THE SCAPULA 141 



Extremities.^ — The sternal extremity of the clavicle presents a triangular 

 articular surface, directed medially, downward, and a little forward, sUghtly 

 concave from before backward and convex from above downward, which articu- 

 lates with a facet on the upper border of the manubrium sterni through an 

 interposed interarticular fibro-cartilage. 



Of the three angles, one is above and two below. The poslero-inferior angle is prolonged 

 backward, and so renders this surface considerably larger than that with which it articulates; 

 the superior angle receives the attachment of the upper part of the fil)ro-cartilage. The lower 

 part of the surface is continuous with a facet on the under aspect of the bone, medial to the 

 costal tuberosity, for the first costal cartilage. The circumference of the extremity is rough, 

 and gives attachment to the interclavicular ligament above and the anterior and posterior 

 sterno-clavicular ligaments in front and behind. 



The acromial extremity presents a smooth, oval, articular facet, flattened or 

 convex, directed slightly downward for the acromion; its border is rough, for the 

 attachment of the capsule of the acromio-clavicular joint. 



Structure. — The clavicle consists externally of a compact layer of bone, much thicker in 

 the middle and thinning out gradually toward the two extremities. There is no true medullary 



Fig. 170. — The Sternal Ends of Two Clavicles with Epiphyses. 



A, right clavicle from below and behind. B, left clavicle from below and behind. 



(From Royal College of Surgeons Museum.) 



Sternal epiphyses 



cavity, for the interior is occupied from end to end by cancellous tissue, the amount in the vari- 

 ous parts of the bone being in inverse proportion to the thickness of the outer compact shell 

 Ossification. — From observations made by F. P. Mall, D. C. L. Fitzwilliams, and E. Faw- 

 cett it seems almost certain that there are two centres of ossification of the shaft of the clavicle, 

 at the juncture of the middle and lateral thirds. They appear very early, about the fifth week 

 of embryonic life, and rapidly fuse. The ossific process extends medially and laterally along the 

 shaft toward the medial and lateral extremities, respectively. About the eighteenth year a 

 secondary centre appears at the sternal end and forms a small epiphysis which joins the shaft 

 about the twenty-fifth year. 



THE SCAPULA 



The scapula (figs. 171, 172) is a large flat bone, triangular in shape, situated 

 on the dorsal aspect of the thorax, between the levels of the second and seventh 

 ribs. Attached to the trunk by means of the clavicle and various muscles it 

 articulates with the lateral end of the clavicle at the acromio-clavicular joint, 

 and with the humerus at the shoulder-joint. The greater part of the bone con- 

 sists of a triangular plate known as the body, from which two processes are 

 prolonged: one anterior in position, is the coracoid; the other, posterior in posi- 

 tion, is the spine, which is continued laterally into the acroniion. 



The body presents for examination two surfaces, three borders, and three 

 angles. The costal (anterior) surface, or venter, looks considerably medialward, 

 is deeply concave, forming the subscapular fossa, and marked by several oblique 

 lines which commence at the posterior border and pass obliquely upward and 

 laterally; these lines or ridges divide the surface into several shallow grooves, 

 from which the subscapularis takes origin, whilst the ridges give attachment to 

 the tendinous intersections of that muscle. The lateral third of the surface is 

 smooth and overlapped by the subscapularis, whilst medially are two small flat 

 areas in front of the upper and lower angles respectively, but excluded from the 

 subscapular fossa by fairly definite lines and joined by a ridge which runs close 

 to the vertebral border. The ridge and its terminal areas serve for the insertion 

 of the serratus anterior {magyius). 



The dorsal (posterior) surface is generally convex and divided by a prominent 

 plate of bone — the spine— into two unequal parts. The hollow above the spine 

 is the supraspinous fossa and lodges the supraspinatus muscle. The part below 



