THE BONES OF THE TRUNK 29 



axis, where the union is delayed until the fourth year, and the neural arch 

 joins the body in the third year. The cartilaginous union between the neural 

 arch and the body at either side is called the neuro-central synchondrosis. In 

 the thoracic vertebrae the superior demi-facets lie behind this, and so they are 

 shown to be placed on the pedicular portion of the body. All vestiges of this 

 svnchondrosis have disappeared prior to the sixth year. The secondary centres, 

 five in number, appear about the sixteenth year, and they are consolidated 

 about the twenty-fifth year. One appears at the extremity of the spinous 

 process, one at the extremity of each transverse process, and the other two 

 take the form of epiphysial plates, one on the upper surface and the 

 other on the under surface of the body. In the case of the seventh cervical 

 vertebra, and sometimes one or two above it, the costal process has a 

 special centre which apj)ears before birth, and it may be developed into 

 a cer\'ical rib. The transverse process (costal element) of the first lumbar 

 has occasionally a special centre, and in these cases it may be developed into 

 a lumbar rib. The lumbar mammillary processes are ossified from special 

 secondary centres. The fifth lambar has sometimes four centres for the 

 neural arch, two at either side, one of which is for the pedicle, transverse 

 process, and superior articular process, and the other for the lamina, 

 inferior articular process, and one-half of the spinous process. These parts 

 may fail to unite, in which cases the neural arch presents a synchondrosis 

 on either side, situated between the superior and inferior articular processe.- 

 (Turner). Sometimes the laminae of the fifth lumbar fail to unite, and so a 

 space is left, bridged over by fibrous tissue. 



The Atlas. — The atlas has three centres of ossification, two for the lateral 

 masses and posterior arch, appearing in the seventh week of intra -uterine Ufe. and 

 one (sometimes two) for the anterior arch, which does not appear until the 



A p;. ears in 7th Week 

 (intra-uterine) 



Fig. 20. — Ossification of the Atlas. 



first year. The two halves of the neural arch usually join towards the end of 

 the third year, there being sometimes a special osseous deposit at the place of 

 junction. The two halves, however, may remain separate throughout life, 

 the interval being bridged over by fibrous tissue. The anterior arch joins 

 the lateral masses in the sixth year. The anterior arch represents the 

 hypochordal brace of the first vertebral bow. 



The Axis. — Excluding the odontoid process, the axis has three primary centres, 

 like an ordinary vertebra, two for the neural arch appearing about the seventh 

 week, and one (sometimes tvvo) appearing in the lower part of the common 

 cartilage of the body and odontoid process in the fourth month. In the upper 

 part of this common cartilage two centres, laterally disposed, appear in the 

 fifth month for the odontoid process, and these unite into one centre about the 

 sixth month. At birth the axis is composed of four osseous parts connected 

 by cartilage, namely, a body, an odontoid process surmounted by cartilage, 

 and two halves of the neural arch. The odontoid process joins the body 

 about the fourth year. The two halves of the neural arch join each other, 

 and the arch joins the body, in the fourth year. The apical part of the odontoid 

 process has a special centre appearing in the fourth year, and it joins the 

 rest of the process in the tweljth year. The body of the axis has the usual 

 epiphysial plate on the under surface of the body, but there is no such plate 



