THE CIRCULATION OF THE BLOOD. 271 



The contraction of any part of the heart is termed the systole] the relaxa- 

 tion, the diastole. As each side of the heart has two cavities the walls of 

 which contract and relax in succession, it is customary to speak of an auricu- 

 lar systole and diastole, and a ventricular systole and diastole. As the two 

 sides of the heart are in the same anatomic relation to each other, they 

 contract and relax in the same periods of time. 



It has also been ascertained that the contraction of the auricles and 

 ventricles as well as their subsequent relaxations, though occurring with 

 extreme rapidity, do not "take place simultaneously but successively; that 

 the contraction process passes over the heart in the form of a wave; that it 

 begins, indeed, at the terminations of the great veins, viz., the vena caves, then 

 passes to and over the auricles, thence to and over the ventricles from base 

 to apex with great rapidity, but occupying in these different regions unequal 

 periods of time; that the relaxation immediately succeeds the contraction, in 

 the same order, and that at the close of the ventricular relaxation there is a 

 period during which the whole heart is in repose, passively filling with blood. 



The immediate cause of the movement of the blood through the vessels 

 is the contraction and relaxation of the muscle-walls of the heart, and more 

 particularly of the walls of the ventricles, each of which plays alternately 

 the part of a force-pump, and possibly to a slight extent of a suction-pump. 

 The motive power is furnished by the heart itself, by the transformation 

 of potential energy, stored up during the period of rest, into kinetic 

 energy i.e., heat and mechanic motion. 



Changes in Position and Form. It is also apparent under the condition 

 of the foregoing observation that the heart during each pulsation undergoes 

 changes of both position and form. In the diastolic condition, during which 

 the heart is in repose, the apex is directed obliquely downward and to the 

 left; the body of the heart is enlarged and its walls relaxed. As the 

 systole begins and reaches its maximum, the apex is tilted upward, the 

 entire heart is rotated on its axis from left to right and forced forward by 

 the expansion and elongation of the pulmonary artery and aorta. As the 

 diastole begins and rapidly passes to its completion a reverse series of 

 movements is presented, viz. : an ascent of the heart due to the recoil and 

 shortening of the pulmonary artery and aorta, a rotation of the heart on 

 its axis from right to left, and a fall of the apex. With the completion of 

 this latter event, the heart for a brief period is in repose. 



It is probable, however, that these movements are not permitted to 

 the same extent in the unopened chest, for the following reasons: the 

 heart is enclosed in the pericardium, is supported posteriorly by the 

 expanded lungs, and both posteriorly and inferiorly by the diaphragm, all 

 of which cooperate in keeping the heart, and more particularly the right 

 ventricle, in close contact with the chest- wall and limiting its movements. 

 By means of needles inserted into the apex of the heart, through the chest- 

 walls, it has been shown by their slight movement that the apex is 

 practically a fixed point. 



In the diastolic condition the shape of the heart near the base is elliptic 

 on cross-section, the long diameter extending from side to side. In the 

 completed systolic condition the shape of the same cross-section approxi- 

 mates that of a circle. In passing from the diastolic to the systolic condition 



