44 



THE SKELETON. 



By 3 centres. 



'or anterior arch (1st year), 



not constant. 



lateral portions of the bodies are formed entirely by the advance of the pedicles. 

 The line along which union takes place between the body and the neural arch is 

 named the neuro-central suture. Before puberty no other changes occur, except- 

 ing a gradual increase in the growth of these primary centres ; the upper and 

 under surfaces of the bodies and the ends of the transverse and spirious processes 

 being tipped with cartilage, in which ossific granules are not as yet deposited. 

 At sixteen years (Fig. 11) three secondary centres appear, one for the tip of 

 each transverse process, and one for the extremity of the spinous process. In 

 some of the lumbar vertebrae, especially the first, second, and third, a second ossi- 

 fying centre appears at the base of the spinous process. At twenty-one years (Fig. 

 10) a thin circular epiphysial plate of bone is formed in the layer of cartilage 



situated on the upper and under sur- 

 faces of the body, the former being 

 the thicker of the two. All these 

 become joined, and the bone is com- 

 pletely formed between the twenty- 

 fifth and thirtieth year of life. 



Exceptions to this mode of de- 

 velopment occur in the first, second, 

 and seventh cervical, and in the 

 vertebrae of the lumbar region. 



The Atlas (Fig. 12). The num- 

 ber of centres of ossification of the 

 atlas is very variable. It may be 

 developed from two, three, four, or 

 five centres. The most frequent ar- 

 rangement is by three centres. Two 

 of these are destined for the two 

 lateral or neural masses, the ossifica- 

 tion of which commences about the 

 seventh week near the articular pro- 



By 7 centres. 



FIG. I'J. Atlas. 



2d year. 



6th month. 



1 for each lateral mass. 



1 for body (4th month). 

 1 for under surface of 



body. 

 FIG. 13. Axis. 



2 additional centres. 



for tubercles on superior articular process. 

 FIG. 14. Lumbar vertebra. 



cesses, and extend backward ; these 

 portions of bone are separated from 

 one another behind, at birth, by a 

 narrow interval filled in with carti- 

 lage. Between the third and fourth 

 years they unite either directly or 

 through the medium of a separate centre developed in the cartilage in the middle 

 line. The anterior arch, at birth, is altogether cartilaginous, and in this a sepa- 

 rate nucleus appears about the end of the first year after birth, and, extending 

 laterally, joins the neural processes in front of the pedicles. Sometimes there are 

 two nuclei developed in the cartilage, one on either side of the median line, which 

 join to form a single mass. And occasionally there is no separate centre, but the 

 anterior arch is formed by the gradual extension forward and ultimate junction 

 of the two neural processes. 



The Axis (Fig. 13) is developed by seven centres. The body and arch of this 

 bone are formed in the same manner as the corresponding parts in the other ver- 

 tebrae : one centre (or two, which speedily coalesce) for the lower part of the 

 body, and one for each lamina. The centres for the laminae appear about the 

 seventh or eighth week, that for the body about the fourth month. The 

 odontoid process consists originally of an extension upward of the cartilag- 

 inous mass in which the lower part of the body is formed. At about the 

 sixth month of foetal life two osseous nuclei make their appearance in the 

 base of this process : they are placed laterally, and join before birth to form 

 a conical bilobed mass deeply cleft above ; the interval between the cleft and 

 the summit of the process is formed by a wedge-shaped piece of cartilage, the base 

 of the process being separated from the body by a cartilaginous interval, which 



