FIBRES 01' THE HEART. 



029 



com pletingtlic wall. Fig. 284 affords an interior 

 view of a section of the right auricle, in which, 



the lining membrane being removed, the fibres 

 are seen arising from the tendinous margin of 

 the annulus venosus AY, forming the internal 

 part of the wall of this auricle, and in their 

 progress up arranged into columns, c, the 

 branches of which are entwined together so as 

 to construct the appendix. These convo- 

 luted columns at the posterior aspect of the 

 appendices are flattened, as shown in fig. 285, r, 

 where their fibres are associating together, and 



Fig. 285. 



in passing round the edges to the anterior sur- 

 face become evenly arranged again, as seen in the 

 appendix A of the right auricle, RA, of fig. 286. 

 Thus far the construction of the two auricles 



Fig. 286. 

 CS 



agrees, the fibres of each arising from its respec- 

 tive annulus, forming first the inner part of the 

 wall of the auricle, and then being arranged 

 into columns which entwine together, forming 

 the whole of the appendix. The fibres of the 

 right auricle, after having formed the wall of 

 this cavity, are prolonged to form the outer part 

 of the wall of the left auricle. As may be seen 



in Jig. 286, the fibres which extend from the con- 

 voluted fibres of the posterior surface of the 

 right auricle, RA, wind evenly arranged, some 

 over the apex, and others round die auricle, 

 marked c, completing the outer part of the wall 

 of the entire auricle : they then meet at the 

 septum S, across which they pass associated 

 together, marked D, and on reaching the left 

 auricle divide into an upper portion and an 

 anterior and posterior band. The tipper portion 

 is composed of the continued fibres D, which 

 proceed up the appendix and encircle its apex. 

 The anterior band E winds round the left au- 

 ricle LA, and on leaching the root of the 

 aorta K, its fibres become more or less at- 

 tached to it iu different hearts ; in its course 

 upwards, marked r, when it has completed a 

 circle it passes behind the fibres which form the 

 first part of the circle to enter into the formation 

 of the fleshy columns of the appendix. The 

 posterior band passes over the left auricle be- 

 tween the appendix A and the vena cava su- 

 perior cs ; and in fig. 285 it can be traced, 

 coming over, marked c, and passing along the 

 posterior surface of this auricle LA, including 

 in its course the posterior edge of the appendix 

 A ; the fibres which pass along the posterior 

 edge of the appendix, on arriving at the ante- 

 rior edge, separate from the band o to pursue 

 their course round the edge of the appendix, 

 now along the anterior edge, ana join the 

 fibres D, which cap the apex. This division 

 of the band which encircles tne appendix is con- 

 stant, and evidently affords particular strength 

 to its edge. The band itself o winds down 

 towards the base, expanding and surrounding 

 the orifices of the pulmonary veins p ; some 

 of its fibres become lost on the surface of the 

 auricle, and the others may be traced to the root 

 of the aorta. 



This band cannot be completely detached in 

 consequence of some of its fibres being inter- 

 woven with its subjacent fibres. 



The left auricle, without the addition of these 

 bands, would nearly balance in substance and 

 strength the right; their addition gives, there- 

 fore, to the left a considerable preponderance 

 in these respects over the right auricle. 



The septum S is, in Jig. 286, shown to;be com- 

 posed, superiorly, of the transverse band of 

 fibres D, which passes from the right to the 

 left auricle ; in its middle part, of the ascending 

 fibres H, which arise from the root of the aorta 

 K, and pass up behind the band D, some 

 joining this band, the others proceeding to the 

 vena cava superior cs ; and lastly, at the infe- 

 rior and posterior part, of a slender fasciculus 

 of fibres which crosses the septum transversely 

 between the root of the aorta K and the vena 

 cava inferior ci, extending from the annulus 

 venosus to the left auricle, but which cannot be 

 seen in this figure. 



In concluding these remarks on the construc- 

 tion of the auricles, it may be mentioned that 

 in the heartsof large animals a great difference ex- 

 ists in the structure of the two vena; cava', the 

 superior beingparticularly fleshy,and the inferior 

 apparently devoid of muscularity. 



(H. Searle.) 



