THE PULSE, 155 



It must be observed, however, that the natural pervious 

 state of an artery here spoken of, is not one of absolute 

 rest; for Mr. Savory has shown that the natural state of 

 all arteries, in regard at least to their length, is one of 

 tension that they are always more or less stretched, and 

 ever ready to recoil by virtue of their elasticity, whenever 

 the opposing force is removed. The extent to which the 

 divided extremities of arteries retract is a measure of this 

 tension, not of their elasticity. 



The Pulse. 



The jetting movement of the blood, which, as just stated, 

 it is one of the offices of the arteries to change into an uniform 

 motion, is the cause of the pulse, and therefore needs a 

 separate consideration. We have already said, that as the 

 blood is not able to pass through the arteries so quickly as 

 it is forced into them by the ventricle, on account of the 

 resistance it experiences in the capillaries, a part of the 

 force with which the heart impels the blood is exercised 

 upon the walls of the vessels which it distends. The 

 distension of each artery increases both its length and its 

 diameter. In their elongation, the arteries change their 

 form, the straight ones becoming curved, or having such a 

 tendency, and those already curved becoming more so ; * 

 but they recover their previous form as well as their dia- 

 meter when the ventricular contraction ceases, and their 

 elastic walls recoil. The increase of their curves which 

 accompanies the distension of arteries, and the succeeding 

 recoil, may be well seen in the prominent temporal artery 

 of an old person. The elongation of the artery is in such 

 a case quite manifest. 



* There is, perhaps, an exception to this in the case of the aorta, of 

 which the curve is by some supposed to he diminished when it is elon- 

 gated ; hut if this he so, it is because only one end of the arch is im- 

 moveahle; the other end, with the heart, may move forward slightly 

 when the ventricles contract. 



