THE HEART. 623 



cular fibres are traced to the fibrous zones or tendinous 

 circles of Lower, which are situated at the auriculo-ventricu- 

 lar, and arterial orifices, and constitute the frame-work of 

 the heart. 



These fibres are divided into the superficial or common, 

 and deep or reflected. The superficial are traced from the 

 base of the heart, taking a spiral course to the apex those 

 on the anterior portion going from right to left, and those 

 on the posterior from left to right, meeting and decussating 

 at the apex. From this latter point, where they turn 

 round upon themselves, they are reflected upward, and 

 constitute the deep layers. Those belonging to the ante- 

 rior superficial set form, by their reflection, the deep layer 

 of the posterior wall, while those of the posterior superfi- 

 cial set constitute the deep layer of the anterior wall. Be- 

 tween these two sets, an intermediate one, called the proper 

 fibres of each ventricle, is seen. They are compared to a 

 small barrel or truncated cone, their superior openings 

 corresponding with the orifices between the auricles and 

 ventricles, while their inferior are observed to leave two 

 considerable spaces, which are simply occupied by the com- 

 mon fibres. This accounts for the apex of the heart being- 

 weaker than any other portion of the ventricles. 



The muscular fibres of the auricles are also divided into 

 a superficial and deep set. The former, the fibres common 

 to both auricles, occupy their anterior surface, and run 

 transversely from right to left. The deep set, or proper 

 fibres form a uniform circular layer. Some of them are 

 also oblique, and constitute a muscular sphincter round the 

 several orifices of the auricles. The muscular layer of the 

 right auricle is not so uniform and continuous as that of 

 the left. The septum of the auricles also contains muscu- 

 lar fibres, which form a ring round the fossa ovalis. At 

 the septum of the ventricles, the right and left hearts are 

 capable of separation, if carefully done. 



The arteries of the heart come from the aorta, and are 

 the first branches given off at its origin. They are the 

 right and left coronary. 



