104 OSTEOLOGY. 



or seven cartilaginous ribs. Paterson, on the other hand, regards the sternum as 

 arising independently of the ribs by the union of a right and left sternal bar in the 

 median ventral line. There are also reasons for supposing that the presternum is 

 intimately associated with the development of the ventral part of the shoulder girdle. 



Ossification of the Vertebrae. The vertebrae are developed by ossification of 

 the cartilage which surrounds the notochord and which passes dorsally over the sides of 

 the vertebral canal. The centres for the bodies first appear in the lower thoracic vertebrae 

 about the tenth week. An oval nucleus develops in each body. At first it is placed 

 dorsal to the notochord, but subsequently surrounds and causes the disappearance of 

 that structure. Occasionally, however, the primitive centre appears to be formed by 

 the coalescence of two primary nuclei. Support is given to this view by the occasional 

 occurrence of vertebrae in which the body is developed in two collateral halves, or in cases 

 where only one-half of the body persists (Turner) ; normally, however, it is impossible to 

 make out this division. From these single nuclei the bodies are developed, the process 

 extending up and down the column until, by the fifth month, all the bodies possess ossific 

 nodules, except the coccygeal segments. About the seventh week a single centre appears in 

 the vertebral arch on either side. These commence first to ossify in the upper cervical region 

 and extend rapidly downwards throughout the column. They first appear near the bases 

 of the superior articular processes, and extend backwards into the laminae, laterally into 

 the transverse processes, and forwards into the roots of the vertebral 

 Centre arches. These latter project anteriorly and form a considerable portion 

 . for of the postero-lateral aspects of the body, from which, however, they 

 " are separated by a cartilaginous strip the neuro-central synchondrosis 

 which does not entirely disappear until about the fifth or sixth year. 

 It is important to note that in the thoracic region the costal facets lie 

 sntre for behind the neuro-central synchondrosis, and are therefore borne on the 

 body lateral aspects of the roots of the vertebral arches. Fusion of the 



laminae in the median plane posteriorly begins, after birth, in the lumbar 

 FIG. 118. OSSIFICATION . -, r , , , 7 , ',. . 



OF VERTEBRAE region and extends upwards, so that by the mteentn month or there- 

 abouts the arches in the cervical region are completed posteriorly. In 

 the sacral region ossification is slower, the vertebral canal not being enclosed till the 

 seventh to the tenth year. The spinous processes are cartilaginous at birth, but they 

 become ossified by the extension into them of the bony laminae. 



At puberty certain secondary or epiphyseal centres make their appearance ; these are 

 five in number. One caps the summit of the spinous process, except in the cervical 

 region. A single- centre on each side appears at the extremity of the transverse 

 process, and in the thoracic region assists in forming the articular surface for the tubercle 

 of the rib. Two epiphysial plates are formed one for the superior, and the second for the 

 inferior surface of the body, including also that part which lies posterior to the neuro-central 

 synchondrosis and is formed by the root of the vertebral arch ; from these the thickened 

 circumference of both upper and lower aspects of the body are derived. Fusion of these 

 centres with the rest of the bone is not complete till the twenty -fifth year. 



In the cervical region independent centres are described as occurring in the anterior 

 roots of the transverse processes of the sixth and seventh vertebrae. These correspond to 

 the costal element, and may occasionally persist in the form of cervical ribs. Elsewhere 

 they are formed by lateral extensions from the root of the vertebral arch. 



In the lumbar region the transverse process of the first lumbar vertebra is occasionally 

 associated with an independent costal centre, which may blend with it, or persist as a 

 lumbar rib. The mamillary processes are derived from separate centres. The vertebral 

 arch of the fifth lumbar vertebra is occasionally developed from two centres on each side, 

 as is demonstrated by the fact that the arch is sometimes divided by a synchondrodial 

 joint running obliquely across between the superior and inferior articular processes. 

 (See ante, p. 91; also Fortschritte auf dem Gebiete der Rontgenstrahlen. Erganzungsheft i. ; 

 "die Entwickelung des menschlichen Knochengerustes wahrend des fotalen Lebens," 

 von Lambertz.) At the eighteenth year there are two epiphyses at the end of the costo- 

 transverse process of the fifth lumbar vertebra ; one caps the transverse element, the other 

 caps the costal element (Fawcett). 



Atlas. The lateral masses, transverse processes, and posterior arch are developed 

 from two centres one on each side which correspond with the centres from which the 

 vertebral arches of the other members of the series are developed. These make their 

 appearance about the seventh week, and do not unite posteriorly till after the third 

 year. Their point of union is sometimes preceded by the formation of a distinct spinal 



