THE STERNUM 133 



ceeding four segments fuse together, forming the body ; whilst the lowest segment, 

 also distinct until middle life, is represented by the xiphoid process. 



In its natural position the sternum is inclined obliquely from above downward and forward, 

 and corresponds in length to the spine from the third to the ninth thoracic vertebra. It is not 

 of equal width throughout, being broader above at the manubrium and narrow at the junction 

 of this piece with the body. Toward the lower part of the body the sternum again widens, and 

 then suddenly contracts at its junction with the xiphoid process which constitutes the narrowest 

 part. 



The manubrium or first piece of the sternum forms the broadest and thickest 

 part of the bone, and is of a somewhat triangular form with the base directed up- 

 ward and the apex downward. It presents for examination two surfaces and four 

 borders. The anterior surface [planum sternale] is largely subcutaneous. It is 

 slightly convex and directed obhquely upward and forward, is smooth and 

 gives origin on each side to the sternal head of the sterno-mastoid and the pedoralis 

 major. The posterior surface, almost flat, and directed downward and backward, 

 affords origin near the lateral margins on each side, to the sterno-hyoid muscle 

 above and the sterno-thyreoid muscle below. Of the four borders, the superior 

 is the longest and much the thickest. In the middle is a curved, non-articular 

 depression, called the jugular (interclavicular) notch, to which the fibres of the 

 interclavicular ligament are attached, and at either end is an oval articular 

 surface [incisura clavicularis], somewhat saddle-shaped and directed upward, 

 backward, and laterally for the reception of the medial end of the clavicle. The 

 circumference of the articular surface gives attachment to the sterno-clavicular 

 ligaments. The lateral borders slope from above downward and medially and 

 each presents an irregular surface above for the first costal cartilage and a small 

 facet below, which, with an adjoining facet on the body, forms a notch for the 

 second costal cartilage. The two articular surfaces are separated by a narrow 

 curved edge in relation with the internal intercostal muscle of the first space. The 

 lower border is thick and short and presents an oval rough surface whicli articu- 

 lates with the upper border of the body, forming the sternal synchondrosis. 

 The two opposed surfaces are separated by a fibro-cartilaginous disc, which may, 

 however, become partially ossified in advanced age, and at the position of the 

 joint there is usually an angle — the angle of the sternum — which can be felt as a 

 transverse ridge beneath the skin. This is useful in locating the position of the 

 second rib in the living subject. 



The body (gladiolus) or second piece of the sternum is longer, narrower, and 

 thinner than the manubrium. It is "widest opposite the notches for the fifth 

 costal cartilages and becomes narrower above and below. The anterior surface 

 is flat, directed upward and forward, and marked by three transverse elevations 

 which indicate the lines of junction of its four component parts. It gives attach- 

 ment on each side to fibres of the pedoralis major, and occasionally presents a 

 foramen — the sternal foramen— situated at the junction of the third and fourth 

 pieces of the bone. The posterior surface is slightly concave, marked by lines 

 corresponding to those on the anterior surface, and below gives attachment on 

 each side to fibres of the iransversus thoracis {triangularis sterni). The lateral 

 borders present four whole notches [incisurse costse] and two half-notches on 

 each side, which articulate with the costal cartilages of the second to the seventh 

 ribs inclusive; the two half -notches are completed by corresponding notches on 

 the manubrium and the xiphoid process. Between the articular depressions the 

 lateral border is curved and in relation to the internal intercostal muscles. 



In order to appreciate the nature of these articular notches, it is advantageous to study 

 the sternum in a young subject. Each typical sternebra presents four angles at each of which is 

 a demi-notch. Between every two sternebrae there is an intersternebral disc so that when in 

 position, each notch for a costal cartilage is formed by a sternebra above and below and an inter- 

 sternebral disc in the middle, thus repeating the relation of the rib-head to the vertebral centre. 

 Later in life these fuse more or less together, except in the case of the first and second sternehrse, 

 which usually remain separate to the end of life. The first (pre-sternum) is the most modified 

 of all the sternebrse, and differs from them in the fact that the costal cartilage of the first rib 

 is continuous with it, and in fact that it supports the clavicles. Occasionally a rounded pisi- 

 form bone is seen on each side, medial to the articular notch for the clavicle; these are the 

 supra-sternal bones. 



The superior border of the sternal body presents an oval facet for articulation 

 (synchondrosis) with the manubrium. The inferior border is short and articu- 



