182 Messrs. S. R Wells and L. Hill. Influence of [Nov. 29, 



readings due to the thickened artery resisting compression, just as an empty- 

 rubber tube does. 



One of us (L. H.) and Martin Flack have found that such differences of 

 pressure are lessened by keeping on the pressure of the armlet, and lowering 

 and raising it so as to take several readings of : (1) the reappearance ; (2) the 

 disappearance of the pulse. Our explanation is, that the artery cut off from 

 the blood relaxes and softens, and therefore the crest of the systolic wave is 

 diminished. It has been shown by Bayliss that compression of an artery is 

 followed by vascular dilatation in the area cut off from the blood. 



In many of these cases the force of the pulse is irregular ; now and again 

 an extra large systolic crest forces its way beneath the armlet, and such 

 large waves are better conducted by the leg arteries, just as happens in the 

 case of aortic regurgitation. 



By means of a circulatory schema, in which two lengths of artery are 

 inserted, one to be compressed, the other to be palpated (the latter gave the 

 index, the disappearance of the pulse), it was easy to demonstrate that the 

 systolic pressure is read more accurately when the palpated artery is made 

 tense (produced in this schema by obstructing the outflow by means of a 

 mercury valve) than when it is soft. In the first case the readings of 

 systolic pressure taken in the pump and in the artery are the same, in the 

 second case the reading taken in the artery is lower. 



In the living animal with its vasomotor nerves, and pressure changes of 

 rapid rate, and output of the heart varying from second to second, it is 

 extremely difficult to study exactly the effect of the various factors on the 

 character of the pulse curve, for one cannot vary at will one of these factors 

 without affecting the others. From these considerations it appeared desirable 

 to one of us (S. B. W.) to investigate the subject by means of non-living 

 elastic tubes. Halls Dally and K. Eckenstein have assisted in this research, 

 which will be published in full later. 



After considerable experiment an apparatus was devised, by means of 

 which fluid at a known rhythmically changing pressure could be passed 

 (a) through elastic tubes of the same calibre, but with walls of various 

 known thicknesses ; (b) through various lengths of the same tube, and 

 (c) keeping to the same tube, the absolute pressure could be varied, while 

 maintaining the same difference between the systolic maximum and the 

 diastolic minimum, or this difference could also be varied at will. 



The tubes used in the experiments were various lengths of rubber tube all 

 of the same internal calibre, but with walls of 0*8, 0'6, 0-4, and 0'2 mm. 

 thickness. The pressure variations of the fluid before flowing through the 

 resilient tube and at the end of it were recorded by Hurthle's manometer. 



