302 MANUAL OF PHYSIOLOGY. 



This is a most important fact, as the suction considerably 

 helps the flow of blood from the veins, and also the current of 

 fluid from the thoracic duct that bears the chyle from the intes- 

 tines and the fluid collected from the tissue drainage back to the 

 blood. 



The pressure of the blood in the veins may then be said to be 

 generally nil, since the veins are nowhere over-filled with blood. 

 The pressures, on the other hand, that can be registered and 

 measured depend upon forces communicated from without, 

 namely : (1) gravity ; (2) the elastic pressure of the surrounding 

 tissue; and (3) the pressure exerted by the muscle during con- 

 traction. This pressure is increased by any circumstance which 

 impedes the flow of blood through the right side of the heart, 

 through any large vein, or through the pulmonary circulation; 

 but when no abnormal obstacle exists to the venous blood-current, 

 the pressure in those vessels can never attain any great height, for, 

 as we have seen, the large trunks are constantly being emptied 

 by the heart's action. 



Most circumstances which tend to lower arterial pressure also 

 tend to raise the pressure in the veins, so that, when the heart's 

 action is weak, or its mechanism faulty, the venous pressure rises. 



In the veins of the extremities the pressure greatly depends on 

 the position of the limb, as it varies almost directly with the effect 

 of gravity. 



In the pulmonary circulation the direct measurement of the in- 

 travascular pressure is rendered extremely difficult, and possibly 

 erroneous, by the fact that to ascertain it the thorax has to be 

 opened. It has been found in the pulmonary artery to be in a 

 dog 29.6 mm., in a cat 17.6 mm., and in a rabbit 12 mm., mercury. 



THE ARTERIAL PULSE. 



Each systole of the ventricle sends a quantity of blood into the 

 aorta, and thus communicates a stroke to the blood in that vessel. 

 The incompressible fluid causes the tense arterial wall to distend 

 still further, and the shock to the column of blood is not trans- 

 mitted onward directly by the fluid, but causes the elastic walls 

 of the arteries to yield locally, and thus it is converted into a wave 



