1094 THE THORAX. 



At its upper part it suddenly tapers off and loses its muscular fibres, consisting 

 only of fibrous tissue covered by two layers of endocardium, and on the right side 

 it is also, during diastole, in contact with the septal flap of the tricuspid valve. It 

 continues upward, and forms the septum between the aortic vestibule and the right 

 auricle. It is derived from the lower part of the aortic septum of the foetus, and 

 an abnormal communication may exist at this part owing to defective development. 

 The aortic vestibule (Sibson) is a small portion of the ventricular cavity immedi- 

 ately under the root of the aorta. 



The Endocardium is a thin membrane which lines the internal surface of the 

 heart ; it assists in forming the valves by its reduplications, and is continuous with 

 the lining membrane of the great blood-vessels. It is a smooth, transparent 

 membrane, giving to the inner surface of the heart its glistening appearance. It 

 is more opaque on the left than on the right side of the heart, thicker in the 

 auricles than in the ventricles, and thickest in the left auricle. It is thin on the 

 musculi pectinati and on the columnae carneae, but thicker on the smooth part of 

 the auricular and ventricular walls and on the tips of the musculi papillares. 



Structure. The heart consists of muscular fibres and of fibrous rings which 

 serve for their attachment. 



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 mass of fibrous tissue, and in some of the 

 larger animals, as the ox and elephant, a nodule 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 ventricular 

 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 

 are dilated to form the sinuses of Valsalva. 



The muscular structure of the heart (myocardiuni) consists of bands of fibres 

 which present an exceedingly intricate interlacement. They are of a deep red 

 color and marked with transverse striae (page 65). 



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 deep fibres proper 

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

 pass upward over each auricle, being attached by two extremities to the corre- 

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

 surround the whole extent of the appendices auricularum, and are continued upon 

 the walls of the venae cavae 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 are arranged in an exceedingly complex man- 

 ner, and the accounts given by various anatomists differ considerably. This is prob- 

 ably due partly to the fact that the various layers of muscular fibres of which the 



