THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



185 



A chondrification center appears in each costal process, shortly after that in 

 the body of the vertebra, and from this point the formation of cartilage 

 gradually extends throughout the entire rib. 



Ossification begins during the third month at a center which is situated near 

 the angle of the rib (Fig. 166). At the age of eight to fourteen years a second- 

 ary center appears in each capitulum and tuber culum and subsequently fuses 

 with the rest of the rib at the age of fourteen to twenty-five years. As the 

 tuberculum develops, the transverse process of the corresponding vertebra 

 grows ventrally and caudally to meet it and form the articulation. 



The ribs reach the highest degree of development in the thoracic region 

 where one develops on each side, corresponding to each vertebra. The first 

 seven or eight thoracic ribs extend almost to the mid- 

 ventral line and are attached to the sternum; the last four 

 or five become successively shorter and are only indirectly 

 or not at all attached to the sternum. In the cervical 

 region the ribs do not reach a high degree of development. 

 Their tips simply fuse with the transverse processes of the 

 vertebras and their heads with the bodies of the vertebrae, 

 leaving a space the foramen transversarium through 

 which the vertebral vessels pass. The seventh cervical rib 

 may, however, reach a fairly high degree of development. 

 In the lumbar region also the ribs are reduced to small 

 pieces of bone which are firmly united with the transverse 

 processes and form the accessory processes. In the sacral 

 region the rudimentary ribs unite to form the lateral part 

 (pars lateralis) of the sacral bone. After the blastemal 

 stage there are no indications of ribs in the coccygeal region. 

 In the blastemal stage, however, there is a small bit of tissue FIG. 169. Sternum of 

 which probably represents the anlage of a rib, but soon 

 fuses with the transverse process. 



The Sternum. The sternum is formed by the fusion 

 of the ventral ends of the first eight or nine thoracic ribs. 

 A longitudinal bar is first formed on each side of the medial 

 line by the fusion of the ventral ends of the ribs on each side; then the two bars 

 unite in the medial line to form a single piece of cartilage (Figs. 168 and 169). 

 Subsequently the last one or two ribs become separated from the sternum, 

 leaving only seven or eight connected with it. At the cephalic end of the 

 sternum two separate pieces of cartilage episternal cartilages appear, with 

 which the clavicles articulate (Fig. 168). These usually unite with the longi- 

 tudinal bar to form a part of the manubrium, but they may remain separate 

 and ossify to form the supr asternal bones (ossa suprasternalia). 



old child, 

 centers of 

 ossification. Seven 

 ribs are attached on 

 the right side, 8 on 

 the left. Markcnvski, 

 Kollmann's Atlas. 



