THE CLAVICLE, OR COLLAR BONE 171 



behind, by the posterior border. It is narrow internally, but gradually increases 

 in width externally, and is continuous with the under surface of the flat portion. 

 Commencing at the sternal extremity may be seen a small facet, the costal facet, 

 for articulation with the cartilage of the rst rib. This is continuous with the 

 articular surface at the sternal end of the bone. External to this is a broad, rough 

 surface, the rhomboid impression (tuberositas costalis), rather more than an inch 

 in length, for the attachment of the costoclavicular (rhomboid) ligament. The 

 remaining part of this surface is occupied by a longitudinal groove, the subclavian 

 groove, broad and smooth externally, narrow and more uneven internally; it gives 

 attachment to the Subclavius muscle, and by its margins to the costocoracoid 

 membrane, which splits to enclose the muscle. Not infrequently this groove is 

 subdivided into two parts by a longitudinal line, which gives attachment to the 

 intermuscular septum of the Subclavius muscle. 



Internal or Sternal Extremity (extremitas sternalis). The internal or sternal 

 extremity of the clavicle is triangular in form, directed inward and a little down- 

 ward and forward; and presents an articular facet (fades articularis sternalis), 

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

 with the sternum through the intervention of an intra-articular fibrocartilage; 

 the circumference of the articular surface is rough, for the attachment of numer- 

 ous 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 intra-articular fibrocartilage, and the lower border is continuous with the 

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

 cartilage of the first rib. 



Outer or Acromial Extremity (extremitas acromialis). The outer or acromial 

 extremity, directed outward and forward, presents a small, flattened, oval facet, 

 acromial surface (fades articularis acromialis), which looks obliquely downward, 

 and which articulates with the acromion process of the scapula. The circum- 

 ference of the articular facet is rough, especially above, for the attachment of 

 the acromioclavicular 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 the female it is 

 placed almost, if not quite, horizontal, while in the male it inclines slightly downward and inward. 

 In those persons who perform considerable manual labor, which brings into constant action the 

 muscles connected with this bone, it becomes thicker and more curved, its ridges for muscle 

 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 by a 

 compact layer much thicker in the middle than at either end. It has no true medullary cavity. 



Development. From two centres, one for the shaft and outer extremity and one for the 

 sternal extremity. The primary centre for the shaft appears very early, before that of any 

 other bone, at about the fifth or sixth week of fetal life. The secondary centre for the sternal 

 end makes its appearance about the fifteenth to the twentieth year, and unites with the rest of 

 the bone about the twenty-fifth year. 



Articulations. With the sternum, scapula, by intra-articular fibrocartilaginous disks, and 

 with the cartilage of the first rib. 



Attachment of Muscles. To six the Sternomastoid, Trapezius, Pectoralis major, 

 Deltoid, Subclavius, and Sternohyoid. 



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, Xvhere the bone 

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

 rounded tendon of the Sternomastoid a V-shaped notch, the presternal 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 Sternomastoid 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 



