2?4 TEXT-BOOK OF PHYSIOLOGY 



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 cava, 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 pulmonic 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 pulmonic 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 

 the transverse diameter diminishes while the antero-posterior diameter 

 increases, and the whole heart becomes somewhat more conic in shape. 

 It is questionable if the vertical diameter perceptibly shortens. During the 

 systole the heart hardens, increases in convexity, and is more forcibly pressed 

 against the chest wall. As this takes place suddenly, it gives rise to a 

 marked vibration of the chest wall, known as 



The Cardiac Impulse. This impulse is principally observed in the 

 space between the fifth and sixth ribs about an inch internal to a line drawn 



