46 



THE SKELETON 



that the pits for the heads of the ribs are situated behind the neuro-central suture, whirh is. 

 directed obhquety backward and medially. The laminae unite during the firSt year after birth; 

 and by the gradual extension of ossification into the various processes, the vertebrae have at- 

 tained almost their full size by the time of puberty. Subsequently the secondary centres 

 appear in the cartilaginous extremities of the spinous and transverse processes, and in the carti- 



FiG. 52.- 



-Cervical Vertebr.\ showing the Epiphysial Plate on the Upper Surface of 



THE Body. 



lage on the upper and lower surfaces of the bodies, forming in each vertebra two annular plates, 

 thickest at the circumference and gradually thinning toward the central deficiency. The 

 epiphyses appear from the fifteenth to the twentieth year and join with the vertebra by the 

 twenty-fifth year. 



Fig. 53. — Lumbar Vertebra at the Eighteenth Year with Secondary Centres. 

 Epiphysial plate or disc 



Mammillary tubercle 



Transverse process 



Spinous process 



Epiphysial plate or disc 



In several vertebrae the mode of ossification differs from the account given above — in some 

 cases considerably — and necessitates separate consideration. 



Atlas. — The lateral portions and posterior arch are formed from two centres of ossification, 

 which correspond to the lateral centres of other vertebra? and appear about the seventh week. 



Fig. 54. 



-Upper Thoracic Vertebra with an Epiphysial Plate Removed and Drawn at 



THE Side. 

 The plate .shows the characteristic deficiency in the centre. (Natural size.) 



The anterior arch is ossified from one centre, which, however, does not appear uiit il a few months 

 after birth. Union of the lateral parts occurs posteriorly in the third year, Ix'iiig sometimes 

 precf(le(i by the ai)p(!arance of a .secondary centre of ossification in th(^ intervening cartilage, 

 and the union of the lateral parts with the anterior arch occurs about the sixth year. 



