134 



potential attending the contraction cf the heart cause the fibre to 

 oscillate; these oscillations are recorded on a moving photographic- 

 plate. The photographic records (electro-cardiograms) obtained with 

 these instruments afford a most beautiful method of recording the 

 rhythm of normal and abnormal hearts in man; they can be obtained 

 by connecting right and left hands, the right hand and left foot, or 

 the left hand and left foot, of a patient with the instrument by means 

 of baths of salt solution into which the wires dip. The second varia- 

 tion or "derivation " (right hand and left foot) is most commonly em- 

 ployed. The heart is placed obliquely across the body, and the 

 wave of contraction and accompanying electrical wave begins in the 

 base, passes to the apex, and thence to the base again. The right hand 

 or mouth is favovrably placed as a lead for indicating the electrical 

 condition of the base, and the left hand or either foot for that of the 

 apex. By recording the electrical variation, and using in turn different 

 leads, favourable and unfavourable e.g., mouth and left hand, and 

 mouth and right hand the axis of the electrical current, and so of 

 the heart in the body, can be determined. By making use of the 

 telephone-wires, there have been recorded the electrical changes of 

 the hearts of patients comfortably seated or in bed in a hospital a 

 mile away. 



The normal electro-cardiogram is seen in Fig. 41 . P is the deflection 

 clue to auricular systole; Q R S T are deflections of ventricular origin, 

 R representing ventricular systole. Fig. 42 represents the condition 

 of heartblock; the ventricles are seen to be beating at a slower rate 

 than the auricles, and quite independent of them. Fig. 43 is an electro- 

 cardiogram showing in the big upward deflection the occurrence at 

 regular intervals of a ventricular extra systole. 



Tissue of Origin and Mode of Conduction of the Excitatory Wave. 

 A long controversy has raged around the question as to the actual 

 tissue in which the excitatory wave of the heart arises and by which 

 it is conducted. It is to be borne in mind that the two questions are 

 really distinct. They are frequently confused. Experiments which 

 bear on the site of origin i.e., the tissue in which the excitatory 

 wave arises have been quoted as evidence of the mode of conduction 

 of this wave, and vice versa. It must be granted that, if the excitatory 

 wave be found to arise in one form of tissue, it is highly probable that 

 it will also be conducted by that tissue, but it is not necessarily the 

 case. It is quite conceivable that the excitatory wave may arise 

 in nerve and be conducted by muscle, or arise in muscle and be con- 

 ducted by nerve, or, arising how it may, the excitatory wave may be 

 conducted both by muscle and by nerve in order that the proper 

 sequence of contraction may be assured. The structure of the nodal 

 tissues suggests that nerve structure there fuses into that of muscle. 

 It is only recently that the claims of the nodal tissue have been ad- 

 vanced as the site of origin of the excitatory wave. For several 

 years a controversy has waged between those who uphold the neuro- 

 genic and myogenic theories. According to the neurogenic theory, 

 nerve is thought to be the supreme tissue, and it is supposed that the 



