THE STEKNUM. 



107 



Incisura jugularis 

 Clavicular facet 



III. Rib cartilag 



the manubrium or handle ; the corpus sterni or body ; and the processus xiphoideus 

 (or xiphoid cartilage). Of 'these the body is formed by the fusion in early life 

 of four segments or sternebrse. 



The manubrium or superior part, usually separate throughout life from the 

 rest of the bone, though occasionally fused with it, is of a flattened triangular 

 form. The anterior surface, slightly saddle -shaped, affords attachment to the 

 fibres of the pectoralis major and sterno-mastoid muscles. It is bounded above by 

 a thick border, the lateral parts of which are hollowed out obliquely to form the 

 facets (incisurse claviculares) for the sternal ends of the clavicles ; around the 

 facets, which have an upward, lateral, 

 and slightly backward direction, the bone 

 is faintly lipped. In the interval be- 

 tween these two facets there is a slight 

 notch (incisura jugularis) which forms the 

 floor of the characteristic hollow seen 

 at the root of the neck anteriorly the 

 supra - sternal notch. The borders are 

 excavated immediately below the clavi- 

 cular facets for the reception of the 

 cartilages of the first ribs. Below this, 

 the margin of the bone slopes medially, 

 and is sharp, except inferiorly, where it 

 presents a facet which supports a part of 

 the second costal cartilage. Around this 

 the bone is usually lipped anteriorly. The 

 upper angles correspond to the ridge 

 separating the clavicular facets from the 

 first costal facets : whilst the lower angle, 

 which may be regarded as cut across trans- 

 versely, forms the surface which is united 

 by cartilage to the body of the sternum. 

 The anterior edge of this surface is usually 

 prominent. The posterior aspect of the 

 manubrium is smoother than the anterior, 

 is pierced by numerous foramina, and IV - Rib cartila e 

 is slightly concave from side to side and 

 above downwards. Here are attached 

 some of the fibres of the sterno-hyoid and 

 sterno-thyreoid muscles. 



The body or middle part of the 

 sternum, usually twice the length and 

 from half to two- thirds the width of 

 the manubrium, displays evidence of its VI . Rib cartilage 

 composite nature. If the planum sternale, 

 (anterior surface) which is slightly convex 

 from above downwards, and faintly con- 

 cave from side to side, is carefully ex- 

 amined, three ill-marked ridges may be 

 seen crossing it transversely ; these corre- 

 spond to the lines of fusion between the 

 four primitive segments. To this surface 

 of the bone the great pectoral muscles are 

 extensively attached on either .side of the 

 median plane. The borders are thick and interrupted at points corresponding 

 to the transverse lines already mentioned by U-shaped hollows, the edges of which 

 are more or less projecting. These are for the reception of the cartilages of the third, 

 fourth, and fifth ribs. The superior border is united to the manubrium above, and 

 forms with it an angle of variable degree the angulus sterni (sternal angle). 

 A small facet is formed at the expense of the lateral extremity oft this 



V. Rib cartilage 



VII. Rib cartilag 



Processus xiphoideus 



FIG. 121. THE STERNUM (anterior view). 



