THE HUMAN STERNUM 59 



In air-breathing vertebrates the sternum is not always present ; nor 

 is its most primitive form to be looked for in animals like Ophidia, without 

 limbs, and with a high development of the ribs. 



Among Reptiles the sternum is found in several different forms. In 

 its most primitive form (Batrachia, Lacertilia) it is a median structure, not 

 connected with the ribs, but associated with the two halves of the shoulder- 

 girdle. 



Chirotes canaliculatus presents a remarkably complete sternum, without 

 costal connexions, and associated with a rudimentary shoulder-girdle. 



Among the higher Reptiles (Crocodilla) the sternum acquires costal 

 connexions in addition to its attachments to the shoulder-girdle, and 

 assumes a character directly comparable with that characteristic of the most 

 primitive Mammals {Omithorhynchus, Echnida) but at the same time it is 

 a simpler structure, as shewn in the want of differentiation of the several 

 elements. 



The study of the sternum in Reptiles encourages the idea that the 

 association of sternum and shoulder-girdle is a primary and fundamental 

 relation ; and that the connexions with the ribs is a secondary event. 



In still higher vertebrates — Birds and Mammals — the modifications 

 of the sternum which occur are directly associated with alterations in the 

 construction and uses of the shoulder-girdle. Costal connexions are always 

 present, but with wide variations in different groups. The differences in 

 the type of shoulder-girdle appear to determine the constitution of the 

 sternum. 



In bipedal animals, or animals whose fore limbs are used for flight, 

 burrowing, etc., in which the coracoid and clavicle (Aves, Ornithodelphia), 

 or clavicle (cat, mole, man) are powerfully developed, the sternum is of a 

 simple type, forming a broad, flat bone, keeled, it may be, for the attach- 

 ment of powerful pectoral muscles. In such cases the presternum is broad 

 and strong, and the mesosternum is in the form of a single bone. 



Among quadrupeds, on the other hand, and Mammals in which the 

 fore limb is more assimilated in type to the hind limb than in birds and 

 bipedal mammals, and is used for supporting weight and for locomotion, 

 mainly or solely, the shoulder-girdle is more rudimentary, the clavicle is 



