THE CLAVICLE. 199 



upper and anterior fibres of the trapezius muscle. The inferior surface, in- 

 clined downwards and backwards, is marked close to the sternal end by 

 an irregular elongated 

 impression (tuberositas 

 costalis), often deeply 

 pitted, for the attach- 

 ment of the costo- 

 clavicular ligament, which 



Unites it tO the Cartilage ^k^SBiiB^SBIII^' Costo-clavicular ligament 



of the first rib. Lateral 

 to this the shaft is 



fharmpllprl V>V < o-rnmrA FlG> 187> ~ THE UNDER SUKFACE OP THE RIGHT CLAVICLE WITH THE 



by a groove ATTACHMENTS OF THE MUSCLES MAPPED OUT. 



which terminates close 

 to the coracoid impression ; into this groove the subclavius muscle is inserted. 



The acromial end of the bone is flattened and compressed from above down- 

 wards, and expanded from before backwards ; its anterior edge is sharp and well 

 defined, and gives attachment to the deltoid muscle, which also spreads over part 

 of its upper surface. Its posterior margin is rougher and more tubercular, and 

 provides a surface for the insertion of the trapezius. The area of the superior 

 surface between these two muscular attachments is smooth and subcutaneous. The 

 lateral edge of this forward-turned part of the bone is provided with an oval facet 

 (facies articularis acromialis) for articulation with the acromion of the scapula ; the 

 margins around this articular area serve for the attachment of the capsule of the 

 joint. The inferior surface of the acromial end of the bone is traversed obliquely 

 from behind forwards and laterally by a rough ridge or line called the trapezoid 

 ridge. The posterior extremity of this ridge, as it abuts on the posterior border 

 of the bone, forms a prominent process, the tuberositas coracoidea; to these, 

 respectively, are attached the trapezoid and conoid portions of the coraco-clavicular 

 ligament. 



The morphology of the clavicle is of special interest. Its presence is associated with 

 the freer use and greater range of movement of the fore-limb, such as are necessary for 

 its employment for more specialised actions than those of mere progression. In conse- 

 quence of these requirements, the limb, and with it the scapula, become further removed 

 from the trunk, and so the support which the blade bone received through the union of 

 its coracoid element with the sternum, as in birds and reptiles, and to some extent in the 

 lowest mammals, is withdrawn. Some substitute, however, is necessary to meet the 

 altered conditions, and in consequence a new element is introduced in the form of a 

 clavicle. The origin of this bone appears to be intimately associated with the precoracoid 

 element met with in amphibia or reptiles, but whereas the precoracoid is always laid 

 down in cartilage, which, however, not infrequently disappears, the clavicle develops in 

 the membrane overlying the precoracoid cartilage. In the course of its development it 

 may become intimately associated with the remains of that cartilage. Thus, it is probable 

 that the articular discs at the sterno-clavicular and acromio-clavicular joints, as well 

 as the sternal articular end of the clavicle, represent persistent portions of the primitive 

 cartilage, whilst it is possible that the supra-sternal ossicles occasionally present may 

 be also derived from it. In this way, in its most specialised form, a secondary support is 

 established between the sternum and scapula, which serves as a movable fulcrum, and 

 greatly enhances the range of movement of the shoulder girdle. 



Nutrient Foramina. The foramina for the larger nutrient vessels, offsets of the transverse 

 scapular artery, of which there may be one or two directed laterally, are usually found about 

 the middle of the posterior border, or, it may be, opening into the floor of the groove for the 

 subclavius muscle. 



Ossification. The clavicle in man is remarkable in commencing to ossify before any 

 other bone in the body ; this occurs as early as the fifth or sixth week of fo3tal life. 

 The shaft is ossified from two primitive centres (Mall). These are preceded by a curved 

 rod of connective tissue on the interior of which are developed two masses of a peculiar 

 precartilaginous nature, one, the sternal, placed medially, lies above and Overlaps in 

 front the acromial mass, which is placed laterally. In each of these near their approxi- 

 mated ends a centre of ossification appears. These, subsequent to the fusion of the 



