308 MANUAL OF PHYSIOLOGY. 



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 

 intravascular pressure is rendered extremely difficult, and possi- 

 bly 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 

 transmitted onward directly by the fluid, but causes the elastic 

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

 a wave which passes rapidly along those vessels. This motion in 

 the walls of the vessel can be felt wherever the artery can be 

 reached by the finger, but best as in the case of the radial and 

 temporal arteries where the vessel is superficial and lies on 

 some unyielding structure, such as bone. 



This motion of the vessel wall is called the arterial pulse. It 

 consists of a simultaneous widening and lengthening of the artery. 

 The arteries near the heart are much more affected by the pulse 

 wave than those more remote, the wave becoming fainter and 

 fainter as it travels along the branching arteries. In the smallest 

 arteries it is hardly recognizable, and under ordinary circum- 

 stances is quite absent in the capillaries and veins. 



The diminution in the pulse wave in the smaller arteries chiefly 



