656 



PRINCIPLES OF GENERAL PHYSIOLOGY 



that such an explanation does not hold. Details may be found in the work of 

 Mines, who followed the course of the excitation wave by leading off from various 

 points on the surface of the ventricle in the frog. No evidence was obtained of 

 such a course of the wave as that supposed by Gotch. The only satisfactory 

 explanation was found to be that the negativity at the base does actually last 

 longer than that at the apex, when the heart is in position in the intact animal. 

 The way in which this fact accounts for the form of the electro-cardiogram will be 

 clear from Fig. 212. 



Now we must remember that, in the mammalian heart, the auricular excitation 

 is transmitted to the ventricle through a system of special muscular fibres, 

 Purkinje's cells, which branch to all parts of the ventricle, and that the muscular 

 structure of the contractile wall consists of strands passing in various directions. 



It seems that Einthoven is inclined 

 to attribute the form of the electro- 

 cardiogram to excitation starting 

 from a place not exactly at the 

 base, but reaching the base before 

 the apex, although various other 

 parts, not necessarily the apex, 

 might be excited immediately after 

 the base. However the excitation 

 wave is conveyed, it seems that at 

 any given spot all the muscular 

 layers must be in contraction 

 s'multaneously, otherwise there 

 would be danger of their tearing 

 apart. Moreover, the fact that 

 simple hearts, such as those of the 

 frog and tortoise, show, in the 

 intact animal, similar forms of 

 electro-cardiogram to that of the 

 mammal, indicates that the develop- 

 ment of the Purkinje system does 

 not alter the general course of the 

 wave. Electro-cardiograms of some 

 of the lower vertebrates are given 

 in Fig. 213 (from Lewis's book). It 

 is probable, as Mines points out 

 (1913, 3, p. 208), that the state of 

 excitation lasts so much longer 

 than the time taken for its trans- 

 mission from one part to another, 

 that a very small difference in the 

 duration at one point or another 



is sufficient to determine the sign of the final phase. In fact, he noticed an 

 alteration of sign in the final phase in a tortoise heart without any obvious 

 difference in the beat. 



A detailed analysis of the course of the excitation wave in the dog and in the 

 toad is given in the papers by Thos. Lewis (1915, 1 and 2). 



From what has been stated, it will be clear that the chief practical value of 

 the electro-cardiogram is in the detection of abnormalities in transmission from 

 auricle to ventricle. Especially is it to be noted that conclusions based on 

 changes in form of the ventricular complex rest on an uncertain basis until we 

 know more about the precise meaning of its components. The very smallest 

 difference between the time at which the excitation wave reaches two points 

 decides which of these becomes negative first, although, as regards the mechanism 

 of the contraction, the fact may be of no significance (see especially Figs. 209 

 and 211). There is, it may be repeated, no evidence that any component of the 

 electro-cardiogram is due to. a process different from any other component. 



FIG. 212. DIAGRAM TO snow MANNER OF PRO- 

 DUCTION OF NORMAL ELECTRO-CARDIOGRAM BY 

 LONGER DURATION, WITH LESS HEIGHT, OF 

 EXCITATORY STATE AT THE BASE THAN AT 

 THE APEX. 



Such a condition is brought about by wanning the apex, for 

 example. 



The uppermost curve represents the excitatory state (nega- 

 tivity) at the base. 



The middle one, that at the apex ; represented in the opposite 

 direction, since the ventricle is supposed to be led off by 

 electrodes at base and apex. 



The lowest curve represents the electrical change which 

 would be seen with the capillary electrometer. 



