112 CIRCULATION 



resistance of friction, and elasticity of bloodvessel walls. But 

 in the capillaries and veins their is no pulse and the pressure 

 is low. The cause of the latter becomes obvious when it is 

 taken into consideration that a part of the force of the heart 

 has been lost in overcoming the friction of the bloodvessels. 

 In addition, the friction which the blood has yet to overcome 

 in its passage back to the heart is but a fraction of the total 

 friction which it encountered at first. Diminished resistance 

 ahead means lowered pressure. The blood in the capillaries 

 has become pulseless, because the elasticity of the arteries 

 displaces the blood in the capillaries at the same rate that 

 the systole of the heart does. 



There are subsidiary forces that assist the heart in propelling 

 the blood. Among these may be mentioned the contractions 

 of the skeletal muscles, the constant pull of the fibers of the 

 lung, and the movements of respiration. The muscles, in 

 contracting, press upon the veins moving the contained blood 

 onward, since the valves prevent all back flow. The fibers 

 of the lungs, through their elasticity, are constantly pulling 

 upon the walls of the heart and the large veins, which tends 

 to draw the blood into them. This effect is increased with 

 each inspiration, and the blood then rushes in at a quicker 

 rate; during the following expiration the blood flows more 

 slowly again. There may in this way arise a distinct pulse 

 in the large venous vessels of the chest, which may extend 

 along the veins to the root of the neck. In this region, in 

 deep respirations, there may be an intermittent flow of blood 

 from a cut vein. The bleeding occurs during each expiration 

 and ceases during each inspiration, when the blood is sucked 

 past the wound and not pressed out of it. Owing to this reason 

 air may be drawn into the vein, an event which causes immediate 

 death. This region is, therefore, known as the dangerous 

 region. 



The Pulse. By the term arterial pulse is meant the fluctua- 

 tions of arterial pressure that correspond to the beats of the 

 heart. The pulse is dependent upon: 



1. The contractions of the heart. 



2. Upon the resistance produced by the friction of the 

 blood in the vessels. 



