ELECTRICAL CHANGES IN TISSUES 



657 



The whole is to be explained by difference in time relations of the ordinary wave 

 of excitation in different directions. Caution must be exercised in drawing 

 conclusions from the signs of the components. The electrical change, as recorded, 

 does not merely indicate the magnitude of the process under one electrode only, 

 since electrical expression of it is cut short according to the time at which the 

 wave arrives at the other electrode. 



The relations in time of the mechanical and electrical effects are of some 



m 



FIG. 213. ELECTRO-CARDIOGRAMS FROM DIFFERENT ANIMALS, 

 TAKEN BY LEADING OFF FROM THE LIMBS WITH THE 

 HEART UNEXPOSED. 



Frog, goldfish, pigeon, and tortoise in order from above downwards. 



The essential similarity to the human electro-cardiogram will be noticed in 

 the cases of the frog and the tortoise. The goldfish shows a diphasic 

 ventricular change, like the exposed heart of the frog and tortoise. 

 That of the pigeon is peculiar. 



(Lewis, 1913, 1, p. 18.) 



interest. Figs. 172, 173, and 214 show that the duration is practically identical. 

 In Fig. 214, from a short article by Piper (1913, 1), we see that the latter begins 

 a little before the pressure change, and ends a little earlier. An interesting point 

 is that the greater part of the final electrical phase appears to take place after the 

 pressure curve has begun to fall, a fact which tends to confirm the view taken 

 above that it represents the last part of the wave of excitation, namely, that part 

 in the fibres which are the latest to relax. 



It has been remarked above (page 215) that Lovatt Evans (1912, 2) found 

 that the heart of the snail, although requiring calcium for its normal activity, is 

 unusually insensitive to these ions. Thus, with 1 per cent, calcium chloride the 

 beat is normal, while the frog's heart is sent into systolic contraction by this 

 concentration. A peculiar effect on the electro-cardiogram is also to be seen. 

 In Fig. 215 we see the effect referred to. The heart is first in tonus ; calcium 



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