BY DR. BURDON-SANDERSON. 229 



the plane of collapse i.e., nearer the axis of the artery. For 

 if it were further from the artery it would be affected by the 

 arterial movement only during its period of expansion, remain- 

 ing the rest of the time motionless. If, on the other hand, it 

 were much nearer, the vessel would be flattened against the 

 bone during the period of collapse, so that in this case, as in 

 the other, there would be no motion (of the spring) during 

 diastole. Hence it is easy to understand how it happens that 

 the tracings obtained with excessive and defective pressure are 

 very similar to each other in their general characters. Stating 

 the same thing in other words, we arrive at the general rule 

 that the spring must be so set that the ivory plate on its under 

 surface is at such a distance from the opposed surface of bone 

 that the artery is pressed upon at all degrees of expansion, yet 

 not so strongly pressed upon as to bring its walls into contact 

 even when it is relaxed. Within these limits, the variations 

 of form of -the tracing in other words, its departure from 

 truth are very inconsiderable ; so that observations made on 

 the same individual at different times yield closely correspond- 

 ing forms. As, however, the results obtained by strong press- 

 ure are less subject to accidental error than those obtained 

 with weaker ones, it is better always to begin with a pressure 

 sufficient to flatten the artery, and then to weaken the spring 

 until the effects of over-compression disappear i. e., until it is 

 found that the lever continues to descend until the very end 

 of diastole. 



39. Use of the Sphyginograpii as a Means of Appre- 

 ciating those Changes of Mean Arterial Pressure 

 which occur in Disease. We have already seen that the 

 sphygmograph is of no use as a gauge of arterial pressure. It 

 is possible, however, by the comparison of observations made 

 at successive periods on the same individual, to determine 

 whether the arterial tension has changed, and in what direc- 

 tion the change has taken place. We have seen that if the 

 spring is so strong that the artery is either partially or en- 

 tirely flattened against the radius, the fact is indicated by the 

 cessation of the motion of the lever. The strength of spring 

 which is required to bring about this result varies with the 

 pressure by which the artery is distended ; so that if in any in- 

 dividual the arterial pressure is increased, a greater tension of 

 the spring is required to compress it than was required before. 

 With Marey's sph^ygrnggraph, as imported, it is not possible 

 for the observer to avail himself of this principle, because the 

 instrument is not graduated i. e., there is no means by which 

 the pressure exerted by the spring at any moment can be ascer- 

 tained. I have therefore modified that instrument as follows 

 (nee Fig. 209, a) : The brass frame, instead of being bound on 

 to the arm by bandages, rests firmly on the bones of the wrist 



