172 SPECIAL ANATOMY OF THE SKELETON 



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

 the varying movements of the shoulder-joint. 



Applied Anatomy. The clavicle is the most frequently fractured 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 fixed outer one-third with the movable inner two-thirds of the bone. This is the weakest 

 and most slender part of the bone. The fracture is generally oblique, and the displacement 

 of the outer fragments is inward, away from the surface of the body ; hence, compound fracture 

 of the clavicle is of rare occurrence. The inner fragment, as a rule, is little displaced. 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. 



The clavicle is not uncommonly the seat of sarcom-atous tumors, rendering the operation 

 of excision of the entire bone necessary. This operation is best performed by exposing the 

 bone freely, disarticulating at the acromial end, and turning it inward. The removal of 

 the outer part is comparatively easy, but resection of the inner part is fraught with difficulty, 

 the main danger being the risk of wounding the great veins which are in relation with its 

 under surface. 



The Scapula, or Shoulder Blade. 



The scapula forms the back part of the shoulder girdle. It is a large flat bone, 

 triangular in shape, situated at. the posterior aspect and side of the thorax, between 

 the second and seventh or sometimes the eighth rib, its internal border or base 

 being about an inch from and nearly but not quite parallel with the spinous pro- 

 cesses of the vertebrae, so that it is rather closer to them above than below. It 

 presents for examination two surfaces, three borders, and three angles. 



Surfaces. The anterior or ventral surface {fades costalis) (Fig. 138) presents 

 a broad concavity, the subscapular fossa (fossa subscapularis). It is marked, in 

 the inner two-thirds, by several oblique ridges (lineae musculares), which pass 

 outward and upward; the outer third is smooth. The oblique ridges give origin 

 to the tendinous intersections, and the surfaces between them to the fleshy fibres, 

 of the Subscapularis muscle. The outer third of the fossa is smooth,- is covered 

 by, but does not afford attachment to, the fibres of this muscle. This surface is 

 separated from the internal border by a smooth, triangular margin at the supe- 

 rior and inferior angles, and in the interval between these by a narrow edge which 

 is often deficient. This marginal surface affords attachment throughout its entire 

 extent to the Serratus magnus muscle. The subscapular fossa presents a trans- 

 verse depression at its upper part, where the bone appears to be bent on itself, 

 forming a considerable angle, called the subscapular angle (angulus subscapularis), 

 thus giving greater strength to the body of the bone from its arched form, while 

 the summit of the arch serves to support the spine and acromion process. It is in 

 this situation that the fossa is deepest, so that the thickest part of the Subscapu- 

 laris muscle lies in a line perpendicular to the plane of the glenoid cavity, and 

 must consequently operate most effectively on the head of the humerus, which 

 is contained in that cavity. 



The posterior or dorsal surface (fades dorsalis) (Fig. 139) is arched from above 

 downward, alternately concave and convex from side to side. It is subdivided 

 unequally into two parts by the spine; the portion above the spine is called the 

 supraspinous fossa, and that below it the infraspinous fossa. 



The supraspinous fossa (fossa supraspinata), the smaller of the two, is concave, 

 smooth, and broader at the vertebral than at the humeral extremity. It affords 

 attachment by its inner two-thirds to the Supraspinatus muscle. 



The infraspinous fossa (fossa infraspinata) is much larger than the preceding; 



