698 THE THORAX. 



margin presents three deep semicircular notches, within which the middle coat 

 of the artery (which presents three convex semicircular segments) is firmly fixed ; 

 the attachment of the artery to its fibrous ring being strengthened by the thin 

 cellular coat and serous membrane externally, and by the endocardium within. It 

 is opposite the margins of these semicircular notches, in the arterial rings, that 

 the endocardium, by its reduplication, forms the semilunar valves, the fibrous 

 structure of the ring being continued into each of the segments of the valve at 

 this part. The middle coat of the artery in this situation is thin, and the sides of 

 the vessel dilated to form the sinuses of Valsalva. 



The muscular structure of the heart consists of bands of fibres, which present 

 an exceedingly intricate interlacement. They are of a deep red color, and marked 

 with transverse striae. 



The muscular fibres of the heart admit of a subdivision into two kinds, those of 

 the auricles, and those of the ventricles, which are quite independent of one another. 



Fibres of the auricles. These are disposed in two layers, a superficial layer 

 common to both cavities, and a deep layer proper to each. The superficial fibres 

 are most distinct on the anterior surface of the auricles, across the bases of which 

 they run in a transverse direction, forming a thin but incomplete layer. Some 

 of these fibres pass into the septum auricularum. The internal or deep fibres proper 

 to each auricle consist of two sets, looped, and annular fibres. The looped fibres 

 pass upwards over each auricle, being attached by both extremities to the corre- 

 sponding auriculo-ventricular rings, in front and behind. The annular fibres sur- 

 round the whole extent of the appendices auricula, and are continued upon the 

 walls of the venas cavas and coronary sinus on the right side, and upon the pul- 

 monary veins on the left side, at their connection with the heart. In the appendices, 

 they interlace with the longitudinal fibres. 



Fibres of the ventricles. These, as in the auricles, are disposed in layers, some 

 of which' are common to both ventricular cavities, whilst others belong exclusively 

 to one ventricle, the latter being chiefly found towards the base of the heart. 

 The greater majority of these fibres are connected by both ends with the auriculo- 

 ventricular fibrous rings, either directly or indirectly through the chorda? ten- 

 dineoe ; some, however, are attached to the fibrous rings surrounding the arterial 

 orifices. 



The superficial fibres are either longitudinal, or, more commonly, oblique or spiral 

 in their direction, and towards the apex are arranged in the form of twisted loops ; 

 the deeper fibres are circular. 



The spiral fibres .are disposed in layers of various degrees of thickness ; the 

 most superficial, on the front of the ventricles, run obliquely from right to left, 

 and from above downwards. On the back of the ventricles they are directed 

 more vertically, and pass from left to right. 



The superficial fibres coil inwards at the apex of the heart, round which they 

 are arranged in a whorl-like lorm, called the vortex, dipping beneath the edge of 

 the deeper and shorter layers. If these fibres are carefully uncoiled, in a heart 

 previously boiled, the cavity ( f the left, and then that of the right ventricle, will 

 be exposed at this point. The layers of fibres successively met with have a simi- 

 lar arrangement ; the more superficial and longer turning inwards, and including 

 the deeper and shorter bands. All these fibres ascend and spread out upon the 

 inner surface of the ventricles, forming the walls, the septum, and the musculi 

 papillares, which project from these cavities ; and they are finally inserted into 

 the auriculo-ventricular fibrous rings, or, indirectly, through the chordaa tendineae. 

 Of these spiral fibres, some enter at the interventricular furrows, and surround 

 either ventricle singly ; others pass across the furrows, and embrace both cavities. 

 On tracing those which form the vortex, back into the interventricular septum, 

 they become interlaced with similar fibres from the right ventricle, and ascend 

 vertically upon the right side of the septum, as far as its base, in the form of a 

 long and broad band. 



Circular fibres. The circular fibres are situated deeply in the substance of 



