THE ORGANS OF THE INTERMEDIATE LAYER OR MESENCHYME. 601 



the median plane. The two sternal bars are separated from each 

 -other by membranous tissue ; later they approach each other in the 

 median plane, and commencing in front, begin to fuse together into 

 an unpaired piece, from which the individual ribs which gave rise to 

 them are afterwards separated by the formation of joints. 



The paired origin of the sternum serves to explain some of its 

 abnormalities. For example, in the adult there is sometimes seen 

 a fissure, which, although closed by connective tissue, passes quite 

 through the sternum (fissura sterni), or a few larger or smaller gaps 

 are found in the body and xyphoid process of the sternum. All 

 these abnormal cases are explained by the complete or partial failure 

 of the two sternal bars to fuse in the 

 usual way during embryonic life. 



The ossification of ribs and sternum 

 is in part accomplished by the develop^ 

 ment of special centres of ossification, 

 that of the ribs beginning as early as 

 the second month, the sternum some- 

 what late, in the sixth foetal month. 



Each rib contains at first one centre of 

 ossification, through the enlargement of 

 which the bony part is formed, while next 

 to the sternum a portion remains cartila- 

 ginous throughout life. In the eighth to 

 the fourteenth year there appear in the 

 capitulum and tuberculum of the rib, ac- 

 cording to SCHWEGEL and KOLLIKER, ac- 

 cessory centres, which fuse with the main 

 piece between the fourteenth and the twenty- 

 fifth year. 



The sternum (fig. 327) ossifies from nu- 

 merous centres, of which one arises in the manubrium, and from six to twelve 

 in its body. Between the sixth and twelfth years the latter begin to fuse 

 together into the three or four large pieces of which the body of the sternum 

 is composed. The xyphoid process remains partly cartilaginous, but acquires 

 a centre of ossification during childhood. 



Regarding the episternal pieces which appear on the manubrium, the text- 

 books of comparative anatomy and the article by HUGE should be consulted. 



Through inequalities in the development of the separate vertebral 

 and costal fundaments and through the fusions which take place here 

 and there are produced the different regions of the skeleton of the 

 trunk : the cervical, dorsal, and lumbar regions of the vertebral 

 column, the sacrum and coccyx. A correct understanding of these 

 skeletal parts is to be acquired only through embryology. 



Fig. 327. Cartilaginous sternum, 

 with portions of the ribs attached 

 and with several centres of ossi- 

 fication (kk), from a child two 

 years old. 



k, Cartilage ; kk, centres of ossifica- 

 tion ; sch, xyphoid process. 



