292 DISSECTION OF THE UPPER LIMB. 



Articular surfaces (fig. 89). The convex articular head of the humerus 

 is two or three times larger than the hollow in the scapula, and forms rather 

 less than the half of a sphere. The head of the bone is joined to the 

 shaft at an angle as it is in the femur, and a rotatory movement is pos- 

 sessed by the humerus in consequence. 



The glenoid surface of the scapula is oval in form with the large end 

 down, and is very shallow ; it is not large enough to cover the head of the 

 humerus. 



Movements. In this joint there is the common motion in four direc- 

 tions, with the circular or circumductory ; and in addition a movement of 

 rotation. 



In the swinging or to and fro movement, the carrying forwards of the 

 humerus constitutes flexion, and the moving it backwards, extension. 

 Flexion is freer than extension ; and when the joint is most bent the 

 scapula, rotating on its axis, follows the head of the humerus, so as to 

 keep the centre of the glenoid fossa applied to the middle of the articular 

 surface of the arm-bone. In extension the articular surface of the scapula 

 does not move after the humerus. 



During these movements the head of the bone rests in the bottom of 

 the glenoid fossa, turning forwards and backwards around a line represent- 

 ing the axis of the head and neck ; and it cannot be dislodged by either 

 the rapidity or the degree of the motion. 



The muscles have more influence than the loose capsule in controlling 

 the swinging motion. 



Abduction and adduction. When the limb is raised, it is abducted, 

 and when depressed, adducted ; and in both cases the humerus rolls on the 

 scapula which is fixed. 



During abduction the head of the humerus, descends to the lower and 

 larger part of the glenoid fossa, and projects beyond it against the cap- 

 sule ; whilst the great tuberosity rubs against the arch of the acromion. 

 In this condition a little more movement down of the head either by 

 muscles depressing it, or by force elevating the farther end of the bone, 

 will throw it out of place, giving rise to dislocation. 



In adduction the head of the bone rises into the socket, the limb meets 

 the trunk, and the tense capsule is set at rest. After the reduction of a 

 dislocation the limb is fixed to the side in this position of security against 

 further displacement. 



In circumduction the humerus passes in succession through the four 

 different states before mentioned, and the limb describes a cone, whose 

 apex is at the shoulder and base at the digits. 



Rotation. There are two kinds of rotatory movement, viz., in and 

 out ; and in each the motion of the head and shaft of the bone has to be 

 considered. 



In rotation in, the great tuberosity is turned forwards, and the head 

 rolls from before back across the glenoid fossa so as to project behind. 

 The shaft is moved forwards round a line lying on its inner side, which 

 reaches from the centre of the head to the inner condyle. 



In rotation out, the osseous movements are reversed : thus the tuberosity 

 turns back, the head rolls forward so as to project in front, and the shaft 

 is carried back around the line before said. 



The upper thickened part of the capsule will be tightened in rotation, 

 but the muscles are the chief agents in checking the movements. 



Dissection. To make the necessary dissection of the ligaments of the 



