THE HEART 



BY MARTIN FLACK 



FROM time immemorial the heart has been the object of great, 

 although perhaps not altogether scientific interest. In recent 

 years many points relating to its anatomy and physiology have 

 occupied the attention of investigators, so that the literature upon 

 these subjects has become very large and in many respects in- 

 tricate. In the following remarks the subject will be treated 

 under these headings : 



A. The microscopic anatomy of the heart. 



B. The morphology of the vertebrate heart. 



C. The nervous elements of the vertebrate heart. 



D. The heart as a muscle. 



E. The site of origin and mode of conduction of the excitatory 

 wave of the heart. 



F. The movements of the heart in situ. 



A. THE MICROSCOPIC ANATOMY OF THE HEART 



The heart musculature must not be regarded as being built up 

 of a number of separate cells fixed together by a cement sub- 

 stance. It is really a network of cells, intimately fused on all sides, 

 or as it is sometimes termed a syncytium (Kolliker, M. Heidenhain). 



In fresh and in well-prepared fixed preparations there is no 

 trace of division of the musculature into short mononuclear seg- 

 ments or cells. The muscle fibre is seen to pass many nuclei 

 without the appearance of a division or cement line between them 

 indeed in the heart of mammals such small segmented portions 

 are hard to find, and seem only to occur in the circular bands of 

 muscle at the heart orifices and in the musculi papillares. 



According to M. Heidenhain, the muscle fibres of the heart are 

 about one-third thinner in diameter than those of voluntary 

 muscle. They possess nuclei, sarcoplasm, fibrillary substance, and 



