THE STERNUM. 



231 



its pointed extremity it gives attachment to the linea alba. This portion of the 

 sternum is very various in appearance, being sometimes pointed, broad, and thin, 

 sometimes bifid or perforated by a round hole, occasionally curved or deflected 

 Considerably to one or the other side. 



Structure. The bone is composed of delicate cancellous structure, covered by 

 a thin layer of compact tissue, which is thickest in the manubrium between the 

 articular facets for the clavicles. 



Development. The sternum, including the ensifonn appendix, is developed by 

 j ntres : one for the first piece or manubrium, four for the second piece or 



for jirst piece, two or more centra. 



for third ] 



for fourth } 2, placed laterally. 



FIG. 181. Development of the sternum by six 

 centres. Time of appearance. 



Arrest of development 

 of lateral piece*, producing 



-Sternal fissure, and 

 -Sternal foramen. 



FIG. 182. Time of union of sternum. 



gladiolus, and one for the ensiform appendix. Up to the middle of foetal life the 

 sternum is entirely cartilaginous, and when ossification takes place the ossific 



I Barely unite, 



i except in old age. 



Between puberty 

 and the Soth year. 



Soon after puberty. 



Partly cartilaginous to 

 advanced life. 



FIG. 184. Peculiarities in mode 

 of union of sternum. 



5 1st year after 

 birth.' 



1 for ensiform } d ^ im 



cartilage J 

 FIG. 183. Peculiarities in number of centres of 



sternum. 



granules are deposited in the middle of the intervals between the articular depres- 

 - for the costal cartilages, in the following order (Fig. 181): In the first 

 piece, between the fifth and sixth months; in the second and third, between the 

 sixth and seventh months; in the fourth piece, at the ninth month; in the fifth, 

 within the first year or between the first and second years after birth ; and in the 

 ensiform appendix, between the second and the seventeenth or eighteenth years, by a 

 single centre which makes its appearance at the upper part and proceeds gradually 

 downward. To these may be added the occasional existence, as described by 

 Breschet. of two small episternal centres, which make their appearance one on each 

 side of the interclavicular notch. They are probably vestiges of the episternal bone 

 of the monotremata and lizards. It occasionally happens that some of the segments 

 are formed from more than one centre, the number and position of which vary 

 (Fig. 183). Thus, the first piece may have two, three, or even six centres. When 



