THE BONES OF THE TRUf^R 



49 



The mesosternum presents two surfaces and four borders. The 

 interior surface is marked by three transverse hnes, which indicate 

 the places of junction of the original four sternebrae. At either 

 side it gives origin to a large portion of the pectorahs major. The 

 posterior surface presents traces 

 of the highest transverse hne, 

 but the lower two have usually 

 become effaced. Adjacent to 

 each lateral border it gives origin 

 to a portion of the triangularis 

 sterni, as high as the level of 

 the third costal facet. Each 

 lateral border presents a series of 

 costal facets, disposed as follows : 

 close to the upper extremity 

 there is a demi-facet for a 

 portion of the second costal 

 cartilage ; opposite each of the 

 three transverse lines there is 

 an entire facet for the third, 

 fourth, and fifth costal carti- 

 lages ; and on the side of the 

 fourth segment of the body 

 there are usually one entire facet 

 and one demi-facet lying close 

 together, the former being for 

 the sixth costal cartilage, and 

 the latter for a portion of the 

 seventh. Altogether there are 

 usually four entire facets and 

 two demi- facets on each lateral 

 border, the demi - facets being 

 situated one at either extremity. 

 Sometimes, however, the inferior 

 demi - facet is replaced by an 

 entire facet for the whole of the 

 seventh costal cartilage. Each 

 of the upper three entire facets 

 is made up of the contiguous 

 demi -facets of two adjacent 

 sternebrae, as in the bodies of 

 most of the thoracic vertebrae. 

 The superior border of the meso- 

 sternum articulates, as stated, 

 with the presternum. The inferior border, which is very narrow, 

 articulates with the metasternum, an intersternebral disc inter- 

 vening until about the fortieth year, when osseous union usually 

 takes place. 

 The metasternum is subject to much variety as regards condition, 



Fig. 34. — The Sternum 

 (Lateral View). 



