478 FIBRES OP THE AURICLES. 



The most inferior of the fibres of the left surface of the septum 

 ventriculorum, after winding spirally around the internal surface of 

 the apex of the left ventricle, so as to close its extremity, form a 

 small fasciculus, which is excluded from the interior of the ventricle, 

 and expands in a radiated manner over the surface of the heart, 

 constituting its superficial layer of fibres. The direction of these 

 fibres is, for the most part, oblique, passing from left to right on the 

 anterior, and from right to left on the posterior surface of the heart, 

 becoming more longitudinal near its base, and terminating by being 

 inserted into the fibrous rings of the auriculo-ventricular openings, 

 and of the pulmonary artery and aorta. Over the right ventricle 

 the superficial fibres are increased in number by the addition of 

 accessory fibres from the right surface of the septum, which pierce 

 the middle layer, and take the same direction with the superficial 

 fibres from the apex of the left ventricle, and of other accessory 

 fibres from the surface of both ventricles. 



From this description it will be perceived, that the superficial 

 layer of fibres is very scanty, and is pretty equally distributed over 

 the surface of both ventricles. The middle layer of both ventricles 

 is formed by the two bands, short and long. But the internal layer 

 of the two ventricles is very differently constituted : that of the left 

 is formed by the spiral expansion of the fibres of the rope, and of 

 the two columnar papillares; that of the right remains to be described. 

 The septum ventriculorum also consists of three layers, a left layer, 

 the radiated expansion of the rope and carneae columnar ; a middle 

 layer, the long band ; and a right layer, belonging to the proper wall 

 of the right ventricle, and continuous both in front and behind with 

 the long band, and in front also with the short band, and with tjie 

 superficial layer of the right ventricle. 



The Internal layer of the rig/it 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 columnse papillares. The fibres from 

 the root of the aorta, associated with some from the carneae columnse, 

 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 bor- 

 der of the septum, while the lower two-thirds of the layer are con- 

 tinuous with the internal 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 columnar papillares, 

 curve forwards from their origin, and wind obliquely downwards 

 and backwards around the internal surface of the wall of the ven- 

 tricle 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 



