462 THE STERNUM. 



the upper ; it is wide in the centre, and narrow at either 

 end; its sides furnish cavities, complete for the third, 

 fourth, fifth, and sixth ribs, and half cavities for the second 

 and seventh ribs. These cavities approach nearer each 

 other as they descend, so that the sixth and seventh are in 

 contact. 



The inferior, or third bone, called ensiform, or xiphoid 

 cartilage, being usually in the cartilaginous state in the 

 adult, is the smallest of the three pieces. It is thin, and 

 varies much as to form and size, being sometimes pointed, 

 sometimes bifid, sometimes thick looking forward, and 

 then backward, and occasionally perforated with a central 

 foramen. Its base is united to the lower extremity of the 

 middle bone ; its sides have the transverse muscles of the 

 abdomen attached to them, and its point is connected with 

 the linea alba. 



The sternum, as a whole, has its anterior surface flat, or 

 a little convex is covered by the aponeurosis of the pecto- 

 ral muscles has the tendons of the sterno mastoid muscles 

 attached to its superior portion, and is crossed by trans- 

 verse lines, marking its original divisions into as many as 

 six pieces. 



Its posterior surface is smooth, somewhat concave, and 

 covered by a shining periosteum. It also presents trans- 

 verse lines, though not so distinct as those in front. The 

 structure of the sternum is spongy, covered by a thin com- 

 pact layer. Its development is from a number of points 

 varying from six to fourteen, which do not unite till late 

 in life. The osseous centres for the first bone of the ster- 

 num are one or two in number, and make their appearance 

 during the fifth and sixth months those of the second and 

 third soon after and the fourth at the close of foetal life or 

 shortly after birth. Ossification of the ensiform cartilage 

 varies from the second to the eighteenth year. The several 

 pieces of the sternum commence uniting from below up- 

 ward. The fourth and third are seen to join^ about puberty, 

 the third and second between twenty and twenty-five, and 

 the second and first between twenty-five and thirty years. 



