64 ELEMENTARY ANATOMY. [LESS. 



A thorax is not a constant character of Vertebrates, for it 

 cannot be said to exist in those Batrachians which have 

 no ribs, neither in true Serpents nor in Fishes, both these 

 groups of animals being utterly destitute of a sternum. 



34. The STERNUM of man represents a part constantly pre- 

 sent in limbed Vertebrates above Fishes, except Chelonians, 

 as also in some forms in which the limbs are absent. Its 

 human condition, however, of serving as a ventral abutment 

 to ribs, though general, is not constant, as not only may ribs 

 exist without a sternum as in Fishes and Serpents but a 

 sternum may exist without ribs, or without forming any 

 cartilaginous or osseous connexion with ribs, as in the class 

 Batrachia. 



The sternum of man, considered as a whole, is neither so 

 broad nor so narrow as in some other forms, and its depth 

 from the surface inwards is much less than may obtain. 



Thus it is broader in proportion to its length in most 

 oviparous animals (from Birds to Batrachians), also in some 

 Mammals, as the Whales, and even in the Siamang Gibbon, 

 belonging to man's own order. In most members of man's 

 own order, however, and in very many of his class, it 

 is, as in the Dog, and in some Reptiles (as the Crocodile 

 and Chameleon), much more narrow in proportion to its 

 length. 



The above exception as to Chelonians not having a ster- 

 num may well excite surprise, for Tortoises and Turtles have 

 not only well-developed limbs, but it has been commonly 

 supposed that part of their " shell," the great ventral shield 

 (or plastron), is one great sternum, or at least a sternum 

 with dermal ossifications added. It appears, however, that 

 this great complex plate does not really include a sternum. 



That threefold division of the sternum which exists in man 

 is normal in his class. In Birds and Reptiles it also exists, 

 though more obscured and difficult to define. 



Even in Mammals, however, this threefold division is not 

 universal, as (e.g. in the Greenland Whale) only the manu- 

 brium may exist, the rest of the sternum aborting ; while 

 in the Dugong we have a xiphisternum (the representative 

 of the xiphoid process of man), together with a manubrium, 

 but no ossified representative of the middle part of the 

 sternum. 



In Tailed-Batrachians and the Slow-worm (Anguis) we have 

 a simple sternum which cannot be said with certainty to re- 

 present any one of the three divisions ; while in many Frogs 



