vin BLOOD -STREAM: MOVEMENT IN VESSELS 267 



of pressure within the artery investigated; it is only proportional 



to the magnitude of the 



local oscillations of the 



pulse, which (as we saw 



on p. 248) are, cetcris 



paribus, in inverse ratio 



to the tension of the ar- 



terial walls. As a matter 



of fact, not only the prim- 



ary, butalso the secondary 



diuretic wave is lower 



with greater, and higher 



with reduced arterial 



pressure. Not even this 



fact, however, can be 



taken as a general law, 



since the varying degree 



of contraction or relaxa- 



tion in theartery explored 



has great influence upon 



the amplitude of the 



sphygmic undulations. 



In order to realise this, 



we have only to consider the marked changes that appear in 



the form and magnitude of the sphygmo- 

 gram after inhalation of amyl nitrite, which 

 immediately produces depression of arterial 

 tone (Fig. 105). The beats of the heart 

 become more frequent, and yet the amplitude 

 of the primary as well as of the dicrotic 

 wave increases. 



Far more interesting and instructive, from 

 the clinical point of view, is the comparison 

 of the sphygmogram with the simultaneously 

 recorded cardiogram, as also the comparison 

 of two or more sphygmograms simultaneously 

 recorded from different arteries. For this 

 purpose, however, the direct sphygmograph 

 is not suitable, and others with air transmis- 

 8ion must beresorted to. These are essentially 

 tacned to a semicircular identical with cardiographs, I tut are different 



spring intended to tit round ,, . . -, L . IIP 



the neck. The pressure of in lorin, since tliey are intended tor use on 



Kio. 107. Burdon-Saiirlerson's cardiograph, which can also 

 be used as a sphygmograph for the carotid, and a 

 pneumograph fXimmermann's type). The apparatus rc>ts 

 on three ebonite feet, which can be adjusted by screws. 

 It is fixed by a band to the thorax or neck, so that the 

 central button, which has a steel spring (the tension of 

 which can be altered by a screw), presses against the 

 spot at which the beat of the heart or carotid pulse is 

 most perceptible. The transmission by air of the move- 

 ments of the spring is effected through a receiving 1 

 tambour, the position of which in regard to the .spring 

 can be regulated by a screw. 



Fit;. io8. Edge's sphygmo- 

 !r a a ll receivSn < 



the several arteries. One 



of these trans- 

 that of Marey 



, 



IOI tllC 1'adial 



piate applied lo the neck, mission sphi/(/ni<nfra]>ks is 



which stretches the spring * -\ . 



more or less according to the (represented 111 Pig. 106), 



position of the screw. i j i ,1 p TT<J 



artery. A simpler model is that ot Edgren 

 for the same artery. Burdon- Sanderson's cardiograph (Fig. 107) 



