise if the lungs are highly inflated, 

 for by this means the flow of blood into the intrathoracic veins is hindered, 

 and it becomes more difficult for the right heart to empty itself. 



On the other hand, the venous pressure falls as a result of all conditions 

 favoring the return of the blood to the right heart or its discharge therefrom 

 e. g., acceleration after section of the vagus provided the heart discharges 

 in unit time a larger quantity of blood than before. 



These influences take effect primarily on the central veins. In the periph- 

 ery the pressure depends mainly upon the variations of blood volume and of 

 resistance in the arteries. If an artery be completely clamped off, the pressure 

 in the corresponding vein sinks to the level of the minimal pressure in the 

 larger vein to which it is tributary. If a vein be clamped off, the pressure 

 increases peripherally to the ligature because in this case the vein represents 

 only a blind end of the artery. 



The variations in pressure in the venae cavae give rise to pulsations in the 

 larger veins of the trunk and the extremities, which are transmitted centrifugally 

 with a velocity of one to three meters per second. The velocity of transmission 

 through the jugular vein is greater than that through the vena cava to the 

 crural vein (Morrow). 



In order that the blood may flow uniformly, it is necessary that the same 

 quantity be delivered by the veins to the heart in unit time as is expelled by 

 the heart into the arteries, and this has been proved by the direct observations 

 of Cyon and Steinmann, and of Burton- Opitz, to be the case. The volume 

 of flow is, therefore, about the same in corresponding arteries and veins; but 

 on account of the greater cross section of the veins, the linear velocity in 

 them is less than in the arteries e. g., in the external jugular of the dog 

 147 mm., in the femoral 62 mm. per second. After section of the vagi, the 

 volume of the current in the jugular vein increases 2.8 times, but decreases 

 about fifty-seven per cent on compression of both carotids ( Burton- Opitz). 



B. AIDS TO THE BLOOD FLOW IN THE VEINS 



The blood flow in the veins can be very easily disturbed by all kinds of 

 external influences; but to offset this we have several special mechanisms 



