CHAPTEE IV 

 THE APPLIED PHYSIOLOGY OF THE HEART 



The Beat of the Heart. 



THE tendency of all recent physiological investigation 

 has been to magnify the importance and independence 

 of the heart muscle. It is now believed that the beat of 

 the heart takes place independently of nerve influence, 

 and as a consequence of the automatic contractility 

 which the cardiac muscle possesses in virtue of its com- 

 paratively embryonic and undifferentiated form.* The 

 beat or contraction starts, as might be expected, in that 

 part of the heart wall which possesses these character- 

 istics in the greatest degree i.e., in the muscle at the 

 mouths of the great veins. Thence the contraction is 

 conducted from muscle cell to muscle cell, sweeping over 

 the auricles and producing their systole, and is trans- 

 mitted to the ventricles by means of the auriculo-ven- 

 tricular bundle of fibres which crosses the auriculo- 

 ventricular groove. t 



* This is sometimes spoken of as the 'myogenic' theory of the 

 heart's action, as opposed to the ' neurogenic ' theory, which supposes 

 the necessity of a local nerve apparatus to maintain the contractions 

 of the heart. 



f A full anatomical description of the auriculo-ventricular bundle 

 in man will be found in a paper by Keith and Flack (Lancet, 



105 



