HEART 



197 



the margin, however, the central mass of dense fibrous tissue is somewhat 

 thickened to form, in each cusp, a dense rim which during valvular 

 closure secures the firm and accurate approximation of the free margins 

 of adjacent cusps. At the apex of the valvular cusp, when- the adjacent 

 fibrous margins of the valve meet, the dense connective tissue, particu- 

 larly in the semilunar valves, is considerably thickened to form a nodule, 

 the corpus arantii. These corpora or noduli, in the aged, are frequently 

 subject to calcareous infiltration. 



Muscular fibers are continued from the adjacent cardiac wall into 

 the dense fibrous tissue at the base of the valve, except in the case of 

 the semilunar valves of the pulmonary and systemic aortae. This muscle 



FIG. 213. THE ENDOCARDIUM. 

 From the ventricular wall of the heart of man. Hematein and eosin. Photo. X 469. 



is generally non-striped, and probably functions as a sphincter. The 

 base of the valve is also surrounded by a ring of fibrous tissue, the 

 annulus fibrosus, whose interlacing bundles are so closely packed as to 

 give them an almost cartilaginous feel. At the auriculoventricular 

 orifices, these fibrous rings are continuous with the auriculoventricular 

 septum, from which the muscle bands of the myocardium take their 

 origin. 



Chordae Tendineae. These are firm, unyielding cords, composed of 

 parallel bundles of dense collagenous fibers, with a few clastic lihers, and 

 covered with a very thin endocardium continuous with that of the ven- 

 tricular wall and cardiac valve. These fibrous bands unite the apices of 

 the papillary muscles to the ventricular surfaces of the mitral and tri- 

 cuspid valves. At the apex of the papillary muscle the fibrous bundles 

 of the chords intermingle with the muscle fibers, and are continued into 

 the endomysial connective tissue, which is especially abundant in those 



