MOTION OF THE HEART AND BLOOD 



the pulse in an artery is due to an impact of blood. 

 Why do the arteries differ so much from veins in the 

 thickness and strength of their walls? Because they 

 must withstand the pressure of the pumping heart 

 and rushing blood. 



Hence, since Nature makes nothing In vain, and 

 does the best everywhere, the nearer arteries are to 

 the heart the more do they differ from veins in struc- 

 ture. Here they are stronger and more ligamentous,^'' 

 but in their terminal branchings, as in the hands, 

 feet, brain, mesentery, and testicles, they are so simi- 

 lar to veins in make up that it is hard to tell one from 

 another by ocular examination of their tunics. This 

 occurs from the following good reason : the farther an 

 artery is away from the heart the less it is reached by 

 the cardiac pressure dissipated by the long space. 

 Since all the arterial trunks and branches must be 

 filled with blood, the cardiac impulse is further dimin- 

 ished, divided in a way by each branching. 



So the terminal arteries appear like veins, not only 

 in structure, but also in function, for they rarely show 

 a perceptible pulse unless the heart beats more vio- 

 lently, or the arteriole dilates or is more open at the 

 particular point.^^ Thus it happens that we may 



tional semi-technical language of the subject, he perhaps never 

 thought of using a word which conveys the meaning he so clearly 

 implies. 



^^ This is the nearest Harvey comes to grasping the idea of the 

 elasticity of blood-vessel walls, a factor of considerable importance 

 in determining blood pressure. 



^* A remarkable explanation, implying vaso-constriction and 



[129] 



