476 



DISSECTION OF THE THORAX. 



membra- 

 nous part of 

 septum. 



Fibres can 

 be separated 

 into layers 

 by dissec- 

 tion. 



Direction of 

 fibres : 



external, 

 middle, 



and internal. 



Course of 

 fibres is 

 obscure. 



Chief sets : 



external 

 oblique 



upper end, in a small area close below the aortic orifice, where there is a 

 very thin part from which muscular fibres are absent (pars membranacea 

 septi. 



Arrangement of fibres. It has been shown by the foregoing dissection 

 that the direction of the muscular fasciculi composing the ventricular wall 

 varies at different depths from the surface, and that at a given spot a number 

 of layers may be separated, which are characterised by the difference in 

 direction of their fibres. Such a division into distinct layers is, however, in 

 great measure artificial, for the change in direction is gradual, and many 

 fibres pass across from one layer to another, and have to be cut to effect the 

 separation. 



Over both ventricles the most superficial fibres are directed very obliquely 

 from base to apex, and from right to left on the anterior surface, from 



left to right on the posterior sur- 

 face. Proceeding inwards, the 

 obliquity gradually diminishes ; 

 and in the centre of the wall the 

 fibres are transverse. Within the 

 last, as the cavity of the ventricle 

 is approached, the fibres become 

 oblique again, but in the opposite 

 direction to the external ones ; 

 and the innermost fibres of all 

 are nearly longitudinal. 



The attempt to trace the whole 

 course of the bundles is, except 

 in the case of the superficial 

 fibres, attended with great diffi- 

 culty, owing to the interlacement, 

 branching, and joining of the 

 fasciculi. The principal groups 

 of fibres that have been dis- 

 tinguished may be arranged as 

 follows ; but it must be under- 

 stood that they are to a great 

 extent intermixed, and that 

 bundles frequently pass from one 

 set into another. 



a. The external oblique fibres 

 (fig. 173) begin at the base of the 

 ventricles, where most of them 

 spring from the auriculo-ventri- 

 cular and arterial fibrous rings, 

 and descend with the spiral 

 course above described to the 

 apex of the heart. On the pos- 

 terior surface they pass without 



FIG. 173. A DIAGRAM OF THE ARRANGE- 

 MENT OF THE FIBRES IN LAYERS IN THE 

 LEKT VENTRICLE. THE DISSECTION is 



CARRIED THROUGH ABOUT TWO-THIRDS 

 OF THE THICKNESS OF THE WALL. 



1, 2, 3. Outer layers, the fibres of which 

 gradually become less oblique. 



4. Middle layer of transverse fibres. 



5. Inner set of oblique fibres. The 

 deepest fibres, corresponding to 1 and 2 of 

 the exterior, are not shown. 



and internal 

 longitudinal 

 are one ; 



annular; 



looped of 

 left, 



interruption from the left to the 

 right ventricle ; but in front the 



fibres crossing the right ventricle in part dip in at the interventricular groove 

 to the septum, while those continued to the left ventricle are joined by others 

 which issue from the septum along the furrow. At the apex of the heart 

 they form a sharp twist, known as the vortex or whorl, and sink in it to 

 become deep and ascend towards the base as the innermost layer of the left 

 ventricle. Some of them are continued to the base and join the auriculo- 

 ventricular and aortic rings ; but others enter the papillary muscles, which 

 are thus formed. 



b. The transverse or annular fibres (fig. 173) are partly special to the 

 left ventricle, and partly common to the two ventricles. Some of them 

 appear to form simple rings round the cavities, but a great many pass from 

 this into the oblique system of fibres. 



c. The looped fibres of the left ventricle spring from the fibrous rings 



