274 THE CIRCULATION OF THE BLOOD 



the base is negative to the apex at the beginning (It-wave) and again at 

 the end (T-wave) of the ventricular contraction. How may this be ex- 

 plained? When electrocardiograms are taken through electrodes placed 

 directly on the base and apex of the ventricle of the exposed heart, it 

 has been found that the contour of the electrocardiogram is like that 

 which is obtained from a strip of muscle when a wave of contraction 

 passes along it: it is diphasic in character (page 188), a result which 

 may be interpreted as indicating that the wave of contraction starts at 

 the base and ends at the apex. This rules out the explanation, at one 

 time suggested for the T-wave, that the wave starts at the base, then 

 proceeds to the apex, and finally ends at the base, following the disposi- 

 tion of the muscular fibers of the ventricle in a folded or loop form, 

 with the bend of the loop at the apex and the free ends at the base. Al- 

 though the explanation seemed at first to conform with the embryo- 

 logical fact that the heart is developed from a folded tube, it can not hold 

 as has been shown by observing the course of the excitation wave se- 

 cured through electrodes placed at various points on the surface of the 

 exposed ventricle (page 194). 



The explanation which is accepted by the majority of observers at the 

 present time is to the effect that the T-wave is caused by the longer con- 

 tinuance of the electric change at the base of the ventricle than at the 

 apex. To test this hypothesis the crucial experiment would evidently 

 be to see whether a T-wave could be induced in an electrocardiogram, 

 such as that of the frog ventricle, in which no T-wave exists, by hurry- 

 ing up the contraction process at the apex without affecting it at the 

 base. This can be done by local warming of the apex, or by applying 

 the ventricular electrode at varying parts of the ventricle in an excised 

 heart beating in Ringer's solution of relatively high H-ion concentra- 

 tion. Mines showed that under these conditions a typical T-wave ap- 

 pears in the electrocardiogram, as shown in Fig. 84.* 



The existence of the small Q-wave, indicating that the contraction 

 does not really start from the base, conforms with the observation that 

 the Purkinje system of fibers ends about the papillary muscles, which 

 therefore would be the first to contract, and with the observations of 

 Lewis, already alluded to above, on the appearance of the negative vari- 

 ation on the surface of the exposed heart. 



The most important clinical application of the electrocardiogram is 

 undoubtedly in connection with the determination of the rate of trans- 

 mission of the excitation wave from auricle to ventricle; thus, the P-R 

 interval, as it is called, indicates the time taken for the impulse to 



*This tracing was found among those left by Professor Mines of McGill University, and for 

 permission to use it the author is indebted to the authorities of that institution. 



