THE CIRCULATION OF THE BLOOD 275 



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 anjd 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 some- 

 what more conic in shape. It is question- 

 able 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 FIG. n6. A CARDIOGRAM. 

 line drawn vertically from the middle of the (After Pockon.) 



clavicle. The cardiac impulse is synchronous with the cardiac systole. 



The cardiac impulse may be recorded with an appropriate apparatus 

 known as a cardiograph; the record obtained with it is known as a cardiogram. 

 A cardiograph consists of a tambour covered with a thin rubber membrane 

 provided with a button. The tambour is supported by a metallic frame 

 which permits of an easy and accurate adjustment of the button over the 

 seat of the cardiac impulse. A rubber tube connects the cardiographic 

 tambour with a second tambour provided with a recording lever and thus 

 transmits all variations in the pressure of the air in the former to the latter. 



When all adjustments are carefully made a tracing similar to that shown 

 in Fig. 116 will be obtained, in which the slight elevation a represents the 

 contraction of the auricle which, completing the filling of the ventricle, 

 causes the apex of the heart to press more vigorously against the chest wall ; 

 b-c represents the contraction of the ventricles, at which moment the apex 

 is suddenly and forcibly driven against the chest wall; c-d represents the 

 systolic plateau, the time during which the ventricle is discharging blood 

 into the aorta; d-e represents the relaxation of the ventricle; while e-f rep- 



