78 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



THE BONES OF THE SHOULDER GIRDLE 



The Clavicle 



The clavicle (key) passes out, back, and slightly upward 

 from the summit of the sternum to the acromion, and con- 

 nects the upper limb to the trunk. It is curved like the letter 

 /. The inner curve is convex forward, and occupies two- 

 thirds of the bone; this part is prismatic. The outer third 

 of the bone is concave in front, and is flattened from above 

 down. 



The superior surfaces of these two portions are continuous; 

 the inferior surfaces are continuous; the anterior border of 

 the outer portion runs into the anterior surface of the inner; 

 and the posterior border of the outer is continuous with the 

 posterior surface of the inner. 



The superior surface is broad externally and largely sub- 

 cutaneous; at its centre it may present a canal for the supra - 

 clavicular nerve; the sternocleidomastoid is attached to the 

 inner part. The (inferior surface is reduced to a rough border 

 on the outer third, where it gives attachment to the deltoid, 

 and may present a deltoid tiihcrcle. The pectoralis major is 

 attached to the inner half. 



The posterior surface is a border externally to one-third 

 the extent of the bone, and gives attachment to the trapezius. 

 In the middle of this surface is the orifice of a medullary canal 

 directed outward. (In bones having but one secondary centre 

 the medullary artery runs from it.) Internally close to the 

 sternal end this surface gives attachment to a part of the 

 sternohyoid muscle. 



The inferior surface is narrow at its sternal end, widening 

 out to the full width of the bone in its outer third. It shows 

 internally a rough impression or costal tuberosity about 1 inch 

 long, for the rhomboid ligament; internal to it is a small facet 

 for articulation with the cartilage of the first rib; external 

 to it, a groove passing beyond the middle third for the sub- 

 clavius muscle; the groove may show a longitudinal ridge for 

 an intermuscular septum. On the posterior border, at the 

 junction of the outer and middle thirds, is the conoid tubercle 



