CIRCULATION. 445 



believe that the excitation-wave under certain conditions returns toward the 

 base after having reached the apex. 1 The speed of the excitation-wave has been 

 measured by the interval between th<f appearance of negative variation in the 

 ventricle when the auricle is stimulated first near and then as far as possible 



FIG. 112. The electrical variation in the spontaneously contracting heart of the frog, recorded by a 

 capillary electrometer, the apex being connected with the sulphuric acid and the base with the mercury 

 of the electrometer. The changes in electrical potential are shown by the line e, e, which is obtained by 

 throwing the shadow of the mercury in the capillary on a travelling sheet of sensitized paper. The con- 

 traction of the heart is recorded by the line h, h ; time, in fa second, by t, t. The curves read from left 

 to right. The electrical variation is diphasic ; in the first phase the base is negative to the apex ; in the 

 second, the apex is negative to the base ; the negative variation passes as a wave from base to apex 

 (Waller, 1887, p. 231). 



from the non-polarizable electrodes. The interval is the time which the excita- 

 tion-wave requires to pass the distance between the two points stimulated. The 

 average rate is at least 50 millimeters per second. 2 The negative variation 

 begins apparently instantly after the application of the stimulus. Its phases 

 and their characteristics have been described by Engelmann. 3 

 The latent period of a frog's heart muscle is about 0.08 second. 4 

 Although the normal course of the excitation-wave is from base to apex, it 

 can be made to travel in any direction. If the frog's ventricle is cut with fine 

 scissors into a number of pieces in such a way as to leave small bridges of 

 heart-tissue between each piece, and any one of the pieces is stimulated, the 

 contraction will begin in the stimulated piece and then run from piece to piece 

 over the connecting bridges until all have successively contracted. The direc- 

 tion in \vhich the excitation- wave travels can thus be altered at the pleasure 

 of the operator. 5 



Whether the excitation is propagated from muscle-cell to muscle-cell or by 

 means of nerve-fibres has given rise to much discussion. Anatomical evidence 

 can be adduced on both sides. On the one hand the rich plexus of nerve- 

 fibres everywhere present in the heart-muscle suggests conduction through 

 nerves ; on the other is the intimate contact of neighboring muscle-cells over 



1 Bayliss and Starling, 1892, pp. 260, 380. 



2 Engelmann, 1878, p. 91 ; Burdon-Sanderson and Page, 1880, p. 426, give 150 millimeters 

 per second 



3 Engelmann, 1878, p. 74. 4 Ibid., 1874, p. 6. 

 5 Ibid., p. 3; compare Bayliss and Starling, 1892, p. 262. 



