VENTRICULAR FIBRES OF HEART. 



323 



arranged into three external (1, 2, 3), three internal (7, 6, o), and a 

 central one (4). All are not prolonged equal distances on the ventricle, 

 for the outermost and the innermost reach farthest towards apex and base ; 

 and the second external and its corresponding inner layer (sixth) extend 

 farther than the third and the fifth. 

 The fourth is the shortest of all. 

 Consequently the wall is thickest 

 about the middle third where all the 

 layers are present, and gradually be- 

 comes thinner upwards and down- 

 wards, until there is only the outmost 

 layer at the apex, and the most ex- 

 ternal and internal (1st and 7th) at 

 the base. (Dr. Pettigrew, Phil. 

 Trans. 1864.) 



Direction of the fibres. Each 

 stratum is formed of fleshy fibres with 

 the undermentioned direction, sup- 

 posing the ventricle standing on the 

 apex, and the anterior surface towards 

 the dissector. 



In the three external strata (1, 2, 

 3) the fibres are inclined downwards 

 from the base and septum to the apex 

 of the ventricle, and become less 

 vertical in each. 



The fourth or mid layer (4) pos- 

 sesses transverse fibres ; and it is 

 nearer the outer than the inner surface 

 of the wall. 



In the three inner strata the fibres change their direction, as is shown 

 by 5, and are directed upwards from the apex and septum to the base of 

 the ventricle ; so that they cross the fibres of the outer layers like the legs 

 of the letter X, and, becoming more oblique in each layer, are almost 

 vertical in the internal. 



Continuity of the fibres. With a piece of muscle rolled into a cone, as 

 before said, the fibres of the different layers would be necessarily continu- 

 ous at the apex ; but in the heart they are united at apex and base. Thus 

 the outermost layer is continuous at the apex and base with the innermost, 

 the fibres being curved in at the tip and out at the base. In like manner 

 the fibres of the second layer are united with those of the sixth, and the 

 third stratum with the fifth. From the three outer layers, fibres are con- 

 tinued to the right ventricle at the back of the heart, forming the " com- 

 mon fibres." 



Each of the three outer layers consists of two sets of fibres, which occupy 

 the front and back of the ventricle. By the turning inwards of the two 

 bundles on opposite sides of the apex, the wall is prevented from having 

 a slanting side, like a piece of paper rolled into a cone. And by the turn- 

 ing outwards of two sets of fibres (anterior and posterior) at the base of 

 the ventricle from each inner layer, the sides of the auriculo-ventricular 

 opening are made level like those of the apex. Many of the fibres of the 

 outer layers are attached to the fibrous ring around the aorta. 



A DlAQRAM OF THE ARRANGEMENT OF THE 

 FIBRES IN LAYERS IN THB LEFT VRNTRI- 

 CLE. 



1. First or external layer. 



2. Second external. 



3. Third external. 



4. Central layer. 



5. The outermost of the three inner strata. 



