156 THE CIRCULATION. 



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. Elourens, 

 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 

 curving ; that which it perceives most plainly, however, is 

 the dilatation, f 



* For this fact, which is contrary to the commonly accepted doctrine, 

 I am indebted to my friend, Dr. Hensley, who has kindly furnished me 

 with the following note on the subject : 



By determining the conditions of equilibrium of a portion of artery 

 supposed cylindrical and filled with blood at a given pressure, it is easily 

 shown that the transverse tension is double the longitudinal. 



Also it may be shown experimentally that, if strips of equal breadth, 

 cut in the two directions from one of the larger arteries, be stretched by 

 equal weights, the stretching of the transverse strip is somewhat greater 

 than that of the longitudinal one. 



(By the word stretching is to be understood amount of stretching, and 

 not increase of length : it may be measured by the ratio which the 

 increase of length bears to the original length : Thus things whose natural 

 lengths are 5 and 10 inches are equally stretched when their lengths are 

 made 6 and 12 inches respectively.') 



Such experiments also show that, within certain limits, the stretching 

 of each strip varies directly as its tension. 



Hence it will be seen that the transverse stretching of an artery, when 



