CIRCULATION OF BLOOD 71 



is produced. This curve describes more accurately the progress 

 of the pulse. The apparatus for the production of a pulse curve 

 is called a sphygmograph. 



The pulse curve (Fig. i) ascends rapidly and then sinks 

 more slowly to the level of the abscissa. In the descent of 



FIG. i. PULSE CURVE (SPHYGMOGRAM) OF THE RADIAL ARTERY. 



the curve there is regularly found a small elevation, called 

 the dicrotic wave. The cause of this wave is not fully 

 known. Often the descending part of the curve shows still 

 other smaller elevations which are supposed to be due to the 

 reflection of the pulse wave in the various parts of the arterial 

 system. 



The blood pressure shows other periodic variations which are 

 synchronous with the respiratory movements; the pressure sinks 

 during inspiration and rises during expiration. This is chiefly 

 due to the fact that during inspiration the intrathoracic pressure 

 and therefore the pressure upon the blood vessels in the thorax is 

 diminished, while during expiration it is increased. Hence the 

 blood vessels in the thorax are better filled with blood during 

 inspiration than during expiration. 



In general the amount of blood pressure and its pulsatory 

 variations depend upon the state of fulness of the vessels, 

 the tonus of the muscles of the blood vessels, and upon the 

 number and strength of the cardiac contractions. 



2. Rate of blood flow. In the arteries the blood flows 

 with a periodically accelerating rate (corresponding to the 

 intermittent entrance of blood into the aorta) ; in the capil- 

 laries and veins the flow is uniform. The change from the 

 intermittent movement of the blood in the arteries to the 

 uniform movement in the capillaries is due to the extensi- 

 bility and elasticity of the arterial walls. In vessels with 

 rigid walls, each systole must force forward the column of 

 blood previously pressed out of the heart. In vessels with 

 elastic walls, the force of the systole is not directly trans- 

 formed into the movement of the blood, but is first changed 

 to the increased tension of the elastic walls, and this stored- 



