CIRCULATION. 6l 



injecting liquid into it at short intervals at a, allowing it 

 to flow out by a small opening at b. In the arterial system, 

 just as in this schema, the momentary distension produced 

 in the aorta by each injection of blood, is propagated to 

 other parts of the system at a rate which is dependent 

 on the previously existing pressure. Every such sudden 

 distension is followed sooner or later by a second, which 

 is called the second beat 



By virtue of the elasticity of the arteries, part of the 

 motion which is communicated to the blood during each 

 ventricular systole is stored as arterial distension, to re- 

 appear as progressive motion during the diastolic interval. 

 If the arteries were not elastic, this motion would lose 

 itself in the shock of the blood against the rigid arterial 

 wall, whereby the arteries would be injuriously strained at 

 each injection of blood, and the effect of the heart's action 

 would be diminished. 



Investigation of Arterial Pressure. The haemadynamo- 

 meter is a mercurial manometer of which one limb can be 

 connected with an artery by a tube containing solution of 

 sodic bicarbonate. In order that the measurement may be 

 accurate, it is necessary (i) that the mean level of the 

 mercury surface in the proximal limb should be the same 

 as that of the arterial aperture ; and (2) that the tube 

 should enter the artery at right angles to its axis. 



Any instrument by which the arterial pressure can be 

 measured, by inscribing its variations on a surface moving 

 horizontally by clockwork at a uniform rate, is called a 

 kymograph. A mercurial kymograph consists of three 

 parts the clockwork and recording cylinder ; the mano- 

 meter and writer ; the tube and cannula by which the 

 manometer is connected with the artery. Its uses are 

 (i) to measure the mean arterial pressure and to record its 

 variations ; (2) to measure the duration of the pulsation 

 intervals. Its chief defect arises from the " proper 

 motion " of the mercurial column. The spring kymograph 



