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HANDBOOK OF PHYSIOLOGY. 



so; 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 thedistention of arteries, and 

 the succeed ing recoil, maybe well seen in the prominent temporal artery 

 of an old person. In feeling the pulse, the finger 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 dila- 

 tation, or return, more or less, to the cylindrical form, of the artery which 

 has been partially flattened by the finger. 



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 ar- 

 tery, 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 arteries 

 never exceeds probably to -J of a second. 



A distinction must be carefully made between the passage of the wave 

 along the arteries and the velocity of the stream (p. 156) of blood. Both 

 wave and current are present; but the rates at which they travel are very 

 different, that of the wave 16.5 to 33 feet per second (5 to 10 metres), 

 being twenty or thirty times as great as that of the current. 



The Sphygmograph. A great deal of light has been thrown on 



spRina 



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Fia. 123. Diagram of the mode of action of the Sphygmograph. 



what may be called the form of the pulse wave by the Sphygmograph 

 (Figs. 123 and 124). The principle on which it acts is very simple (see 

 Fig. 123). 



The small button replaces the finger in the 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 that time it 

 is just strong enough to resist its making any sudden jerk, and in the in- 

 terval 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. 124). 



It is evident that the beating of the pulse with the reaction of the 

 spring will cause an up-and-down movement of the lever, the pen of 



