SECTION IIA. 

 CHAPTER V. 



CARDIAC MUSCLE. 



Cardiac muscle resembles voluntary muscle in its cross stria- 

 tion. The individual cells, however, are joined end to end and 

 many of them interconnected by fork-like projections. Anasto- 

 mosis is so complete that there is continuity throughout the myo- 

 cardium of both auricles and ventricles. Even in mammals there 

 is evidence of a complete syncytium in the form of muscle-fibrils 

 passing from cell to cell. The connection between the auricles and 

 ventricles consists of a narrow bundle of somewhat modified 

 muscular tissue, the bundle of Kent. 



Cardiac muscle contracts much more rapidly than smooth 

 muscle, but more slowly than voluntary muscle. 



The heart is involuntary in action, and undergoes rhythmic 

 contraction throughout life. This rhythmicity is so well estab- 

 lished that it is impossible to tetanize a herat by artificial stimula- 

 tion. Herein lies the most essential difference between cardiac 

 and skeletal muscles. If a stimulus is sent into the heart during 

 systole there is no result, but when stimulated during diastole 

 an extra contraction follows. The whole period of systole is 

 refractory to stimulation. In case an extra contraction is induced 

 during diastole, the relaxation which follows is prolonged to com- 

 pensate for the pause which was missed. 



All-or-none Principle. The all-or-none principle applies 

 to cardiac muscle as well as to voluntary muscle, in fact it was 

 first observed in the former. Due perhaps to the syncytium-like 

 structure of the heart muscle, all of its fibres have the same threshold 

 of stimulation, so if one contracts all contract. Moreover as in 

 voluntary muscle, those fibres which contract, do so to their full 

 extent if at all. Therefore the whole heart contracts to the maxi- 

 mum or not at all. 



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