244 



PHYSIOLOGY OF THE HEART 



[CH. XXI. 



electrometer, and the different phases of the action current can in 

 the former case be ascertained by watching the movements of the 

 magnetic needle, and in the latter case by watching under the 

 microscope the movements of the meniscus of mercury in the capil- 

 lary tube. If the eyepiece of the microscope is removed, and the 

 image of the mercurial column allowed to fall on a moving photo- 

 graphic plate, a graphic record is obtained, in which the to-and-fro 

 movements of the mercury are shown as waves. Such a graphic 

 record is termed an electro-cardiogram, and one of these is shown 

 in the next figure (fig. 225). 



FIG. '225. Electrocardiogram from frog's ventricle. Diphasic variation. (Simultaneous photograph 

 of a single beat (upper black line), and the accompanying electrical change indicated by the level 

 of the black area, which shows the varying level of mercury in a capillary electrometer. The time- 

 tracing at the top marks tenths of a second. (Waller.) 



It is, however, possible (as Waller first demonstrated) to obtain 

 an electro- cardiogram in the intact animal, and even in man. If a 

 dog is placed with a fore paw in a basin of salt solution, and a hind 

 paw in another, and the two basins are led off to the electrometer, 

 the electrical changes produced by the beating heart will be con- 

 ducted through the body of the animal and through the electrometer, 

 and the movements of the mercury can be watched with a microscope 

 or recorded on a travelling photographic plate. By the use of this 

 method Miss Buchanan has succeeded in performing what otherwise 

 would have been the impossible task of counting the heart rate of 

 small mammals such as mice. The photographic plate must travel 

 at great speed, and the notches in the shadow of the mercurial 

 column, which correspond to the heart-beats, were found in the 

 mouse to occur at the rate of 700 per minute. In a corresponding 

 way the human electro-cardiogram can be registered, as shown in 

 fig. 226. In that particular experiment, the "lead-offs" were from 

 mouth and left foot. It is more usual to employ one hand and 

 one foot. 



We must, however, recognise that the heart muscle is not a 

 simple longitudinal strip like a sartorius, but is arranged in a com- 

 plex way, and in the mammalian heart is arranged around four 



