566 THE VASCULAR SYSTEMS 



ring on the left side is closely connected by its right margin with the aortic arterial ring. Be- 

 tween these and the right auriculoventricular ring is a mass of fibrous tissue (trigonum fibrosum) , 

 and in some of the larger animals, as the ox and elephant, a nodule of bone, the 05 cordis. 



The fibrous rings surrounding the arterial orifices serve for the attachment of the great vessels 

 and semilunar valves. Each ring receives, at its ventricular margin, the attachment of the 

 muscle 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, strengthened by white fibrous tissue, forms the semilunar valves, the fibrous 

 structure of the ring being continued into each of the segments of the valve. The middle coat 

 of the artery in this situation is'thin, and the sides of the vessels are dilated to form the sinuses 

 of Valsalva. 



The myocardium of the heart consists of bands and layers of muscle tissue which present an 

 exceedingly intricate interlacement. It consists of (a) the fibres of the auricles, (b) the fibres 

 of the ventricles, and (c) the auriculoventricular bundle of His. 



FIG. 421. The arrangement of the muscles of the auricles. (Poirier and Charpy.) 



Fibres of the Auricles (Fig. 421). 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 

 atriorum. 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 extrem- 

 ities to the corresponding auriculoventricular rings in front and behind. The annular fibres 

 surround the auricular appendices, and form annular bands around the terminations of the 

 veins and around the fossa ovalis. 



The fibres of the ventricles are arranged in a complex manner, and various accounts have 

 been given of their course and connections. The following description is based on the work 

 of McCallum. 1 They consist of superficial and deep layers, all of which, with the exception of 

 two, are inserted into the papillary muscles of the ventricles. The superficial layers consist 

 of the following: (a) Fibres which spring from the tendon of the conus arteriosus and sweep 

 downward and toward the left across the anterior interventricular furrow and around the apex 

 of the heart, where they pass upward and inward to terminate in the papillary muscles of the 

 left ventricle. Those which spring from the upper half of the tendon of the conus arteriosus 

 pass to the anterior papillary muscle, those from the lower half to the posterior papillary muscle 

 and the papillary muscles of the septum, (b) Fibres which arise from the right auriculo- 

 ventricular ring and run diagonally across the back of the right ventricle and around its right 

 border on to its anterior surface, where they dip beneath the fibres just described, and, crossing 

 the interventricular groove, wind around the apex of the heart and terminate in the posterior 

 papillary muscle of the left ventricle, (c) Fibres which spring from the left auriculoventricular 

 ring, and, crossing the posterior interventricular furrow, pass successively into the right ventricle 



1 Johns Hopkins Hospital Reports, vol. ix. 



