SEC. 4. THE PULSE. 



121. We have seen that the arteries, though always dis- 

 tended, undergo, each time that the systole of the ventricle drives 

 the contents of the ventricle into the aorta, a temporary additional 

 expansion so that when the finger is placed on an artery, such 

 as the radial, an intermittent pressure on the finger, coming and 

 going with the beat of the heart, is felt, and when a light lever 

 is placed on the artery, the lever is raised at each beat, falling 

 between. 



This intermittent expansion, which we call the pulse, cor- 

 responding to the jerking outflow of blood from a severed artery, 

 is present in the arteries only, being, except under particular 

 circumstances, absent from the veins and capillaries. The expan- 

 sion is frequently visible to the eye, and in some cases, as where 

 an artery has a bend, may cause a certain amount of locomotion 

 of the vessel. 



We may, by applying various instruments to the interior of an 

 artery, study the temporary increase of pressure which is the cause 

 of the temporary increase of expansion. This makes itself felt, as 

 we have seen, in the curve of arterial pressure taken by the mercury 

 manometer ; but the inertia of the mercury prevents the special 

 characters of each increase becoming visible. In order to obtain 

 an adequate record of these special characters we must have 

 recourse to other instruments. 



The membrane-manometer, of which we have already spoken ( 113), 

 and on the results gained by which when applied to the root of the 

 aorta by means of a catheter we have dwelt ( 116), may also be applied 

 to other arteries, the tube leading to the tambour of the manometer being 

 connected with the artery by means of a cannula in the ordinary way. 



In Fick's spring-manometer, in its original form, Fig. 56, the artery 

 is connected by means of a cannula and a rigid tube containing fluid 

 with the interior of a curved spring ; an increase of pressure unfolds 

 the curve of the spring, the movements of the end of which may be 

 recorded by means of a lever. In Fick's improved form the membrane 

 of a small air-tambour works against a horizontal slip of steel which 

 acts as a spring; this instrument, like Frey and Erehl's manometer 



