100 THE VASCULAR SYSTEM 



Valves. At the cardiac orifices the entire thickness of the en- 

 docardium is folded upon itself to form a double layer, between 

 the folds of which an intervening stratum of dense fibrous tissue 

 is inserted. These endocardial folds are the cardiac valves. The 

 number and shape of their cusps are dependent upon the location. 

 The semilunar valves of the aortic and pulmonary orifices consist 

 of three crescentic endocardial folds ; at the auriculo-ventricular 

 orifices the tricuspid valve consists of three large cusps, the mitral 

 of two, together with an equal number of small intervening folds 

 of the endocardium. 



The margin of the valvular cusp is extremely thin ; just within 

 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 approxima- 

 tion of the free margins of adjacent cusps. At the apex of the 

 valvular cusp, where the adjacent fibrous margins of the valve 

 meet, the dense connective tissue, particularly in the semilunar 

 valves, is considerably thickened to form the corpus Aurantii. 

 These corpora, in the aged, are frequently subject to calcareous 

 infiltration. 



Muscular fibres are frequently continued from the adjacent 

 cardiac or arterial wall into the dense fibrous tissue at the base of 

 the valve. The base of the valve is also surrounded by a ring of 

 fibrous tissue, the annula fibrosa, whose interlacing bundles are so 

 closely packed as to give them an almost cartilaginous feel. At 

 the auriculo-ventricular orifices, these fibrous rings are continuous 

 with the auriculo-ventricular septum, from which the muscle bands 

 of the myocardium take their origin. 



Chordae tendinese. These are firm, unyielding cords, composed 

 of parallel bundles of dense white fibres, and covered with a very 

 thin endocardium continuous with that of the ventricular wall 

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

 papillary muscles to the ventricular surfaces of the mitral and 

 tricuspid valves. At the apex of the papillary muscle the fibrous 

 bundles of the chordae intermingle with the muscle fibres, and are 

 continued into the endomysial connective tissue, which is especially 

 abundant in those portions of the myocardium. At their valvular 

 attachment the fibrous bundles of the chordae tendinese turn almost 

 at right angles, and spread out, in a somewhat radial manner, to 

 become continuous with the dense fibrous tissue which forms the 

 interior of the valve. 



