FIBRES OP THE VENTRICLES. 477 



For the sake of clearness of description the fibres of the ventri- 

 cles have. been divided into three layers, superficial, middle, and 

 internal all of which are disposed in a spiral direction around the 

 cavities of the ventricles. The mode of formation of these three 

 layers will be best understood by adopting the plan pursued by Mr. 

 Searle in tracing the course of the fibres from the centre of the 

 heart towards its periphery. 



The left surface of the septum ventriculorum is formed by a 

 broad and thick layer of fibres, which proceed backwards in a spiral 

 direction around the posterior aspect of the left ventricle, and 

 become augmented on the left side of that ventricle, by other fibres 

 derived from the bases of the two columnar papillares. The broad 

 and thick band formed by the fibres from these two sources, curves 

 around the apex and lower third of the left ventricle, to the anterior 

 border of the septum, where it divides into two bands, a short or 

 apicial band, and a long or basial band. 



The Short or apicial band is increased in thickness at this point 

 by receiving a layer of fibres (derived from the root of the , aorta 

 and carneae columnar) upon its internal surface, from the right sur- 

 face of the septum ventriculorum ; it is then continued onwards in 

 a spiral direction from left to right, around the lower third of the 

 anterior surface, and the middle third of the posterior surface of 

 the right ventricle to the posterior border of the septum. From the 

 latter point the short band is prolonged around the posterior and 

 outer border of the left ventricle to the anterior surface of the base 

 of that ventricle, and is inserted into the anterior border of the left 

 auriculo-ventricular ring, and the anterior part of the root of the 

 aorta and pulmonary artery. 



The Long or basial band, at the anterior border of the septum, 

 passes directly backwards through the septum, forming its middle 

 layer, to the posterior ventricular groove, where it becomes joined 

 by fibres derived from the root of the pulmonary artery. It then 

 winds spirally around the middle and upper third of the left ven- 

 tricle to the anterior border of the septum, where it is connected by 

 means of its internal surface with the superior fibres derived from 

 the aorta, which form part of the right wall of the septum. From 

 this point it is continued around the upper third of the anterior and 

 posterior surface of the right ventricle to the posterior border of 

 the septum, where it is connected with the fibres constituting the 

 right surface of the septum ventriculorum. At the latter point the 

 fibres of this band begin to be twisted upon themselves, like the 

 - strands of a rope, the direction of the twist being from below up- 

 wards. This arrangement of fibres is called, by Mr. Searle, " the 

 rope ;" it is continued spirally upwards, forming the brim of the 

 left ventricle, to the anterior surface of the base of that ventricle, 

 where the twisting of the fibres ceases. The long band then curves 

 inwards towards the septum, and spreads out upon the left surface 

 of the septum into the broad and thick layer of fibres with which 

 this description commenced. 





