THE PHENOMENA NOTED DURING EACH CARDIAC CYCLE 285 



as a means of diagnosis, because it frequently fails to represent the 

 conditions as they actually are. It must be granted, however, that the 

 fault does not always lie with the instrument, but more frequently 

 with the experimenter. If properly, applied, it registers the different 

 beats with accuracy, but does not allow definite conclusions being 

 made regarding the character of the contractions, because its mechan- 

 ism is easily affected by various factors such as changes in the position 

 of the body, or alterations in the resistance under which it is made to act. 



Fig. 143. — Marey's Tambour. 

 a. Axis of lever; b, metal tray covered with rubber membrane, and communicating 

 by tube / with the receiving drum shown in Fig. 142. (Starling.) 



Moreover, the conspicuousness of the impulse differs even in perfectly 

 normal individuals, owing to differences in the thickness of the chest 

 wall. 



Under ordinary conditions, the cardiogram consists of a series of 

 upstrokes and downstrokes. The former indicate the successive sys- 

 tolic and the latter the successive diastolic movements of the ventricles. 

 In complete agreement with the general character of the contraction 

 of the cardiac muscle, these two limbs of the curve are generally joined 



Stc 



Fig. 144. — Cardiogram. 

 AB, Systole ;BC, plateau; CD, diastole; DA, pause; time in seconds. 



by a "plateau," the implication being that this muscle does not relax 

 immediately upon having attained its state of maximal shortening, 

 but remains in this condition for a brief period of time. The curve 

 may also present an initial slight rise which is caused by the systole of 

 the auricles, and a small peak upon its downstroke which occurs 

 synchronously with the closure of the semilunar valves.^ 



1 For purposes of diagnosis, it is necessary to ascertain not only the location 

 of the impulse but also its strength. A displacement of it is brought about by 

 accumulations of air (pneumothorax), serum (hydrothorax), blood, and pus, as 

 well as by tumors of the thoracic and abdominal viscera. Hypertrophy and 



