THE CIRCULATION OF THE BLOOD. 14> 



are inseparable. 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 circulation ; and dimin- 

 ished by the reverse conditions. 



The dicrotic wave is increased by : 1. Sudden contraction of the 

 Heart ; 2. Low blood pressure ; 3. Increased outflow or rapid capillary 

 circulation ; 4. Elasticity of the aorta; 5. Eelaxationof muscular coat ; 

 and diminished by the reversed conditions. 



One very important precaution in the use of the sphygmograph lies 

 in the careful regulation of the pressure. If the pressure be too great, 

 the characters of the pulse may be almost entirely obscured, or the ar- 

 tery 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 charac- 

 ers may be represented on the tracing. 



The Pressure of the Blood within the Arteries (producing 



arterial tension). 



It will be understood from all that has been said about the ar- 

 teries in a normal condition (a) that they are during life continually 

 "on the stretch," even during the cardiac diastole, and that in con- 

 sequence of the injection of more blood at each systole of the ven- 

 tricle into the elastic aorta, that this stretched condition is exagge- 

 rated each time the ventricle empties itself. This state of distention 

 of the arteries is due to the pressure of blood within them, and 

 arises in consequence of the resistance presented by the smaller arteries 

 and capillaries (peripheral resistance) to the sudden emptying of the ar- 

 terial system between the contractions of the ventricle. It is called the 

 condition of arterial tension. It will be further understood (b) that, as 

 the blood is forcibly injected into the already full arteries against their 

 elasticity, it must be subjected to the pressure of the arterial walls, so 

 that, when an artery is cut across, the blood is projected forwards by 

 this force for a considerable distance. Thus, although the blood distends 

 the arteries and produces tension, yet the elasticity of the arteries reacts 

 upon the blood, and subjects it to pressure. We have therefore to remem- 

 ber that we have to do with two things related but not identical, viz., the 

 pressure which the blood exerts upon the arterial walls tending to stretch 

 them, and the pressure to which the blood is subject by the arteries tend- 



