DEVELOPMENT OF THE STERNUM. 



157 



Fig. 181. 

 B 



Development and Subsequent Changes. — The cartilaginous bars repre- 

 senting the ribs in the early embryo end in front in a strip connecting them from the 

 first to the ninth, which approaches its fellow above and recedes from it below. 

 The union of these two strips, which begins above, forms the future sternum as far 

 as the ensiform cartilage. Thus at this early stage there are nine sternal ribs. While 

 the mesosternum is forming by the union of the lower part, a portion of the ninth 

 strip separates itself from the rest to fuse with its fellow for the ensiform cartilage, 

 and the remainder of the ninth joins the eighth, which, as a rule, itself later recedes 

 from the sternum. 



The original cartilaginous strips having fused, points of ossification first appear 

 in the manubrium about the sixth month of foetal life. There is one chief one and 

 a varying number of small ones variously disposed. Sometimes it ossifies in a larger 

 upper and a smaller lower piece. In the latter months, before birth, several points 

 appear in the mesosternum. The first piece generally has a single centre, those below 

 two in pairs. At birth one usually finds ossification begun in the first three pieces 

 of the body. The centre for the last piece of the body begins to ossify at a very 

 variable time. We have seen bone in it at thirteen days and have found none at 

 seven years. Perhaps three years is not far from the average. The centre, or cen- 

 tres, for this last piece of the body are placed in its upper part. Its cartilage is 

 directly continuous with that of the 

 ensiform, the line of demarcation 

 being determined by the difference 

 in thickness, the ensiform being 

 thinner and continuing the plane of 

 the posterior surface. Thus, the 

 lower part of the last piece may 

 continue cartilaginous for a con- 

 siderable time. A centre in the 

 ensiform is sometimes seen at three, 

 but may not come for several years 

 later. The four pieces of the meso- 

 sternum join one another from be- 

 low upward, the union being com- 

 pleted on the posterior surface first. 

 The process is extremely variable. 

 The only points regarding which we 

 are certain are that it is more rapid 

 than is usually stated and that the 

 body is almost always in one piece 

 at twenty. The fourth piece of the body joins the third at about eight, the third 

 joins the second at about fifteen, and the second unites with the first usually at 

 eighteen or nineteen. We once saw all four pieces distinct at eighteen, but in 

 one or two instances only have we found the body incomplete after twenty. The 

 amount of bone in the ensiform at twenty is still small. The adult condition, except 

 that the ensiform gradually becomes wholly bone, may persist to extreme old age. 

 The ensiform often joins the body after middle age, rarely before thirty. The 

 union of the manubrium and the body is rare, and appears to be the result of a con- 

 stitutional tendency rather than of age, as in our observations we have repeatedly 

 found it under fifty, and have seen all three pieces united at twenty-five. The 

 different pieces are more apt to fuse in man than in woman. 



o c 



Ossification of the sternum, /i, at sixth fcetal month ; a, centre 

 for manubrium. B, at birth ; a, for manubrium ; b, c, d, for seg- 

 ments of body. C, at about ten years ; a, manubrium ; b, c, d, seg- 

 ments of body ; <?, ensiform cartilage. 



ARTICULATIONS OF THE THORAX. 



The joints uniting the bones taking part in the formation of the bony thorax 

 constitute two general groups, the Anterior and the Posterior Thoracic Articula- 

 tions. The former include the joints between the pieces of the sternum, those be- 

 tween the sternum and the costal cartilages, and those between the costal cartilages ; 

 the latter, or the costo-vertebral articulations, include those between the vertebrae 

 and the ribs. 



