182 STUDIES IN ELECTRO-PHYSIOLOGY: 



CHAPTER XII 

 CARDIAC MUSCLE 



THE problem of the structure and precise function- 

 ing of cardiac muscle is not easy of solution, owing, in 

 the main, to the absence of diagrams illustrating its 

 connections. Several facts, however, stand out promin- 

 ently. 



It is said (1) to be intermediate in structure and 

 properties between voluntary and involuntary muscle ; 



(2) to contract more slowly than ordinary striped muscle ; 



(3) to be striated ; and (4) to have no true sarcolemma, 

 " although there is a thin superficial layer of non-fibrillated 

 substance." (Schafer.) 



Considering, as we must do, each segment as a sarco- 

 mere, it will be seen that the segments differ in length and 

 in diameter (permitting of infinite variation of tension) ; 

 that some are non-nucleated, and that there are branch, or 

 shunt, circuits which no doubt play their part in the 

 inductive regulation of tension in an automatic neuro- 

 electrical system, because, although the heart's action 

 may be subject to psychological influences, it must 

 be supplied, from within or without, with energy un- 

 intermittently, and therefore must form part of an 

 automatic system. 



And here, perhaps, we may begin to appreciate the 

 beautiful regulation exercised by the vagus nerves. The 

 energy, partly self-contained or not, which, in life, is 



