THE BODY WALL, RIBS AND STERNUM. 287 



buds from the ventral ends of the ribs, to which they correspond, 

 and his conclusions are supported by the evidence of comparative 

 anatomy. 



In its development the sternum passes through three stages — 

 fibrous, cartilaginous and bony. 



1. Fibrous Stage.— At the 6th week (Fig. 232) the costal 

 cartilages are already chondrified. The mesoblast on each side of 

 the median line, in which they end, has become condensed, and 

 forms the membranous basis of the two sternal bars (Paterson). 

 The bars begin to fuse together in front. 



2. Cartilaginous Stage. — The mesoblast of each sternal bar 

 begins to chondrify in the intervals between the ends of the 

 costal cartilages. The process of chondrification and fusion proceed 

 apace, and by the commencement of the third month the segments 

 of each side have united to form the cartilaginous sternal bars 

 (Paterson). At the same time, the two bars gradually unite, the 

 cellular tissue between them becoming chondrified. Thus a single 

 cartilaginous plate formed out of the two cartilaginous bars is 

 formed. Fibrous joints are subsequently formed between the 

 presternum and mesosternum and between the mesosternum and 

 ensiform process. A fibrous and synovial joint is also developed 

 at the union of the costal cartilages with the sternum, except in 

 the case of the first pair. 



3. Ossification. — A centre appears for each sternebra ; those for 

 the third and fourth of the mesosternum are usually double, oue 

 being placed on each side. The centres for the 4th segment are 

 more frequently absent than present (Paterson). The centre for 

 the presternum (there may be two or even more) appears about 

 the 4th month ; the centres behind appear in order ; that for the 

 4th sternebra of the mesosternum appearing about the time of 

 birth, that for the ensiform after birth. The process of fusion of 

 segments begins behind about puberty; the segments of the 

 mesosternum are united together by the 30 th year. Occasionally 

 a median foramen may be seen in the sternum ; it is due to im- 

 perfect union of the sternal bars. 



The Presternum is complex in nature. It probably represents 

 more than the simple sternebra between the first and second pairs 

 of ribs. Part of it may belong to the segment in front, the last 

 cervical. When the last (7th) cervical rib is fully developed it 



