60 ELEMENTARY PHYSIOLOGY 



veins ; to overcome a resistance work must be done, 

 and to do work, force must be employed and energy 

 expended. Now blood-pressure is the force available for 

 overcoming the resistance, and if it be thus used up there 

 is less of it left, or in other words the pressure falls. In 

 the veins the l)lood-pre.ssure is still less than in the 

 capillaries, and diminishes gradually along their course 

 towards the heart. 



These differences of pre.s.sure in the several parts of the 

 vascular system determine the How of blood along the 

 vessels ; the blood is always flowing from a higher to a 

 lower pressure ; the main work of the heart is to estjiblish 

 the large blood-pressure existing in the larger arteritis. 



When a vein is cut the blood does not spurt out as it 

 does from a cut artery but oozes or trickles out gently, 

 the reason being that the pressure in the veins is small. 

 Further the flow is in this case continuous and not jerky 

 as it is from a cut artery, in cfnrespondence with the 

 fact that there is no "pulse" in the veins as there is in 

 the arteries. But this stiitement recjuires that we should 

 next consider the nature and causes of the pulse. 



12. The Pulse. — If the finger be placed on an artery 

 which lies near the surface of the body, such as the radial 

 artery at the wri.st, what is known as the pulse will be 

 felt as a slight throbbing jtressure on the finger, coming 

 and going at regular intervals which correspond to the 

 successive ])eats of the heart. What is felt is in reality 

 the intermittent risu and fall of that piece of the arterial 

 wall which lies innnediately under the finger. This fact 

 may be easily proved by placing a light lever so as to rest 

 over the artery, whereupon its end may be seen to rise 

 and fall at the same regular intervals. This movement of 

 the arterial wall is due to that distension of the arteries, 

 of which we have already spoken, which is started at each 

 beat of the heart by the extra quantity of blood driven 

 into them by the ventricle, then travels in the form of a 

 wave from the larger to the smaller arteries, and corre- 



