540 FIBRES OF THE AURICLES. 



The Internal layer of the right ventricle is formed by fasciculi of 

 fibres which arise from the right segment of the root of the aorta, from 

 the entire circumference of the root of the pulmonary artery, and from 

 the bases of the columnae papillares. The fibres from the root of the 

 aorta, associated with some from the carneae columnar, constitute a 

 layer which passes obliquely forwards upon the right side of the 

 septum. The superior fibres coming directly from the aorta join the 

 internal surface of the long band at the anterior border of the septum, 

 while the lower two-thirds of the layer are continuous with the in- 

 ternal surface of the short band, some of its fibres piercing that band 

 to augment the number of superficial fibres. The fibres derived from 

 the root of the pulmonary artery, conjoined with those from the base 

 of one of the columnae papillares, curve forwards from their origin, and 

 wind obliquely downwards and backwards around the internal surface 

 of the wall of the ventricle to the posterior border of the septum, 

 where they become continuous with the long band, directly that it has 

 passed backwards through the septum. 



Fibres of the Auricles. The fibres of the auricles are disposed in 

 two layers, external and internal. The internal layer is formed of 

 fasciculi which arise from the fibrous rings of the auriculo-ventricular 

 openings and proceed upwards to enlace with each other, and con- 

 stitute the appendices auricularum. These fasciculi are parallel in 

 their arrangement, and in the appendices form projections and give 

 rise to the appearance which is denominated musculi pectinati. In 

 their course they give off branches which connect adjoining fasciculi, 

 and form a columnar interlacement between them. 



External Layer. The fibres of the right auricle having completed 

 the appendix, wind from left to right around the right border of this 

 auricle, and along its anterior aspect, beneath the appendix, to the 

 anterior surface of the septum. From the septum they are continued 

 to the anterior surface of the left auricle, where they separate into 

 three bands, superior, anterior, and posterior. The superior land 

 proceeds onwards to the appendix, and encircles the apex of the 

 auricle. The anterior band passes to the left, beneath the appendix, 

 and winds as a broad layer completely around the base of the auricle, 

 and through the septum to the root of the aorta, to which it is partly 

 attached, and from this point is continued onwards to the appendix, 

 where its fibres terminate by interlacing with the musculi pectinati. 

 The posterior band crosses the left auricle obliquely to its posterior 

 part, and winds from left to right around its base, encircling the open- 

 ings of the pulmonary veins ; some of its fibres are lost upon the 

 surface of the auricle, others are continued onwards to the base of the 

 aorta ; and a third set, forming a small band, is prolonged along the 

 anterior edge of the appendix to its apex, where it is continuous with 

 the superior band. The septum auricularum has four sets of fibres 

 entering into its formation ; 1 . The fibres arising from the auriculo- 

 ventricular rings at each side ; 2. Fibres arising from the root of the 

 aorta, which pass upwards to the transverse band, and to the root of 



