144 THE CIRCULATION. 



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. 



The dilatation or increase of the diameter of the artery 

 is less evident. In several reptiles, it may be seen without 

 aid, in the immediate vicinity of the heart, and it may be 

 watched, with a simple magnifying glass, in the aorta of 

 the tadpole. Its slight amount in the smaller arteries, the 

 difficulty of observing it in opaque parts, and the rapidity 

 with which it takes place, are sufficient to account for its 

 being, in Mammalia, imperceptible to the eye. But in 

 these also experiment has proved its occurrence. Flourens, 

 in evidence of such dilatation, says he encircled a large 

 artery with a thin elastic metallic ring cleft at one point, 

 and that at the moment of pulsation the cleft part became 

 perceptibly widened. 



This dilatation of an artery, and the elongation producing 

 curvature, or increasing the natural curves, are sensible to 

 the finger placed over the vessel, and produce the pulse. 

 The mind cannot distinguish the sensation produced by 

 the dilatation from that produced by the elongation and 



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

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

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

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

 when the ventricles contract. 



