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external layers posteriorly are found to be for the most part common 

 to both ; in other words, the fibres on the back part of the left 

 ventricle cross over the posterior coronary tract, and pass on to the 

 right ventricle ; whereas, in front, with the exception of a large cross 

 band at the base, the fibres of the right and of the left ventricle 

 respectively dip inward at the anterior coronary tract, as if altogether 

 independent of each other : an arrangement which induced Winslow 

 to regard the heart as consisting of two muscles enveloped in a third. 

 When, moreover, the so-called common fibres, posteriorly, are dis- 

 sected layer by layer simultaneously with the independent anterior 

 fibres, it is found that both pass through the same changes of direc- 

 tion ; and the same rule holds good with the fibres of the septum. 



Another possible mode of explaining the septum, as the Lecturer 

 showed, is to regard the layers entering into the formation of the 

 left ventricle as splitting up posteriorly, the one half of each layer 

 winding round to form the right ventricle, and then dipping in front 

 to form the right half of the septum, whilst the other half proceeds 

 immediately forwards to form the left half of the septum. 



Both ventricles thus appear to be formed on the same general plan, 

 but they differ materially in the structure of their apices ; and the 

 question arises which is the primary or typical ventricle ? Now, 

 while the fibres of the left ventricle enter its apex in a spiral manner 

 by a species of involution similar to that which would be produced 

 by rolling a sheet of muscle into a cone, those of the right ventricle 

 simply bend or double on themselves. Moreover, as the Lecturer sug- 

 gested, were we to split the septum into two, assigning to each ven- 

 tricle its proper share, and then apply the cut ends of the common 

 fibres (which cross from the left to the right ventricle posteriorly) to 

 their corresponding fibres in the left half of the septum, we should find 

 that we had still a perfect whole in other words, a complete system 

 of external and internal spirals ; whereas the fibres of the right 

 ventricle and its half of the septum, treated in the same way, would 

 represent only a part of a more complete system a portion nipped 

 off, as it were, from the side of the perfect cone. Accordingly, if 

 we would dissect the left ventricle, and especially its apex, sym- 

 metrically, we must detach the right ventricle as if it were of no 

 account, and dissect layer after layer of the septum pari passu 

 with the layers of the left ventricular wall generally ; on the other 



