THE CIRCULATION OF THE BLOOD. 



355 



six equal parts indicated by the letters a to g. In accordance with this 

 subdivision of the artery the systole of the heart may be also divided into six 

 parts, during the first three of which the heart increases in power, and during 

 the last three of which it decreases in power, gradually falling to zero. The 

 effect on the arterial wall of the discharge of blood from the ventricle is 

 illustrated in the figure. During the first one-sixth of the systole a certain 

 volume of blood is forced into the artery, which at this moment is already 

 full of blood. Of this volume a portion moves forward while another 

 portion moves sideways as the arterial wall begins to expand under the 

 pressure of the heart. At the end of the first one-sixth of the systole the 

 condition of the arterial wall may be represented by the lines ib. During 



FIG. 168. DIAGRAM SHOWING THE DEVELOPMENT OF A PULSE WAVE. (Rollet.) 



the second one-sixth the artery expands still more as the volume of blood 

 increases under the increasing force of the heart, so that at the end of the 

 second period the expansion of the arterial wall is not only greater at the 

 point a but in addition has extended over a greater length of the artery so 

 that the condition of the artery may be represented by the lines 2 c. Dur- 

 ing the third sixth the same process continues; the incoming volume of blood 

 still further expands the artery at a, as well as successive portions further on 

 as far as d, so that at the height of the systolic power the condition of the 

 artery may be represented by the lines 3 d. 



The force of the heart now begins to decline and from this moment on, the 

 elastic force of the artery preponderates and in consequence the arterial wall 

 begins to recoil at the point a. At the end of the fourth sixth of the systole, 

 therefore, the arterial wall at a, has recoiled to 2, while the expansion at a 

 has advanced to b 3 where the force of the heart and the elastic force of the 

 artery are equal. At this moment the condition of the artery may be repre- 

 sented by the lines 2, 63, e. During the two remaining sixths of the cardiac 

 systole, the same process continues until, through elastic recoil, the artery 

 has returned to its original condition at a, and the expansion has extended 

 as far as g, while the height of the expansion has advanced to ^3 where the 

 force of the systole and the force of the elastic recoil balance each other. At 

 the end of the systole the condition of the arterial wall may be represented 

 by the lines a, d$, g, which indicates that the expansion and recoil of the 

 artery, which together constitute the pulse, partake of the form of a wave the 

 length of which is represented by the line o, 0, and the height by the distance ^3. 



This expansion and recoil which thus pass from the beginning to the 

 end of the arterial system assumes the form of a wave and therefore is known 

 as the pulse-wave or pulse. Preceding and causing the expansion and recoil 

 of the arterial system there is an alternate increase and decrease of the gen- 



