THE SHOULDER-GIRDLE. 229 



The inner two-thirds of the clavicle is convex forward. Above this portion is 

 the subclavian triangle in the supraclavicular fossa. The outer third of the clavicle 

 is convex backward and from its upper surface the trapezius muscle can be felt pro- 

 ceeding upward. This leaves the middle third of the bone free from muscle. 



Under the middle of the bone passes the subclavian artery. It curves upward 

 about 2. 5 cm. ( i in. ) above the clavicle to descend again to the sternoclavicular joint. 

 The arch so formed indicates the apex of the lung because the subclavian artery 

 rests on the pleura. The internal jugular vein passes down opposite the interval 

 between the sternal and clavicular heads of the sternomastoid muscle. 



Just above the clavicle, a little internal to its middle, and behind the clavicular 

 origin of the sternomastoid muscle is seen the external jugular vein. It terminates 

 in the subclavian vein, which lies to the inner (anterior) side of the artery. To the 

 outer side of the artery the cords of the brachial plexus pass upward and inward. 

 They become prominent in emaciated subjects when the head is turned forcibly 

 toward the opposite side. The posterior belly of the omohyoid muscle varies much 

 in its position, sometimes it lies behind the clavicle, at others two or three centimetres 

 above it. 



Immediately below the clavicle is the infraclavicular fossa. At its inner extremity 

 can be felt the first rib. As it is exceedingly easy to mistake the ribs, it is best, in 

 counting them, to locate the second rib by recognizing the angle of the sternum, 

 (angle of Ludwig) to which it is opposite, on the surface of the sternum about 5 cm. 

 (2 in.) below its upper edge. Attached to the lower edge of the inner half of the 

 clavicle is the pectoralis major muscle and to the outer third the deltoid muscle. 



This leaves one sixth of the lower edge of the clavicle free from muscular attach- 

 ments. This forms the base of the subclavicular triangle and its two sides are formed 

 by the adjacent edges of the pectoralis major and deltoid muscles. Beneath this 

 triangle runs the first portion of the axillary artery with the vein to its inner side 

 and the cords of the brachial plexus to its outer side. Deep pressure at this point 

 compresses it against the second rib. 



Just to the outer side of the junction of the middle and outer thirds of the 

 clavicle, in front of the deepest part of the concavity of the clavicle and about 2. 5 cm. 

 ( I in. ) below it, is the coracoid process. It is better felt by pressing the fingers flat 

 on the surface than by digging them in. It is somewhat obscured by the edge of 

 the deltoid muscle, which covers it. Running from the coracoid to the acromion 

 process is the sharp edge of the coraco-acromial ligament. An incision midway 

 between the two processes would open the joint and strike the long biceps tendon 

 as it winds over the head of the humerus to reach the upper edge of the glenoid 

 cavity. 



Beneath the acromion process is felt the greater tuberosity of the humerus. If 

 the arm is placed alongside of the body with the palm facing forward, a distinct groove 

 can be felt to the inner side of the acromion process passing downward on a line with 

 the middle of the arm. It is the bicipital groove for the long tendon of the biceps 

 muscle. The bony process of the humerus to its outer side is the greater tuber- 

 osity and that to its inner side, between it and the coracoid process, on a slightly 

 lower level, is the lesser tuberosity. It will be noted that the greater tuberosity pro- 

 jects beyond the acromion process and forms the prominence of the shoulder. On 

 rotating the arm the tuberosities can be distinctly felt moving under the deltoid muscle. 



Following the acromion process around toward the back it turns abruptly where 

 it joins the spine of the scapula, forming a distinct angle. This angle is quite prom- 

 inent, can be readily seen and felt, and can be used as a landmark for measuring the 

 length of the humerus. If the spine of the scapula is followed still farther it ends 

 in its root at the posterior border of the bone opposite the upper edge of the fourth 

 rib and third thoracic spine. This marks the upper extremity of the fissure of the 

 lung; with the arm to the side, the lower angle of the scapula lies over the seventh 

 interspace. 



Axilla. On raising the arm directly out from the body the armpit and axillary 

 folds become visible. The rounded edge of the anterior axillary fold is formed by 

 the pectoralis major muscle. It follows the fifth rib and its upper end merges with 

 the lower edge of the deltoid muscle. 



