THE BONES OF THE UPPER LIMB 177 



into an antero-snperior and aniero-infefiot border, which enclose 

 between them the anterior surface. The posterior border bifurcates, 

 over the inner two-thirds, into a postero-superior and posfero- 

 inferior border, the latter forming the posterior lip of the sub- 

 clavian groove. The antero-superior and postero-superior borders 

 limit the superior surface, the postero-superior and postero-inferior 

 limit the posterior surface, the postero-inferior and antero- inferior 

 limit the inferior surface, and, as stated, the antero-inferior and 

 antero-superior limit the anterior surface. 



The sternal extremity is enlarged and covered by cartilage. As 

 viewed on end, it is somewhat triangular, and presents a prominent 

 posterior angle which is directed downwards, inwards, and back- 

 wards. The surface is concave from before backwards, and convex 

 from above downwards, and it articulates \nth the clavicular 

 impression on the upper border of the presternum, an inarticular 

 fibro-cartilage interv^ening. The circimiference of the sternal end 

 is rough for the stemo-clavicular and interclavicular ligaments, 

 except inferiorly, where there is a narrow strip for the first costal 

 cartilage. 



The acromial extremity presents an oval facet for the acromion 

 process of the scapula. 



The clavicle receives its blood-supply from the suprascapular 

 artery- and the thoracic axis. 



Articulations. — Internally with the presternum cind first costal 

 cartilage, and externally v^ith the acromion process of the scapula. 



Structnre. — The exterior is composed of compact bone which is 

 thickest towards the centre, and the interior of coarse cancellated 

 tissue, the principal lamellae being disposed longitudinally. The 

 clavicle has no medullary canal, but, towards the centre of the 

 shaft, the medullary spaces of the cancellated tissue are of large size. 



Varieties. — (i) There may be a deltoid spine. (2) The superior surface 

 may present a small sUt-Uke aperture, N^ariously situated, for one of the 

 descending branches of the cervical plexus of nerves. 



The Clavicle of the Female. — The bone is smoother, more slender, 

 straighter, and more cylindrical over its inner two-thirds, than that 

 of the male. 



Ossification. — The clavicle, which is the earliest bone to ossify, has two 

 primary centres and one secondary centre. It is preceded by a rod of con- 

 Appears in the 18U1 year, 

 and joins about 25 



I 

 L 



I 



Appears in the 6th week 

 (intra-uterine) 



FiG.loa— OssiFic.vrioN of the Cl.wicle. 



nective tissue. Within the two halves of this rod collections of ' precartila- 

 ginous tissue ' are formed, and within these at their contiguous ends the 

 primary centres appear about the 6th week. Subsequently the precartilag- 



12 



