THE CLAVICLE. 137 



inward and a little downward and forward ; and presents an articular facet, 

 concave from before backward, convex from above downward, which articulates 

 with the sternum through the intervention of an interarticular fibro-cartilage ; the 

 circumference of the articular surface is rough, for the attachment of numerous 

 ligaments. The posterior border of this surface is prolonged backward, so as to 

 increase the size of the articular facet ; the upper border gives attachment to the 

 interarticular fibro-cartilage, and the lower border is continuous with the costal 

 facet on the inner end of the inferior or subclavian surface, which articulates with 

 the cartilage of the first rib. 



The outer or acromial extremity, directed outward and forward, presents a 

 small, flattened, oval facet, which looks obliquely downward, for articulation with 

 the acromion process of the scapula. The circumference of the articular facet is 

 rough, especially above, for the attachment of the acromio-clavicular ligaments. 



Peculiarities of the Bone in the Sexes and in Individuals. In the female the 

 clavicle is generally shorter, thinner, less curved, and smoother than in the male. 

 In those persons who perform considerable manual labor, which brings into con- 

 stant action the muscles connected with this bone, it becomes thicker and more 

 curved, its ridges for muscular attachment become prominently marked. The right 

 clavicle is generally longer, thicker, and rougher than the left. 



Structure. The shaft, as well as the extremities, consists of cancellous tissue, 

 invested in a compact layer much thicker in the middle than at either end. The 

 clavicle is highly elastic, by reason of its curves. From the experiments of Mr. 

 Ward it has been shown that it possesses sufficient longitudinal elastic force to 

 project its own weight nearly two feet on a level surface when a smart blow is 

 struck on it ; and sufficient transverse elastic force, opposite the centre of its 

 anterior convexity, to throw its own weight about a foot. This extent of elastic 

 power must serve to moderate very considerably the effect of concussions received 

 upon the point of the shoulder. 



Development. By two centres : one for the shaft and one for the sternal 

 extremity. The centre for the shaft appears very early, before any other bone 

 according to Be'clard, as early as the thirtieth day. The centre for the sternal end 

 makes its appearance about the eighteenth or twentieth year, and unites with the 

 rest of the bone about the twenty -fifth year. 



Articulations. With the sternum, scapula, and cartilage of the first rib. 



Attachment of Muscles. To six : the Sterno-cleido-mastoid, Trapezius, Pecto- 

 ralis major, Deltoid, Subclavius, and Sterno-hyoid. 



Surface Form. The clavicle can be felt throughout its entire length, even in persons who 

 are very fat. Commencing at the inner end, the enlarged sternal extremity, where the bone 

 projects above the upper margin of the sternum, can be felt, forming with the sternum and the 

 rounded tendon of the Sterno-mastoid a V-shaped notch, the pre-sternal notch. Passing out- 

 ward, the shaft of the bone can be felt immediately under the skin, with its convexity forward 

 in the inner two-thirds, the surface partially obscured above and below by the attachments of 

 the Sterno-mastoid and Pectoralis major muscles. In the outer third it forms a gentle curve 

 backward, and terminates at the outer end in a somewhat enlarged extremity which articulates 

 with the acromial process of the scapula. The direction of the clavicle is almost, if not quite, 

 horizontal when the arm is lying quietly by the side, though in well-developed subjects it may 

 incline a little upward at its outer end. Its direction is, however, very changeable with the 

 varying movements of the shoulder-joint. 



Surgical Anatomy. The clavicle is the most frequently broken of any single bone in the 

 body. This is due to the fact that it is much exposed to violence, and is the only bony connec- 

 tion between the upper limb and the trunk. The bone, moreover, is slender, and is very super- 

 ficial. The bone may be broken by direct or indirect violence or by muscular action. The most 

 common cause is, however, from indirect violence, and the bone then gives way at the junction 

 of the outer with the inner two-thirds of the bone ; that is to say at the junction of the two 

 curves, for this is the weakest part of the bone. The fracture is generally oblique, and the dis- 

 placement of the outer fragments is inward, away from the surface of the body; hence com- 

 pound fracture of the clavicle is of rare occurrence. The inner fragment as a rule is little dis- 

 placed. Beneath the bone the main vessels of the upper limb and the great nerve-cords of the 

 brachial plexus lie on the first rib, and are liable to be wounded in fracture, especially in fracture 

 from direct violence, when the force of the blow drives the broken ends inward. Fortunately, 

 the Subclavius muscle is interposed between these structures and the clavicle, and this often 

 protects them from injury. 



