500 THE RABBIT CHAP.] 



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. 130). 



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 (compare pp. 498 and 499). 



The sternum, which is developed in the embryo by the 

 fusion of the ventral ends of the ribs (and therefore has a 

 different morphological significance from the sternum of 

 Amphibians, see p. 48), consists of six segments or 

 sternebr&, the first of which, or manubrium, is larger 

 than the rest, and has a ventral keel. With the last is 

 connected a rounded, horizontal, cartilaginous plate, the 

 xiphisternum. The ribs articulate between the successive 

 sternebrae except in the case of the first pair, the 

 articulations 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 



