THE CIRCULATION OF THE BLOOD 



293 



cessation of the auricular contraction the string returns to its position of 

 rest. The more or less horizontal line between P and Q indicates that both 

 auricles and ventricles are now at rest and in the same state of electric 

 potential. The passage of the excitation process through the conduction 

 system, which is now taking place, from its beginning to its terminations, 

 is, in and of itself, too feeble to cause a deviation of the thread under the 

 conditions of tension necessary for the registration of the other features of 

 the electrocardiogram. 



The system of points or waves, negative and positive, Q, R, S in the first 

 part of the ventricular portion of the electrocardiogram permits of the infer- 

 ence that the stimulus arrives and the contraction arises, at once or almost 

 at once in different places of the ventricular masses. The first point Q, indi- 

 cates that the stimulus arrives first at a point lying near to the apex and 

 calls forth a contraction of moderate degree establishing a certain degree 

 of electronegativity toward the base which is now electropositive. The 

 direction of the point, below the horizontal portion, is due to the direction 

 of the current through the heart and galvanometer, which is of course the 

 reverse of the current that originates in the auricle and which gives rise to P. 

 The point R, which immediately follows, indicates that the stimulus has 

 arrived at the base of the ventricle calling forth a more pronounced contrac- 

 tion and a greater degree of electronegativity than that present at the apex. 

 The point S indicates that soon there- 

 after, the stimulus reaches regions 

 lying nearer to the apex of the ven- 

 tricle calling forth a contraction and 

 establishing an electronegativity which 

 soon gains the upper hand. The 

 horizontal portion of the electrocar- 

 diogram, between the system of points 

 Q, R, S, on the one side, and the point 

 T, on the other side, represents a con- 

 traction state in which the entire 

 muscle mass of the two ventricles 

 participates in the same measure and 

 hence all portions are in a condition 

 of equal negativity for which reason 

 the string remains stationary. It is during this period that the ventricles 

 are engaged in driving the blood out of their cavities into the aorta and 

 pulmonic artery. 



T, the last peak of the ventricular portion of the electrocardiogram, is 

 directed upward and hence reveals the fact that the base of the ventricle is, 

 at this moment, in a condition of greater electronegativity than are por- 

 tions of the heart lying nearer the apex. The origin of the T peak has been 

 a subject of much discussion but the investigations of Gotch have made it 

 apparent that it is due to the contraction of muscle-fibers which have not 

 before been brought into action, that is, fibers near the root of the aorta, 

 in that portion of the ventricle which is the homologue of the bulbus arteriosus 

 in the lower animals. The contraction of this region completes the discharge 

 of the blood from the ventricular cavity. 



FIG. 131. SCHEME AND INTERPRETATION OF 

 THE ELECTROCARDIOGRAM. (Nicolai.) 



