208 ESSENTIALS OF PHYSIOLOGY. 



aorta continues to move in virtue of its momentum, thereby producing 

 a slight fall of pressure at the root of the aorta ; and the wall of the 

 artery shrinks a little. This slight fall of pressure immediately causes 

 a reflux of blood against the semilunar valves, closing them, and from 

 the closed valves the blood rebounds, thereby producing a small 

 secondary expansion of the aorta ; this expansion travels along the 

 arterial system and forms the dicrotic wave. The height of both the 

 primary and dicrotic waves is largely determined, first, by the elasticity 

 of the arteries, and, secondly, by the degree of distension of the arteries 

 between successive heart beats. If they have become rigid (from old 

 age or disease), their capacity for expansion will obviously be diminished ; 

 and if they are already greatly distended by a high mean arterial blood 

 pressure, their capacity for further distension will also be decreased. 

 The conditions most favourable to the appearance of a marked dicrotic 

 pulse, therefore, are (1) a strongly beating heart, (2) a moderate blood 

 pressure, (3) highly elastic arteries. These conditions are often very 

 fully realised in young adults during fever. 



The rise of pressure caused by the entrance of blood into the arterial 

 system with each heart beat produces its maximum effect where it 

 first enters it, namely, at the root of the aorta, and the wave of ex- 

 pansion is largest at this point. Some of this rise of pressure is used 

 up in expanding the first section of the aorta, and the force tending 

 to distend the next segment will be slightly less ; this process continues 

 from segment to segment along the arterial system, the wave gradually 

 becoming smaller and smaller, until in the capillaries it has entirely 

 disappeared and no trace of any pulse is visible. 



VENOUS PULSE. 



A venous pulse is normally present in the great veins near the 

 heart, arid direct observation of the jugular vein shows two visible pulse 

 waves for each heart beat. In order to obtain a record of the venous 

 pulse and to interpret it, a simultaneous tracing of the venous pulse 

 and of the radial pulse is obtained by means of the polygraph. This 

 consists of a clockwork arrangement, whereby a continuous record of 

 the venous and radial pulses can be obtained on a moving sheet of 

 paper. It is provided also with a time-marker, which records on the 

 paper. A small metal cup with an opening at its base is pressed on to 

 the skin over the jugular vein just above the clavicle, and is connected 

 with a tambour attached to a lever which writes on the recording 

 surface. A sphygmograph, attached to the wrist, is also connected by 

 rubber tubing with a similar tambour and lever, and the two levers are 



