66 



ELEMENTAR Y ANA TOMY. 



[LESS. 



The middle part of the sternum (mesosternuni) may be 

 much shorter relatively than in man, as is the case in the 

 Sperm Whale, and it may abort, as just said, in the Dugong. 

 Generally, however, in Mammals it is large and more per- 

 manently segmented than in him, and may (as in the Orang) 

 show to a much later period its median division. The divi- 

 sion into two lateral halves (one piece on each side only) 

 transitorily exists in the Struthious Birds. 



FIG. 76. STERNUM OF A HOWLING MONKEY (Mycetes). 

 m, bones representing "episternal granules." 



The segments of the mesosternum may send down pro- 

 cesses together forming a sort of keel, and each furnished 

 with two articular surfaces for the ribs, as is the case in the 

 Tamandua Ant-eater. 



The xiphoid cartilage, or its osseous or cartilaginous repre- 

 sentative, may abort altogether as a distinct part, as in the 

 Tailed-Batrachians and the Right Whale. It maybe long and 

 pointed, as in the Two-toed Ant-eater, or enlarged and rounded, 

 as in very many species (e.g. Sorex). It may even in Mammals 

 be enormously produced into two elongated horns, far exceed- 

 ing all the rest of the sternum in size, as in the long-tailed Pan- 

 golin ; and a similar but more reduced form may exist in 

 Lizards (e.g. Iguana, Draco (Fig. 78), and still more Stellio}. 



It is in Birds, however, that the xiphisternum attains its 

 maximum of size and importance, forming all that part of the 

 sternum post-axial to the attachment of the ribs. It may 

 consist of one sheet (as in the Qstrich and Cassowary), or be 



