viii 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, ceteris 



paribus, in inverse ratio 



to the tension of the ar- 

 terial walls. As a matter 



Of fact, not only the prim- 

 ary, but also the secondary 



dicrotic 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 the artery 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 sphygniogram 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 

 IDS. Bdgren's sphygmo- is not suitable, and others with air transmis- 

 a f receivTg^iSuht- sion musfc beresorted to. These are essentially 

 tached to a" semicircular identical with cardiographs, but are different 



ng intended to tit round ./. ,, j! j j j? 



neck. The pressure of in form, sin ce .they are intended for use on 



FIG. 107. Burdon-Sanderson's cardiograph, which can also 

 be used as a sphygmograph for the carotid, and a 

 pneumograph (Zimmermann's type). The apparatus rests 

 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 

 tambour, the position of which in regard to the spring 

 can be regulated by a screw. 



the exploring button upon 

 the artery is regulated by a 

 plate applied to the neck, 

 which stretches the spring 



the several arteries. One of these trans- 

 mission sphygmographs is that of Marey 



more or less according to the (represented in Fig. 106), for the radial 

 artery. A simpler model is that of Edgren 

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



