CLAVICLE. 121 



OF THE UPPER EXTREMITY. 



The upper extremity consists of the arm, the forearm, and the hand. Its con- 

 tinuity with the trunk is established by means of the shoulder, which is homo- 

 logous with the innominafe or haunch bone in the lower limb. 



OF THE SHOULDER. 



The shoulder is placed upon the upper part and side of the chest, connecting 

 the upper extremity to the trunk ; it consists of two bones, the clavicle, and the 

 scapula. 



THE CLAVICLE. 



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

 shoulder. 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 internally with the upper border of the sternum, 

 and with the acromion process of the scapula by its outer extremity ; serving to 

 sustain the upper extremity in the various positions which it assumes, whilst, at 

 the same time, it allows it great latitude of motion. The horizontal plane of the 

 clavicle is nearly straight ; but in the vertical plane it presents a double curvature, 

 the convexity being, in front, at the sternal end ; and, behind, at the scapular end. 

 Its outer third is flattened from above downwards, and extends, in the natural 

 position of the bone, from the coracoid process to the acromion. Its inner two- 

 thirds are of a cylindrical form, and extend from the sternum to the coracoid 

 process of the scapula. 



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

 wards, 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 in front, for the attachment of 

 the Deltoid ; behind, for the Trapezius ; between these two impressions, externally, 

 a small portion of the bone is subcutaneous. The under surface is flattened. At 

 its posterior border, at the junction of the prismatic 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 

 forwards and outwards to near the outer end of the anterior border ; it is called 

 .the oblique line, and affords attachment to the trapezoid ligament. The anterior 

 border'is concave, thin, and rough; it limits the attachment of the Deltoid, and 

 occasionally presents, near 'the centre, a tubercle, the deltoid tubercle, which is 

 sometimes distinct in the living subject. The posterior border is convex, rough, 

 broader than the anterior, and gives attachment to the Trapezius. 



Internal or Cylindrical Portion. The cylindrical 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 unoccupied interval 

 between the attachment of the Pectoralis major and Deltoid muscles ; about the 

 centre of the clavicle it divides to inclose an elliptical space for the attachment 

 of the clavicular portion of the Pectoralis major. This space extends inwards as 

 far as the anterior margin of the sternal extremity. 



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 towards the inner third for the attachment 

 of the Sterno-cleido-mastoid muscle, and terminates at the upper angle of the 

 sternal extremity. 



