AXIAL SKELETON 



3" 



the vertebral bodies, thus leaving intervals — the transverse foramina — through 

 which the vertebral vessels course. In the lumbar region the ribs are again dimin- 

 utive and are fused to the transverse processes. The rudimentary rib^ of the 

 sacral vertebra are represented by flat plates which unite on each side to form a 

 pars lateralis of the sacrum. With the exception of the first coccygeal vertebra, 

 ribs are absent in the most caudal vertebrae. 



Sternum.— The sternal anlages arise as paired mesenchymal bands, with 

 which the first eight or nine thoracic ribs fuse secondarily (Whitehead and Wad- 

 dell, Amer. Jour. Anat., vol. 12, 1911). After the heart descends into the thorax, 

 these cartilaginous sternal bars, as they may now be termed, unite in a cranio-cau- 

 dal direction to form the sternum, at the same time incorporating a smaller mesial 

 sternal anlage (Fig. 314). Ultimately one or two pairs of the most caudal ribs 



Clavicle 



Fig. 314. — Formation of the sternum in a 

 human fetus during the third month (modified 

 after Ruge). 



Fig. 315. — Sternum of a child, showing centers of 

 ossification. 



lose their sternal connections, the corresponding portion of the sternum consti- 

 tuting the xiphoid process in part. At the cranial end of the sternum there are 

 two imperfectly separated episternal cartilages with which the clavicles articulate. 

 These usually unite with the longitudinal bars and contribute to the formation 

 of the manubrium. Variations in the ossification centers are not uncommon, al- 

 though 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 mesen- 

 chyme which envelops the cranial end of the notochord and extends cephalad into 

 the nasal region. Laterally it forms wings which enclose the neural tube. Except 

 in the occipital region, where there are indications of the incorporation into the 

 skull of three or four vertebras, the skull is from the first devoid of segmentation. 



