764 THE THORAX. 



The fibrous rings surround the auriculo-ventricular and arterial orifices; they 

 are stronger upon the left than, on the right side of the heart. The auriculo- 

 ventricular rings serve for the attachment of the muscular fibres of the auricles 

 and ventricles, and also for the mitral and tricuspid valves; the ring on the 

 left side is closely connected, by its right margin, with the aortic arterial ring. 

 Between these and the right auriculo-ventricular ring, is a fibre-cartilaginous 

 mass ; and in some of the larger animals, as the ox and elephant, a portion of 

 bone. 



The fibrous rings surrounding the arterial orifices, serve for the attachment 

 of the great vessels and semilunar valves. Each ring receives, by its ventricu- 

 lar margin, the attachment of the muscular fibres of the ventricles; its opposite 

 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 striaB. 



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 more 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 <//> 

 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 ex- 

 tremities to the corresponding auriculo-ventricular rings, in front and behind. 

 The annular fibres surround the whole extent of the appendices auricula, and 

 are continued upon the walls of the vena3 cavaB and coronary sinus on the right 

 side, and upon the pulmonary 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 exclu- 

 sively 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 

 chordre tendinea3; 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 arc; 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, ami pass from left to right. 



The superficial fibre's coil inwards :it the apex of the. In-art, round which they 

 are arranged in a whorl-like form, called the vorlc.r, dipping beneath the edge 

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

 heart previously boiled, the cavity of the left, and then that of the right ven- 

 tricle, will be exposed at this point. The layers of fibres successively met with 



