X STERNUM AND PECTORAL ARCH 483 



There are twelve or occasionally thirteen pairs of ribs, 

 which have the form of curved rods, situated in the walls of 

 the thorax, and articulating with the thoracic vertebrae 

 above and — in the case of the first seven — with the breast- 

 bone or sternum below ; the remaining ribs do not reach 

 the sternum (Fig. 121). 



Each rib consists of a bony, dorsal, vertebral portion, and of a ventral, 

 sternal portion consisting of cartilage which is calcified or only 

 incompletely ossified. The dorsal end — the head or capitulum of the 

 rib — articulates with the capitular facet on the centra, and the first nine 

 have also a tubercle, a short distance from the capitulum, which articulates 

 with the tubercular facet ; just externally to the tubercle is a short, 

 vertical process. 



The sternum, which is developed in the embryo by the 

 fusion of the ventral ends of the ribs, consists of six seg- 

 ments or sternebrce, the first of which, or manubrium, is larger 

 than the rest, and has a ventral keel. AVith the last is con- 

 nected a rounded, horizontal, cartilaginous plate, the xiphi- 

 sternum. The ribs articulate between the successive 

 sternebrae except in the case of the first pair, the articula- 

 tions of which are on the manubrium. 



The chief bone of the pectoral arch is the flat, triangular 

 scapula, the coracoid portion (compare p. 47) becoming early 

 fused with it and forming a small, inwardly curved, coracoid 

 process, situated anteriorly to the glenoid cavity at the lower 

 end or apex of the scapula : the apex lies over against the 

 first rib, and the bone inclines upwards and backwards to 

 its dorsal base, which in the fresh condition, consists of a strip 

 of cartilage, the supra-scapula. On its outer surface is a 

 prominent ridge or spine, the free ventral edge of which is 

 called the acromion, from which a process, the metacromion, 

 projects backwards. The collar-bone or clavicle is never 

 strongly developed in Mammals in which the fore-limb only 



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