io6 Motion of the 



And many things having reference to the arteries 

 farther illustrate and confirm this truth. Why does 

 not the arteria venosa pulsate, seeing that it is num- 

 bered among the arteries ? Or wherefore is there a 

 pulse in the vena arteriosa ? Because the pulse of the 

 arteries is derived from the impulse of the blood. 

 Why does an artery differ so much from a vein in 

 the thickness and strength of its coats ? Because it 

 sustains the shock of the impelling heart and streaming 

 blood. Hence, as perfect nature does nothing in vain, 

 and suffices under all circumstances, we find that the 

 nearer the arteries are to the heart, the more do they 

 differ from the veins in structure ; here they are both 

 stronger and more ligamentous, whilst in extreme parts 

 of the body, such as the feet and hands, the brain, the 

 mesentery, and the testicles, the two orders of vessels 

 are so much alike that it is impossible to distinguish 

 between them with the eye. Now this is for the 

 following very sufficient reasons : for the more remote 

 vessels are from the heart, with so much the less force 

 are they impinged upon by the stroke of the heart, 

 which is broken by the great distance at which it is 

 given. Add to this, that the impulse of the heart 

 exerted upon the mass of blood, which must needs 

 fill the trunks and branches of the arteries, is diverted, 

 divided, as it were, and diminished at every sub- 

 division ; so that the ultimate capillary divisions of 

 the arteries look like veins, and this not merely in 

 constitution but in function ; for they have either no 

 perceptible pulse, or they rarely exhibit one, and never 

 save where the heart beats more violently than wont, 

 or at a part where the minute vessel is more dilated or 

 open than elsewhere. Hence it happens that at times 

 we are aware of a pulse in the teeth, in inflammatory 

 tumours, and in the fingers ; at another time we feel 

 nothing of the sort. Hence, too, by this single symptom 

 I have ascertained for certain that young persons, 

 whose pulses are naturally rapid, were labouring under 



