Anatomy of Skeleton 



of the glenoid cavity that is the main object of such rotation is further increased by 

 sterno-clavicular movement : it will perhaps give a clearer conception of these actions 

 if the motion of raising the arm above the head is analysed. The arm is raised to a 

 right angle by the Deltoid, acting from a scapula fixed by Serratus magnus and Trapezius 

 (see Fig. 60) . An arm held in full abduction by the Deltoid could be raised to a higher 

 level by active contraction of the same muscles that would fix the scapula to enable the 

 Deltoid to act that is, the Serratus magnus and Trapezius, which rotate the scapula 

 outwards, are also of service in holding it against the Deltoid and weight of the arm, 

 which tend to rotate it inwards. The rotation appears to take place at first round an 

 axis passing through the front part of the acromio-clavicular joint, but the consequent 



depression of the coracoid 

 quickly tightens the conoid 

 ligament and the axis is trans- 

 ferred to the upper attachment 

 of this. Now the acromial sur- 

 face slides up. on the clavicular, 

 but owing to the oblique plane 

 of the surfaces this entails some 

 outward displacement of the 

 scapula, therefore tightens the 

 inner part of the conoid band, 

 and hinders further movement. 

 By this time the alteration in 

 direction of the glenoid cavity 

 has only progressed a certain 

 way, and further change is ob- 

 tained by carrying the shoulder 

 back through antero-posterior 

 movements at the sterno-cla- 

 vicular joint : this, when the 

 angle is fixed as far forward as 

 possible by Serratus magnus, 

 has the effect of further rotating 

 the scapula. At the same time 

 the clavicle is elevated, and 

 probably a very slight upward 

 rotation occurs round its longi- 

 tudinal axis, thus associating 



it with the rotating scapula and at the same time allowing elevation to the full extent, 

 as the rhomboid ligament is attached behind this longitudinal axis. 



In life the various actions are combined, and go on more or less at the same 

 time. On starting to raise the arm, the lower fibres of the Trapezius begin to rotate 

 the scapula, by this action also, of course, holding it against the Deltoid. When the 

 limb has come out a little distance from the side, the Serratus magnus joins in the 

 movement, and so the raising of the humerus by the Deltoid goes on with simultaneous 

 rotation of the scapula, though this last movement is not proportionately so rapid. 

 In the same way the " terminal " movements of elevation and rotation of the clavicle, 

 with drawing back of the scapula, begin to make their appearance fairly early in the 

 whole action. 



The movements are reversed as the arm comes down, and internal rotation of 



arm 



FIG. 60. Scheme of the directions in which the muscles act in 

 rotating the scapula. The centre round which the bone 

 rotates is put approximately at the acromio-clavicular 

 joint, and it is clear that any muscles pulling on the circle in 

 the direction A. must rotate the bone in, while those pulling 

 in the direction B. have the opposite effect. The upper 

 fibres of Trapezius are mainly suspensory, the lower are 

 external rotators, while the intermediate ones help in the 

 retraction of the scapula which occurs in the later stages of 

 elevation of the limb. 



