THE PULSE. 123 



the supply of blood to each part, in accordance with its de- 

 mands. 



The Pulse. 



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

 it is one of the offices of the arteries to change into a uni- 

 form 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 resist- 

 ance 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 elonga- 

 tion, the arteries change their form, the straight ones becoming 

 curved, or having such a tendency, and those already curved 

 becoming more so ; l but they recover their previous form as well 

 as their diameter when the ventricular contraction ceases, and 

 their elastic walls recoil. The increase of their curves which 

 accompanies the distension of arteries, and the succeeding re- 

 coil, 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 Mam- 

 malia, imperceptible to the eye. But in these also experi- 

 ment 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 dilata- 

 tion from that produced by the elongation and curving ; that 

 which it perceives most plainly, however, is the dilatation. 2 



1 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- 

 movable ; the other end, with the heart/may move forward slightly 

 when the ventricles contract. 



8 For this fact, which is contrary to the commonly accepted doc- 



