THE CLAVIC'LE. 135 



The Clavicle. 



The Clavicle (clavis, a key), or collar-bone, forms the anterior portion of the 

 shoulder girdle. It is a long bone, curved somewhat like the italic letter/, and 

 placed nearly horizontally at the upper and anterior part of the thorax, immediately 

 above the first rib. It articulates by its inner extremity with the upper border of 

 the sternum, and by its outer extremity with the acromion process of the scapula, 

 serving to sustain the upper extremity in the various positions which it assumes, 

 whilst at the same time it allows of great latitude of motion in the arm. 1 It 

 presents a double curvature when looked at in front, the convexity being forward 

 at the sternal end and the concavity at the scapular end. Its outer third is flat- 

 tened from above downward, and extends, in the natural position of the bone, from 

 a point opposite the coracoid process to the acromion. Its inner two-thirds are of 

 a prismatic form, and extend from the sternum to a point opposite the coracoid 

 process of the scapula. 



External or Flattened Portion. The outer third is flattened from above down- 

 ward, so as to present two surfaces, an upper and a lower ; and two borders, 

 an anterior and a posterior. The upper surface is flattened, rough, marked by 

 impressions for the attachment of the Deltoid in front and the Trapezius behind ; 

 between these two impressions, externally, a small portion of the bone is sub- 

 cutaneous. The under surface is flattened. At its posterior border, a little 

 external to the point where the prismatic joins with the flattened portion, is a rough 

 eminence, the conoid tubercle ; this, in the natural position of the bone, surmounts 

 the coracoid process of the scapula and gives attachment to the conoid ligament. 

 From this tubercle an oblique line, occasionally a depression, passes forward and 

 outward to near the outer end of the anterior border ; it is called the oblique line 

 or trapezoid ridge, and affords attachment to the trapezoid ligament. The anterior 

 border is concave, thin, and rough, and gives attachment to the Deltoid; it occa- 

 sionally presents, at its inner end, at the commencement of the deltoid impression, 

 a tubercle, the deltoid tubercle, which is sometimes to be felt in the living subject. 

 Tho posterior border is convex, rough, broader than the anterior, and gives attach- 

 ment to the Trapezius. 



Internal or Prismatic Portion. The prismatic portion forms the inner two- 

 thirds of the bone. It is curved so as to be convex in front, concave behind, and 

 is marked by three borders, separating three surfaces. The anterior border is 

 continuous with the anterior margin of the flat portion. At its commencement it 

 is smooth, and corresponds to the interval between the attachment of the Pectoralis 

 major and Deltoid muscles; at the inner half of the clavicle it forms the lower 

 boundary of an elliptical space for the attachment of the clavicular portion of the 

 Pectoralis major, and approaches the posterior border of the bone. The superior 

 border is continuous with the posterior margin of the flat portion, and separates 

 the anterior from the posterior surface. At its commencement it is smooth and 

 rounded, becomes rough toward the inner third for the attachment of the Sterno- 

 mastoid muscle, and terminates at the upper angle of the sternal extremity. The 

 posterior or subclavian border separates the posterior from the inferior surface, and 

 extends from the conoid tubercle to the rhomboid impression. It forms the pos- 

 terior boundary of the groove for the Subclavius muscle, and gives attachment to a 

 layer of cervical fascia covering the Omo-hyoid muscle. The anterior surface is 

 included between the superior and anterior borders. It is directed forward and a 

 little upward at the sternal end, outward and still more upward at the acromial 

 extremity, where it becomes continuous with the upper surface of the flat portion. 

 Externally, it is smooth, convex, nearly subcutaneous, being covered only by the 



1 The clavicle acts especially as a fulcrum to enable the muscles to give lateral motion to the arm. 

 It is accordingly absent in those animals whose fore limbs are used only for progression, but is present 

 for the most part in those animals whose anterior extremities are clawed and used for prehension, 

 though in some of them as, for instance, in a large number of the carnivora it is merely a rudi- 

 mentary bone suspended among the muscles, and not articulating either with the scapula ot 

 sternum. 



