THE VASCULAR MECHANISM. 177 



cusps, tricuspid valve, arising from the fibrous junction between the 

 auricle and ventricle and hanging down into the ventricle. The cusps 

 consist of connective and elastic tissue, and are so arranged as to permit 

 the flo\v of blood from auricle to ventricle, but not in the reverse 

 direction. From each cusp a number of tendinous threads, chordae 

 tendinece, pass to be attached to projections of the ventricular wall, 

 known as the papillary muscles. 



The right ventricle possesses two openings : ( 1 ) the auriculo-ven tricular 

 just described, and (2) the opening into the pulmonary artery, which 

 conveys blood from the heart to the lungs. The latter opening is 

 provided with a valve having three semilunar cusps composed of strong 

 fibrous and elastic tissue. In the centre of the free border of each cusp 

 is a small fibrous nodule, the corpus Arantii, which serves to strengthen 

 the valve. When the valve is closed, the free borders of the cusps come 

 into contact with each other and are pressed together, thereby pre- 

 venting the return of blood from the pulmonary artery to the ventricle. 



The general arrangement of the left side of the heart is very 

 similar to that of the right. Two pulmonary veins from each lung 

 open into the left auricle. The auriculo-ventricular valve possesses 

 only two cusps ; it somewhat resembles a bishop's mitre, and is known 

 as the mitral valve. The cusps, like those of the right auriculo- 

 ventricular valve, are connected by tendinous cords with the papillary 

 muscles and the wall of the left ventricle. Opening out of the left 

 ventricle is the aorta, which is provided with a valve having three 

 semilunar cusps similar in structure to those of the pulmonary artery. 



The cavities of the heart are lined by a smooth membrane, the endo- 

 cardium, composed of delicate connective and elastic tissue covered by 

 flattened endothelial cells. 



The substance of the heart .consists of muscular tissue bound 

 together by connective tissue and supplied with blood from the coronary 

 arteries. It is arranged in sheets composed of fibres, which are built 

 up of short cylindrical segments or cells ; each cell has an oval nucleus 

 and shows an indistinct longitudinal and transverse striation. The 

 fibres are branched, the branches of adjacent fibres uniting with one 

 another so that the heart muscle forms a continuous network of cells, 

 known as a syncytium. 



The wall of the auricles is composed of (a) superficial fibres common 

 to both auricles, and (b) deep fibres, both looped and annular, proper to 

 each auricle ; the annular fibres form muscular rings around the open- 

 ings of the great veins. The auricles are joined to the ventricles by 

 strong fibrous rings, which encircle the auriculo-ventricular orifices, and 

 by a band of modified muscular tissue, known as the auriculo-ventricular 



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