THE ARCHETYPAL STERNUM 



57 



In the memoir on the sternum Geoffrey's first care is to 

 arrive at a definition of what a sternum is. He defines it 

 partly by its functions, partly by its connections, as the 

 system of bones which covers and protects the thorax, and 

 gives attachment to certain groups of muscles. 



The most highly developed sternum (according to this 

 definition) is the plastron of the tortoise, whose structure 

 it dominates (p. 103). It is important, therefore, to determine 

 of how many bones the plastron is composed, since the full 

 number of elementary parts of which an organ is composed 

 is best seen when the organ is at the maximum of its 

 development. There are nine bones in the plastron of 

 the tortoise. " The conclusion to be drawn from this is that 

 every sternum, provided that it is not inhibited in its 

 development by some obstacle, is composed of nine elementary 

 parts" (p. 105). These nine bones are in Geoffrey's 

 nomenclature, the episternals, the hyosternals, the hypo- 

 sternals, the xiphisternals, which are all paired bones, and 

 the entosternal, which is unpaired. The arrangement of 

 them is in the tortoise : 



Episternal 



Episternal 



Hyosternal 



Hyosternal 



Hyposternal 



Hyposternal 



Xiphisternal 



Xiphisternal. 



The articulations in the tortoise are indicated by the 

 connecting lines. Geoffrey tries to show that the sternum in 

 other animals is composed of these nine bones, or at least of 

 a certain number of them, always in the same invariable 

 relative positions. Thus in birds the sternum consists of five 

 pieces, of a huge keeled entosternal, and of two " annexes "" on 

 either side, which are the hyo- and hyposternals. These are 

 separate only in young birds. Occasionally, especially in 



