THE CLAVICLE, OR COLLAR BONE 169 



I 



THE SHOULDER GIRDLE. 



The shoulder girdle consists of the clavicle and the scapula. 



The Clavicle, or Collar Bone (Clavicula). 



The clavicle 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 over 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, while at the same 

 time it allows of great latitude of motion in the arm. 1 It presents a double curva- 

 ture when looked at in front, the convexity being forward at the sternal end and 

 the concavity at the scapular end. Its outer third is flattened from above downward, 

 and extends, in the natural position of the bone, from a point opposite the cora- 

 coid 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. 



Outer 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. 



Surfaces. The upper surface is flat, rough, marked by impressions for the 

 attachment of the Deltoid in front and the Trapezius behind; between these two 

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

 flattened. At its posterior border, a little external to the point where the pris- 

 matic joins with the flattened portion, is a rough eminence, the conoid tubercle 

 (tuberositas coracoidea) ; 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. 



Borders. The anterior border is concave, thin, and rough, and gives attachment 

 to the Deltoid; it occasionally presents, at its inner end, at the commencement of 

 the deltoid impression, a tubercle, the deltoid tubercle. The posterior border is 

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



Inner 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. 



Borders. The anterior border is continuous with the anterior margin of the flat 

 portion, and separates the anterior surface from the posterior; at the inner half 

 of the clavicle it forms the lower boundary of an elliptical space for, the attach- 

 ment of the clavicular portion of the Pectoralis major, and approaches the pos- 

 terior border of the bone; it coincides with the anterior margin of the subclavian 

 groove. The superior border is continuous with the posterior margin of the flat 

 portion, and separates the anterior from the posterior 1 surface. At its commence- 

 ment it is smooth and rounded, becomes rough toward the inner third for the 

 attachment of the Sternomastoid muscle, and terminates at the upper angle of 

 the sternal extremity. The posterior or subclavian border separates the posterior 



most . f ^ ui M=> _ 



of them as, for instance, in a large number of the carnivora it is merely a rudimentary bone suspended among 

 the muscles, and not articulating with the scapula or sternum. 



