THE AXIAL SKELETON 311 



The ribs, originating as ventro-lateral outgrowths from the vertebral 

 bodies, reach their highest development in the thoracic region. In the 

 cervical region they are short; their tubercles fuse with the transverse 

 processes and their heads with the vertebral bodies, thus leaving intervals, 

 the transverse foramina, through which the vertebral vessels course. In 

 the lumbar region the ribs are again diminutive and are fused to the 

 transverse processes. Tke rudimentary ribs of the sacral vertebra are 

 represented by flat plates which unite on each side to form a pars ateralis 

 of the sacrum. With the 'exception of the first coccygeal vertebra, ribs 

 are absent in the most caudal vertebras. 



The Sternum. The sternal anlages arise as paired mesenchymal 

 bands, with which the first eight or nine thoracic ribs fuse secondarily 

 (Whitehead and Waddell, 1911). After the heart descends into the 

 thorax, these cartilaginous sternal bars, as they may now be termed, unite 

 in a cranio-caudal direction to form the sternum, at the same time incor- 

 porating a smaller mesial sternal anlage (Fig. 314). Ultimately,! one or 

 two pairs of the most caudal ribs lose their sternal connections, the cor- 



cartilage 



FIG. 314. Formation of the sternum in a FIG. 315. Sternum of a child, showing centers 

 human fetus during the third month (modified of ossification, 



after Ruge). 



responding portion of the sternum constituting the xiphoid process in part . 

 At the cranial end of the sternum there are two imperfectly separated epi- 

 ternal cartilages with which the clavicles articulate. These usually unite 

 with the longitudinal bars and contribute to the formation of the manu- 

 brium. Variations in the ossification centers are not uncommon, although 

 a primitive, bilateral, segmental arrangement is evident (Fig. 315). In 

 the two cranial segments, however, unpaired centers occur. 



The Skull. The earliest anlage of the skull consists in a mass of dense 

 mesenchyme which envelops the cranial end of the notochord and extends 

 cephalad into the nasal region. Laterally, it forms wings which enclose 



