CIRCULATORY SYSTEM 



49 



almost entirely absent. The valve itself, represented in fig. 

 30, really consists of two parts, which are distinguished by 

 the insertion of a large papillary muscle, which ties the entire 

 valve to the free wall of the ventricle. It is only rarely that 

 any other representative of the generally numerous papillary 

 muscles and chordae tendinese of the mammalian heart occur, 

 but occasionally a few muscular threads in addition to the 

 single papillary muscle are to be found. Their existence has 

 been noted, for instance, in Apteryx australis and in a few 

 other birds. It has been usually held that the muscular 

 right auriculo- ventricular valve of the bird's heart represents 



Fig. 30. — Heabt of Fowl, Intebiob 

 OF Eight Venhucle. 



papillary muscle ; &, c, valve. (After 

 Lakkesteb.) 



Fig. 31. — Heart of Apteryx, Intebioe 

 OF B.IGHT Ventbiole, with Attach-. 



MENT OF PaPILLAEY MuSCLE . COT 

 THROUGH. 



X, flap of veutriculai' wall removed with 

 muscle. (After Lankester.) 



only one half of the complete valve of the mammalian and 

 crocodilian hearts. In these last-mentioned animals the 

 entire circumference of the ostium, which leads from the 

 auricle into the ventricle, is surrounded by the valve, which 

 thus forms a complete collar. There is, however, an excep- 

 tion in the case of the Monotremata, where the septal flap of 

 the valve (i.e. that lying on the side of the ostium which 

 abuts upon the interventricular septum) is partially or 

 entirely absent. On a careful comparison, however, betw:een 



