124 THE CIRCULATION. 



The pulse due to any given beat of the heart is not per- 

 ceptible at the same moment in all the arteries of the body. 

 Thus it can be felt in the carotid a very short time before it is 

 perceptible in the radial artery, and in this vessel again before 

 the dorsal artery of the foot. The delay in the beat is in pro- 

 portion to the distance of the artery from the heart, but the 

 difference in time between the beat of any two arteries never 

 exceeds probably % to of a second. 



A great deal of light has been thrown on what may be called 

 the form of the pulse by the sphygmograph (Figs. 42 and 43). 

 The principle on which the sphygmograph acts is very simple 

 (see Fig. 42). The small button replaces the finger in the 

 ordinary act of taking the pulse and is made to rest lightly on 

 the artery, the pulsations of which it is desired to investigate. 

 The up-and-down movement of the button is communicated to 

 the lever, to the hinder end of which is attached a slight spring, 

 which allows the lever to move up, at the same time that it is 

 just strong enough to resist its making any sudden jerk, and 

 in the interval of the beats also to assist in bringing it back to 

 its original position. For ordinary purposes, the instrument 

 is bound on the wrist (Fig. 43). 



trine, I am indebted to my friend, Dr. Hensley, who has kindly fur- 

 nished 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 tbe transverse slip 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 wliose 

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

 lengths are wade 6 and 12 inches respectively.) 



Such experiments also show that, within certain limits, the stretch- 

 ing of each strip varies directly as its tension. 



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

 when filled with blood, must be somewhat more than double its longi- 

 tudinal stretching. 



This being true for different blood pressures, the difference between 

 the transverse stretchings for different pressures must be somewhat 

 more than double the difference between the corresponding longitu- 

 dinal stretchings; and thus we can hardly be justified in saying that 

 the increase of longitudinal stretching which takes place with the 

 pulse is greater than the increase of transverse stretching. 



It must also be remembered that the arteries are, under all circum- 

 stances, naturally in a state of tension longitudinally, and that their 

 length, therefore, cannot be increased at all until the blood pressure 

 is increased beyond a certain point. ED.) 



