472 CIRCULATION OF BLOOD AND LYMPH. 



from the other. The latter condition would seem to be more nor- 

 mal, since the blood-flow and normal tension of the walls are 

 probably less disturbed. An increase in rigidity of the arteries 

 causes the velocity to rise; in elderly people, therefore, the velocity 

 is distinctly greater. In arterial sclerosis with hypertrophy of the 

 heart the velocity may increase to as much as 11 or 13 m. Any 

 marked dilatation of the arteries such as occurs, for instance, 

 in aneurysms, retards the pulse wave markedly; so that the 

 existence of an aneurysm may be detected in some cases by this 

 fact. If we know the velocity of the wave and the time that it takes 

 to pass any given point the length of the wave is given by the 

 formula /= vt. In an adult the duration of the wave (t) at the radial 

 may be taken as 0.5 to 0.7 sec.; so that if the velocity of the wave 

 were uniform throughout the arteries the length of the wave would 

 be from 3.25 m. to 4.5 m. We can imagine, therefore, that before 



Fig. 197. The Dudgeon sphygmograph in position. 



the wave has disappeared at the root of the aorta it has reached the 

 most distant arteries. 



The Form of the Pulse Wave Sphygmography. The pulse 

 wave may be felt upon any superficial artery in consequence of the 

 distension of the vessel. By the tactile sense alone the experienced 

 physician may distinguish some of the characters of the wave, its 

 frequency, its force, etc. The details of the form of the wave, 

 however, were made evident only when the variations in size of the 

 artery were recorded graphically by placing a lever upon it. Any 

 instrument suitable for this purpose is designated as a sphygmo- 

 graph, and very numerous forms have been devised. The move- 

 ment of the artery is very small and to obtain a distinct record it is 

 necessary to magnify this movement greatly by a properly con- 

 structed lever. 



The form of lever that is perhaps most frequently employed is shown in the 

 accompanying figures. The instrument is strapped upon the arm so that the 



