368 



THE HEART. 



known as the os cordis. This central fibro-cartilage is placed in the angle between 

 the aortic and the two auriculo-ventricular openings (see fig. 313), and from it 

 processes pass in various directions. One of these, extending downwards to meet the 

 fleshy septum of the ventricles, separates the left ventricle from the right auricle, 

 forming the right boundary of the aortic vestibule (p. 365). These processes form 

 the bases of what have been described by authors as the fibrous or tendinous rings 

 of the auriculo-ventricular and arterial openings. The fibrous tissue of the rings is 

 continuous with that which is found in the segments of the valves, strengthened on 

 the sides next the septum by the processes from the fibro-cartilage. The rings of 

 the arterial orifices give attachment below to some of the muscular fasciculi of the 

 ventricle, whilst above they project between the flaps of the valves, with the curved 

 border of which they are continuous, as well as with the middle coat of the artery. 

 The fibres of the middle coat of the artery, here comparatively thin, are not arranged 

 annularly, as in other parts of the vessel, but converge to the intervals between the 

 sinuses of Valsalva, to be attached to the projections of the fibrous rings. 



The tendinous rings of the aortic and left auricular orifices are confluent, so 

 that when the fibrous tissue is destroyed by boiling the two apertures run into one. 



MYOCABDIUM. 



Arrangement of the muscular fasciculi. The arrangement of the fasciculi 

 (or "fibres," as they are ordinarily termed) in the auricles and ventricles must be 

 considered separately, for the muscular bundles of the two are not continuous, being 



Fig. 320. POSTERIOR VIEW OP THE SAME 



PREPARATION AS IS REPRESENTED IN THE 



PRECEDING FIGURE. (Allen Thomson. ) | 



a, right ventricle ; J, left ventricle ; e, 

 posterior interventricular groove ; d, right 

 auricle ; e, the left ; /, superior vena cava ; 

 9> </ pulmonary veins cut short ; h, sinus 

 of the great coronary vein covered by mus- 

 cular fibres ; h', middle cardiac vein joining 

 the coronary sinus ; i, inferior vena cava ; 

 i', Eustachian valve. 



only connected by the fibrous tissue 

 around the auriculo-ventricular ori- 

 fices : in conformity with this it is 

 seen that, after boiling the heart, 

 the auricles may be easily separated 

 from the ventricles. 



Fibres of the auricles. These 

 consist of a superficial set, common 

 to both cavities, and of deeper fibres 

 proper to each. The superficial fibres 

 run transversely over both venous 



sinuses, and are most numerous on the anterior surface ; some pass in at the inter- 

 auricular septum (fig. 319). Of the deeper fibres, which are proper to each auricle, 

 some pass over the auricle, and seem to be attached by both extremities to the 

 corresponding auriculo-ventricular rings (looped fibres). Others, which are termed 

 annular fibres, encircle the auricular appendages (fig. 319 d", 320 e'), some longitu- 

 dinal fibres running within them. Annular fibres also surround the terminations of 

 the vense cavse (/, *') on the right, and of the coronary vein and the pulmonary 



