274 TEXT-BOOK OF PHYSIOLOGY. 



that the contraction process passes over the heart in the form of 

 a waveVthat it begins, indeed, at the terminations of the^great 

 A^eins, then passes to and over the auricles, thence to and overtfre 

 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. 



Changes in Position and Form. In passing from the diastolic 

 to the completed systolic condition the exposed heart undergoes 

 changes both of position and form as the contraction rises to its 

 maximum. This having been attained, the heart undergoes reverse 

 changes until the original diastolic condition is regained. Thus 

 at the time of the ventricular systole the apex is tilted upward, the 

 entire heart is twisted on its axis from left to right and forced down- 

 ward by the expansion and elongation of the pulmonary artery and 

 aorta. At the time of the diastole, the reverse movements take place. 



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 

 is that of a circle. In passing from the diastolic to the systolic con- 

 dition the transverse diameter diminishes while the antero-posterior 

 diameter increases, while the whole heart becomes somewhat more 

 conic in shape. It is questionable if the vertical diameter per- 

 ceptibly 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 is principally ob- 

 served in the space between the fourth and fifth ribs, between the 

 left edge of the sternum and a line drawn vertically through the nipple. 

 The cardiac impulse is synchronous with the cardiac systole. 



The Cardiac Cycle. The entire period of the heart's pulsation 

 may be divided into three phases, viz. : 



1. The auricular contraction. 



2. The ventricular contraction. 



3- The pause or period of repose, during which both auricles and 

 ventricles are at rest. 



