536 



AXGIOLOGY 





ance to the inner surface of the heart; it assists in forming the valves by its reduplications, and 

 is continuous with the lining membrane of the large bloodvessels. It consists of connecthe 

 tissue and elastic fibers, and is attached to the muscular structure by loose elastic tissue which 

 contains bloodvessels and nerves; its free surface is covered by endothelial cells. 



The fibrous rings surround the atrioventricular and arterial orifices, and are stronger upon 

 the left than on the right side of the heart. The atrioventricular rings serve for the attachment 

 of the muscular fibers of the atria and ventricles, and for the attachment of the bicuspid and 

 tricuspid valves. The left atrioventricular ring is closely connected, by its right margin, with 

 the aortic arterial ring; between these and the right atrioventricular ring is a triangular mass of 

 fibrous tissue, the trigonum fibrosum, which represents the os cordis seen in the heart of some of 

 the larger animals, as the ox and elephant. Lastly, there is the tendinous band, already referred 

 to, the posterior surface of the conus arteriosus. 



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 some 

 of the muscular fibers of the ventricles; its opposite margin presents three deep semicircular 

 notches, to which the middle coat of the artery is firmly fixed. The attachment of the artery 

 to its fibrous ring is strengthened by the external coat and serous membrane externally, and 

 by the endocardium internally. From the margins of the semicircular notches the fibrous structure 

 of the ring is continued into the segments of the valves. The middle coat of the artery in this 

 situation is thin, and the vessel is dilated to form the sinuses of the aorta and pulmonary artery. 



FIG. 499. Anastomosing muscular fibers of the heart seen 

 in a longitudinal section. On the right the limits of the 

 separate cells with their nuclei are exhibited somewhat dia- 

 grammatically. 



FIG. 500. Purkinje's fibers from the sheep's 

 heart. A. In longitudinal section. B. In 

 transverse section. 



Cardiac Muscular Tissue. The fibers of the heart differ very remarkably from those of other 

 striped muscles. They are smaller by one-third, and their transverse striae are by no means so 

 well-marked. They show faint longitudinal striation. The fibers are made up of distinct quad- 

 rangular cells, joined end to end so as to form a syncytium (Fig. 499). Each cell contains a clear 

 oval nucleus, situated near its center. The extremities of the cells have a tendency to branch or 

 divide, the subdivisions uniting with offsets from other cells, and thus producing an anastomosis 

 of the fibers. The connective tissue between the bundles of fibers is much less than in ordinary 

 striped muscle, and no sarcolemma has been proved to exist . 



Purkinje Fibers (Fig. 500). Between the endocardium and the ordinary cardiac muscle are 

 found, imbedded in a small amount of connective tissue, peculiar fibers known as Purkinje fibers. 

 They are found in certain mammals and in birds, and can be best seen in the sheep's heart, where 



