THE SPHYGMOGRAPH. 157 



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

 perceptible 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 proportion to the distance of the 

 artery from the heart, but the difference in time between 

 the beat of any two art'eries never exceeds probably ^ to 

 of a second. The pulse, moreover, is but the maximum 

 distension and elongation of the vessel ; and it has been 

 shown by M. Marey that the commencement of the act 

 occurs at the same moment in all the arterial system, but 

 that the highest point of distension and lengthening, which 

 alone we recognise as the pulse, is reached more rapidly 

 in proportion to the proximity of the artery to the heart. 



This observation has been made by means of a special 

 modification of the sphygmograph, an instrument invented 

 by Yierordt and much improved by M. Marey and others, 

 which has thrown a great deal of light on what may be 

 called the structure of the pulse. 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 



filled with, blood, must be somewhat more than double its longitudinal 

 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 longitudinal stretch- 

 ings ; 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. (Eo.) 



