214 CIRCULATION OF THE BLOOD 



Since both these reflected waves travel through the tube with the same velocity 

 and naturally interfere with each other, it depends on the degree of constriction 

 whether the algebraic sum of the two will be a positive or a negative wave, or 

 will be nil (Grashey). 



If from a simple tube A, a side branch B be given off, every wave which 

 runs through A will traverse also the branch B ; and, it matters not whether the 

 wave arises in the wide or in the narrow tube, it will traverse both. This state- 

 ment holds also for a complex system of tubes, and we may say in general that 

 when a wave starts from any point of a branching system of vessels, it is 

 propagated to all the branches. 



Reflection takes place in such a system at every dividing place. But if the 

 velocity with which the waves are propagated changes at any point in the same 

 proportion as the cross section changes, no reflection occurs (v. Kries). 



All the conditions for the origin of primary and reflected waves and of inter- 

 ference are found in the arterial system. The difficulty consists only in isolating 

 from among the theoretically possible movements, those which cause the peculi- 

 arities of the arterial pulse. 



B. THE PULSE 



The ancient physicians distinguished in the pulse a number of different 

 qualities, which can be reduced to four : frequency, size., velocity and hardness. 



With respect to frequency, the rapidly repeated pulse (pulsus frequens) is 

 to be distinguished from the less frequent (rarus). With respect to size we 

 have the large (magnus) in which the expansion of the artery under the finger 

 is large, and the small pulse (parvus) in which the expansion is small. With 

 respect to velocity the quick pulse (celer) in which the artery presses against 



the finger suddenly and disappears sud- 

 denly, can be distinguished from the 

 sluggish pulse (tardus} in which the im- 

 pact is more prolonged. And with re- 

 spect to hardness, the pulse (durus} 

 which can be compressed with difficulty, 

 can be distinguished from the one (mol- 

 lis} which can be easily obliterated by 

 pressure. On the basis of these four 

 fundamental qualities a series of other 

 qualities can be named, but we shall 

 not discuss them further. 



FIG. 86. The spring of Marey's sphygmo- Knowledge of the pulse has made 



graph. rapid progress since E. H. Weber gave 



it a mechanical explanation (1850) 



and Vierordt showed that it could be graphically recorded (1855). The first 

 pulse recorder (sphygmograph) to give correct pulse curves was constructed 

 somewhat later (1860) by Marey. 



The most important part of the sphygmograph (Fig. 86) is the steel spring 

 (p) with a contact surface to be placed over the artery. This spring responds 

 to the movements of the artery and transmits the movements to the writing 

 lever, which magnifies and records them on a writing surface driven by a small 

 clockwork. The movements of the spring are transmitted to the lever by means 

 of the screw s, jointed to the contact surface, and the tension of the spring can 



