THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



187 



of each vertebra, and following this a center in each half of the vertebral arch 

 (Fig. 1 66). Osseous tissue then gradually replaces the cartilage. The two 

 halves of an arch fuse dorsal to the spinal cord during the first year of post- 

 natal life, thus completing the bony arch. The arch fuses with the body of the 



Med. ossif. center 



FIG. 1 66. Thoracic vertebra and ribs of human embryo of 55 mm. (middle of 



3rd month). Kollmann's Atlas. 

 Cartilage indicated by stippled areas, ossification centers by irregular black lines. 



vertebra between the third and eighth years. Thus it is seen that the process of 

 ossification is a slow one, and this is even more striking when one considers the 

 formation of the secondary centers. For at about the age of puberty a secondary 

 center appears in each of the cartilages that cover the ends of the vertebrae, pro- 



Spinous process 

 Transverse process 

 Articular process 



Body of vertebra 



Upper epiphyseal 

 plate 



FIG. 167. Lumbar vertebra (lateral view) showing secondary centers of ossification. Sappey. 



ducing disks of bone the epiphyses. A secondary center also appears in the 

 cartilage on the tip of each spinous process and transverse process, and in the 

 lumbar vertebrae one appears also on the tip of each articular process (Fig. 167). 

 The epiphyses unite with the vertebras any time between sixteen and twenty- 



