2l6 ESSENTIALS OF PHYSIOLOGY. 



ventricle so as to lessen or abolish its conductivity. The mammalian 

 ventricle differs, however, from that of the frog in possessing a more 

 pronounced rhythmic power, and when functionally isolated from the 

 auricles, it immediately begins to beat with its own rhythm. 



It is evident that the bundle of His is essential for the propagation 

 of the wave of contraction from the auricles to the ventricles. 



The Electrical Changes in the Heart. The time relations of, and 

 the course taken by, the wave of contraction, as it travels from the 

 sino-auricular node over the heart, can be easily demonstrated, not 

 only in the lower animals, but even in man, by studying the electri- 

 cal changes which take place at the same time. These changes 

 may be recorded by connecting two parts of the heart, for instance 

 the auricles and ventricles, with some form of galvanometer. For 

 this purpose, the string galvanometer (p. 24) is now most gener- 

 ally used. 



The resting heart is isoelectric, that is to say, the auricles and the 

 ventricles are at the same potential, and the thread of the galvanometer 

 is at rest. When the auricles contract, a difference of potential is set 

 up between them and the ventricles, and a current passes in the heart 

 from auricle to ventricle, and through the galvanometer from ventricle 

 to auricle. During systole of the ventricles and diastole of the auricles, 

 the current passes in the opposite direction. Cardiac muscle thus 

 resembles skeletal muscle in that its contraction is preceded by an 

 electrical change, the contracting part being galvanometrically negative 

 (but electro-positive) to the resting part. The difference in character 

 of the galvanometric tracings of the electrical changes in the heart 

 and in skeletal muscle is due, partly to the prolonged contraction of 

 cardiac muscle fibres, partly to the complex arrangement of these 

 fibres in the heart wall. 



In order to obtain a record of the electrical changes in the human 

 heart, one end of the thread of the galvanometer is connected with a 

 vessel containing salt and water into which the right arm of the subject 

 is placed. The left leg is placed in a 'similar vessel connected with the 

 other end of the thread. The right arm is regarded as conducting the 

 electrical changes at the base of the heart, while the changes occurring 

 at the apex are conveyed down the left leg to the apparatus. The 

 apparatus is so arranged that in these circumstances an upward move- 

 ment of the shadow of the thread on the photographic record means 

 that the base of the heart is negative to the apex, and is therefore 

 contracting, while the apex is at rest. 



Fig. 80 represents an electrocardiogram obtained with this instru- 

 ment, and shows three upstrokes for each cardiac cycle. The first 



