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HAND-BOOK OF PHYSIOLOGY. 



Of the three main parts then of a pulse-tracing, viz., the percussion 

 wave, the tidal, and the dicrotic, the percussion wave is produced by 

 sudden and forcible contraction of the heart, perhaps exaggerated by an 

 excited action, and may be transmitted much more rapidly than the tidal 

 wave, and so the two may be distinct; frequently, however, they are in- 

 separable. The dicrotic wave may be as great or greater than the other 

 two. 



According to Mahomed, the distinctness of the three waves depends 

 upon the following conditions: 



The percussion wave is increased by: 1. Forcible contraction of the 

 Heart; 2. Sudden contraction of the Heart; 3. Large volume of blood; 

 4. Fulness of vessel; and diminished by the reversed conditions. 



The tidal wave is increased by: 1. Slow and prolonged contraction 

 of the Heart; 2. Large volume of blood; 3. Comparative emptiness of 

 vessels; 4. Diminished outflow or slow capillary circu- 

 lation; and diminished by the reversed conditions. 



The dicrotic wave is increased by: 1. Sudden con- 

 traction of the Heart; 2. Comparative emptiness of 

 vessels; 3. Increased outflow or rapid capillary circu- 

 lation; 4. Elasticity of the aorta; 5. Relaxation of mus- 

 cular coat; and diminished by the reversed conditions. 



One very important precaution in the use of the 

 sphygmograph lies in the careful regulation of the pres- 

 sure. If the pressure be too great, the characters of the 

 pulse may be almost entirely obscured, or the artery may 

 be entirely obstructed, and no tracing is obtained; and 

 on the other hand, if the pressure be too slight, a very 

 small part of the characters may be represented on the 

 tracing. 



THE PRESSURE OF THE BLOOD WITHIN THE ARTERIES 

 (PRODUCING ARTERIAL TENSION). 



It will be understood from the foregoing that the 

 arteries in a normal condition, are continually on the 

 stretch during life, and in consequence of the injection 

 of more blood at each systole of the ventricle into the 

 elastic aorta, this stretched condition is exaggerated each time the ventricle 

 empties itself. This condition of the arteries is due to the pressure of 

 blood within them, because of the resistance presented by the smaller ar- 

 teries and capillaries (peripheral resistance) to the emptying of the arterial 

 system in the intervals between the contractions of the ventricle, and is 

 called the condition of arterial tension. On the other hand, it must be 

 equally clear that, as the blood is forcibly injected into the already full 



ter. 



