HEART 



387 



The muscle fibres of the ventricles are arranged essentially 

 in three layers : — 



(1) The outmost layer takes origin from the auriculo- 

 ventricular and pulmonic rings, and passes downwards and 

 to the left till it reaches the apex of the heart. Here it. 

 turns inwards, forming a sort of vortex, and becomes con- 

 tinuous with the inmost layer, 



(2) The middle layer is composed of fibres running hori- 

 zontally round each ventricle. It is the thickest layer of 



'Fig. 165. — The Right Ventricle and Tricuspid Valve to show the relationship 

 of the Papillary Muscles and Chordse Tendinese to the Cusps of the 

 Valve. (See text.) 



the heart, and in contracting it pulls the walls of the 

 ventricles towards the septum ventriculi. 



(8) The inmost layer is continuous with the outmost 

 layer, as it turns in at the apex, and it is mixed with 

 primitive fibres of the auriculo-ventricular band. It may be 

 considered as composed of two parts — (a) A layer of fibres 

 running longitudinally along the inside of each ventricle 

 from the apex upwards to the auriculo-ventricular ring. 

 These fibres are raised into fleshy columns, the columnse carnese. 

 (6) A set of fibres, constituting the papillary muscles (fig. 165, 

 F.M.), which, taking origin generally from the apical part of 



