A series of regular figures is normally obtained in which are depressions and elevations and regular spacing of these elevations and depressions. The waves so-called have been arbitrarily designated _P_, _Q_, _R_, _S_, _T_. There is some difference in the three leads. "The wave _P_ is positive in _all leads_. _P_ to _R_ interval varies slightly in the _three leads_. All the waves of _Lead II_ are greater than those of _Leads I_ and _III_. The wave _R_ is positive in _all leads_. _T_ is usually positive in _all leads_, but is occasionally negative in Lead III. Even in normal individuals there is a considerable range of variation in the electrocardiogram which is within the limits of the normal." (Hart.) (Fig. 39.) [Illustration: Fig. 39.--Normal electrocardiogram. (After Hart.)] The _P_ wave is admitted to be the wave of auricular contraction. _Q_, _R_, _S_, is the ventricular complex caused, it is thought, by the current passing over the ventricles. _T_ wave is not yet definitely settled. It has been thought by some that it represented actual ventricular contraction and its height and shape had some meaning in heart force. This is denied by others. Hart defines it as "The final activity of the ventricle." The _T_ wave is usually increased in size during exercise. The _P-R_ interval is almost the most important feature of the tracing. It is the actual conduction time in fractions of a second of the impulse from s-a node to the ventricles. Normally this is about 0.2 second or slightly less. Much that was hoped for from the electrocardiograph in the clinic has not been forthcoming. Its greatest value is in states of abnormal conductivity, such as various grades of heart block, extrasystoles, whether originating in auricles or in either ventricle, abnormalities of rhythm, as flutter and fibrillation. It has, however, aided materially in the intelligent interpretation of many phenomena heretofore not well understood, and has enormously increased our knowledge of the physiology and pathologic physiology of the heart. It is not possible to enter farther into the subject here. This brief discussion must suffice. The reader is referred to works on this subject in connection with diseases of the heart. CHAPTER IV IMPORTANT CARDIAC IRREGULARITIES ASSOCIATED WITH ARTERIOSCLEROSIS