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TEXT-BOOK OF PHYSIOLOGY. 



latter, shut. For these reasons this structure is known as the tri- 



cuspid valve. This valve is 

 formed of fibrous tissue de- 

 rived from the fibrous ring, 

 some muscle-fibers, covered 

 over by a reduplication of 

 the endocardium. To the 

 under surface and to the 

 edges of this valve the ten- 

 dinous cords of the papillary 

 muscles are firmly and intri- 

 cately attached. These cords 

 are just sufficiently long to 

 permit closure of the valve 

 and to prevent their being 

 floated into the auricle. 



The orifice of the pul- 

 monary artery is also sur- 

 rounded by a ring of fibrous 

 tissue to which are attached 

 three semilunar or pocket- 

 shaped membranes, the 



semilunar valves. Each valve is formed by a reduplication of the 



endocardium strengthened by fibrous tissue. In the center of the 



free edge of the 'valve there 



is a small nodule of fibro- 



cartilage (the corpus Aur- 



antius). The outer edge 



of the valve is strengthened 



by a delicate fibrous band. 



A similar band strengthens 



the convex attached por- 



tion of the valve just where 



it is joined to the fibrous 



ring. A third set of fibers 



pass toward the nodule, in- 



terlacing in all directions. 



Two narrow crescentic- 



shaped areas (the lunulae) 



near the free edge are de- 



void of these fibers. Dur- 



ing the period of relaxation 



FIG. 116. RIGHT CAVITIES or THE HEART. 

 Auriculo-ventricular valves open, arte- 

 rial valves closed. (Dalton.} 



of the heart the edges of 

 the valves are in close 

 apposition and prevent a 



FlG * ll ^~ RlGHT CA VITIES or THE HEART. 



