6i8 HUMAN ANATOMY. 



attributed, (a) to the gripping of the neck of the metacarpal bone between the 

 flexor brevis poUicis and the obhque portion of the adductor poUicis (these often 

 being considered as the two heads of the flexor brevis poUicis) ; [b) to a similar 

 entanglement of the head and neck in the sht in the capsule ; (r) to the winding of 

 the tendon of the flexor longus poUicis around the neck of the bone ; and (d) to 

 the interposition of the gleno-sesamoid plate. Of these theories the last two seem 

 to of!er the most satisfactory explanation of the difficulties met with in attempts at 

 replacement. 



The Surface Landmarks of the Upper Extremity.— The axilla (page 574) 

 is very distinctly l)ounded anteriorly by the lower border of the pectoralis major, which 

 runs in the line of the fifth rib from the sixth costal cartilage to the external bicipital 

 ridge ; posteriorly by the lower edge of the latissiums dorsi and teres major, extend- 

 ing to the bicipital groove. The shape of the axillary fossa varies with the position 

 of the arm, becctning deeper when the arm is raised at a right angle to the trunk or 

 when the great pectoral and latissimus are contracted. With the arm still farther 

 elevated, the depth of the space decreases as traction on those muscles appro.ximates 

 the axillary borders and the humeral head enters and partly obliterates the cavity. 

 With the arm close to the thorax, the third rib may be reached by the. exploring 

 finger. The concavity of the space is lessened or effaced by glandular tumors, effu- 

 sions of blood, or collections of pus (page 582). In opening an axillary abscess it 

 should be remembered that the inner or thoracic wall is the direction of safety so far 

 as the great vessels are concerned. 



In the region of the shoulder the rounded surface is produced by the thick 

 deltoid muscle spread over the greater tuberosity of the humerus. It is fuller anteri- 

 orly than posteriorly, partly on account of the presence of the lesser tuberosity in 

 the former position, but chiefly because the hinder portion of the muscle is thinner 

 than the fore part and because of its close attachment to the infraspinatus fascia and 

 muscle. The greatest width of the shoulders does not correspond to the points at 

 which the deltoid muscles overlap the head of the humerus, but is at the level of the 

 lower border of the anterior axillary fold, — i.e., on the level of the point at which 

 the various bundles of deltoid fibres are gathered together to pass to their insertion 

 (Thomson). The bony points in this region have been described (pages 270, 279, 

 280). The anterior border of the deltoid presents a rounded eminence bounded 

 internally above by the infraclavicular fossa {vide infra) and below by the closely 

 applied outer margin of the pectoralis major. In the shallow groove between these 

 two muscles the cephalic vein and a branch of the acromio-thoracic artery are to be 

 found. Just external to the groove under the inner fibres of the deltoid is the cora- 

 coid process (page 255). The infraclavicular fossa is the triangular interval bounded 

 by the outer fibres of the pectoralis major internally, the inner fibres of the deltoid 

 externally, and the clavicle above. The surface depression known by this name may 

 be much larger than this intermuscular interval, and may almost correspond in extent 

 to the roof of the superficial infraclavicular triangle (page 581). It is not very marked 

 in muscular subjects. It is effaced — owing to tension of fascia and muscles — in sub- 

 coracoid luxation of the humerus, or in fracture of the clavicle with marked displace- 

 ment «f the fragments. It may be converted into a rounded elevation by glandular 

 growths extending upward from the axiUa, or by the head of the humerus in intra- 

 coracoid (infraclavicular) luxation. At the bottom of this fossa, just within the cora- 

 coid process, — i.e., not far from the middle of the clavicle, — the first portion of the 

 axillary artery may be compressed against the second rib by pressure directed back- 

 ward and a little inward, the patient being supine. 



The posterior border of the deltoid above is tendinous, is closely attached to 

 the infraspinatus muscle beneath it, and is scarcely discernible. Below it is thicker 

 and presents a well-marked rounded eminence which inclines from behind forward to 

 meet the anterior border at the middle of the outer side of the arm, where a distinct 

 depression indicates the insertion of the deltoid (Fig. 595). This depression is a valu- 

 able practical landmark for the reasons that : ( i ) It corresponds to the middle of the 

 shaft of the humerus, where the two curves of the bone unite and where the cylin- 

 drical joins the {)rismatic part of the shaft, which is there smallest, hardest, and least 

 elastic (page 272), and hence ir> most frequently broken. ( 2) It indicates the region 



