GENERAL STRUCTURE. 



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known as sternal ribs. In the anterior dorsal region the capitular 

 head of the ribs articulates with the vertebrae in a pit at the junction 

 of two centra, while the tubercular head joins the transverse process 

 of the second of these two vertebrae ; but in the posterior dorsal 

 region the two heads generally coalesce. Dr Baur regards the 

 intervertebral articulation of the capitula in the anterior region as a 

 direct survival of the intercentral attachment of the ribs of primitive 

 forms allied to Theriodont Anomondontia, which has been totally 

 lost among other Reptiles ; and the mode of costal articulation is 



" 



Fig. 1128. — A, Sternum and right costal cartilages of Man ; b, Sternum and left costal cartilages 

 of Dog. /, Presternum ; m, Mesosternum ; x, Xiphisternum. 



undoubtedly very near to that of those Anomodonts which have 

 lost the intercentra. Uncinate processes are never present on the 

 ribs. 



In all Mammals the pectoral limb is well developed. The pec- 

 toral girdle is, however, usually simpler than in lower forms ; the 

 coracoid persisting as a distinct bone only in the Monotremata, 

 where, however, it anchyloses to the scapula. An interclavicle is 

 present only in the last-named order ; where it articulates with the 

 clavicles after the Reptilian manner. In the Eutheria the clavicles 

 only retain their complete development in those groups, such as the 

 Insectivora, many Rodentia, the Chiroptera, and the Primates, 

 which use the pectoral limb for flight, burrowing, or prehension. 

 The humerus frequently has an entepicondylar foramen, like that of 

 the Anomodont Reptiles. The radius and ulna retain their original 



