272 TEXT-BOOK OF PHYSIOLOGY. 



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 

 vertically from the middle of the clavicle. The cardiac impulse is synchron- 

 ous 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. 123 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 relaxa- 

 tion of the ventricle ; while e-f represents the 

 time of the diastole, during which the heart 

 FIG 123. A CARDIOGRAM. cavities are enlarging with the incoming of a 

 (After Pachon.) new volume of blood in consequence of which 



the heart is pressing against the chest walls. The systolic plateau is charac- 

 terized by one or more elevations and depressions, the true cause of which is 

 unknown. 



The Cardiac Cycle. The term cardiac cycle is employed to express the 

 sequence of events from the beginning of one auricular systole and the 

 beginning of the auricular systole which immediately follows it. An examina- 

 tion of the heart shows that each pulsation may be divided into three phases, 

 viz. : 



1. The auricular systole. 



2. The ventricular systole. 



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

 tricles are at rest. 



For the purpose of obtaining accurate information as to the sequence of events, their time 

 relations, as well as of the pressure within the heart cavities during each phase of its activity, it 

 is necessary to obtain graphic records of the entire cardiac cycle. 



This was first successfully accomplished by Chauveau and Marey, by means of sounds or 

 tambours (Fig. 124) introduced through the jugular vein into the cavities of the right heart. Each 



