THE HEART BEAT. 541 



excitation toward the base of tiie ventricle. Following this method 

 of interpretation the electrocardiogram has been used to trace out 

 the course of the wave of excitation and contraction in the ventricle, 

 but there has been little agreement in tire conclusions reached by- 

 various workers. If we can accept the description, given in the 

 preceding paragraph, of the course of the wave of excitation and 

 contraction in the ventricle it would seem that the whole of the 

 ventricle should be in a condition of equal potential during systole, 

 and this appears indeed to be the case in the period between the S 

 and T waves. But the rapid oscillations in potential at the begin- 

 ning of systole indicated by the Q, R, and S waves have not been 

 explained satisfactorily with reference to the course of the waves of 

 excitation and contraction. The T wave occurs at the end of sys- 

 tole. According to Einthoven this wave may be positive or nega- 

 tive or be absent altogether. He interprets it to mean that the 

 condition of contraction does not cease simultaneously in all parts 

 of the ventricle. When, as is usually the case, it is positive it in- 

 dicates a longer continuance of excitation and contraction in the 

 basal portion of the ventricle.* While the precise significance of 

 the ventricular waves of the electrocardiogram is not yet de- 

 termined, it has been found by observation that variations in the 

 character or sign of these waves under pathological conditions can 

 be used in diagnosis, particularly in the diagnosis of distm-bances of 

 the normal rhythm. For details in regard to this point, reference 

 must be made to clinical works, j 



Change in Form of the Ventricle During Systole. — Much 

 attention has been paid to the external change of form of the 

 ventricle during systole. Does it diminish in size in all diameters 

 or only in certain diameters? The question is one that cannot 

 be answered definitely for all normal conditions, owing to the 

 fact that the form of the heart during diastole varies with the 

 posture of the body. During diastole the heart muscle is 

 quite soft and relaxed, and consequently its shape is influenced 

 by gravity. The exact change of form that it undergoes in 

 passing from diastole to systole will vary with its shape, what- 

 ever that may happen to be, in diastole. During systole the 

 musculature, on the contrary, is hard and resisting and the form 

 of the heart in this phase is probably constant. The change 

 from the variable diastolic to the constant systolic form will natu- 

 rally be different in different positions. With an excised frog's 

 heart one can show that the ventricle is elongated in passing from 

 diastole to systole or one can show the reverse. If the heart is laid 



*ror discussion and literature see Kahn, "Ergebnisse d. Physiol.," 14, 

 1914; Eyster and Meek, "Physiological Reviews," 1, 1, 1921. 



t Barker, "The Clinical Diagnosis of Int. Diseases," vol. i, 787, 1916. 



