THE CONNECTIVE TISSUES AND THE SKELETAL SYSTEM. 



153 





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. 128). At the age of eight to fourteen years a second- 

 ary center appears in each capitulum and tuberculum 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 

 vertebrae 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. 131. Sternum of 

 which probably represents the anlage of a rib, but soon 

 fuses with the transverse process. 



The Sternum. The sternum, according to Hanson's 

 recent contribution, originates independently of the ribs. 

 On each side, some distance from the midventral line, the 

 sternal band or bar arises as a mesenchymal condensation in the body 

 wall. These bars then approach the midventral line and fuse with each 

 other to form a single cartilaginous structure. Meanwhile the ventral 

 ends of the first seven ribs extend far enough to come into contact with and 

 join the sternal bar (Fig. 130). Before the two bars have united a medial 

 unpaired rudiment appears opposite their anterior ends to form the pre- 

 sternum with which the paired rudiments subsequently unite. The pre- 

 sternal component, with which the clavicles articulate, probably represents 

 the ventral part of the primitive vertebrate shoulder girdle. 



12 year old child, 

 showing centers of 

 ossification. Seven 

 ribs are attached on 

 the right side, 8 on 

 the left. Markowski, 

 Kollmann's Atlas. 



