Limbs : Upper Limb 79 



the scapula is chocked by the trapezoid fibres. The rotators of the scapula may be 

 again enumerated : 



Turning the angle out are . Serratus magnus ; 



Trapezius. 



Turning the angle in are . Rhomboids ; 



Weight of limb (Deltoid) 

 (? Levator anguli scapulae). 



\Yhen the limb is fixed on an external object, so that muscles may act from it on 

 the scapula, these rotators are, of course, not used in this way. 



In its build the bone is very thin in most of its extent ; in fact it may be perforated 

 in old subjects. But though thin it is rendered very rigid by the stout bar of bone 

 already mentioned as existing near the axillary border for supporting the strain of the 

 Serratus magnus, by the thickening along the vertebral border, and by the strong 

 spine placed across it. The strongest part of the spine is the thick outer edge, and 

 this supports the acromion and resists the downward pull of the Deltoid from this 

 process. In structure the bone only presents cancellous tissue in its thicker parts : 

 in the young bone there is more of this tissue between two enclosing plates of compact 

 bone, but the middle layer disappears in the adult except along the vertebral and 

 axillary borders and in the spine. 



The scapula is sometimes described as consisting of three bony plates diverging 

 from a central axis : the spine is called the mesoscapula, the bone above the spine the 

 prescapula, and the infraspinous plate the postscapula. These form angles with each 

 other along the axis, which corresponds with the attached border of the spine : the 

 angle between the meso- and pre- scapula is about 100 degrees, and those between 

 pre- and post- scapula and meso- and post- scapula about 130 degrees. 



A scapula is present in all mammals, whether they possess a clavicle or not : the 

 human scapula is remarkable for the length of its postscapular part, which gives the 

 bone its characteristic elongation. The proportionate length of this part increases 

 after birth. The proportion of breadth to height is taken to form a " scapular index," 

 and is lowest in the adult, highest in mid-fcetal life higher in negroes than in Europeans. 



Development. 



The scapula is recognisable as a mesenchymal condensation early in the second 

 month, and is preformed in large part in cartilage, and a centre appears in the peri- 

 chondrium on its ventral aspect about its middle, before the end of the second month. 

 From this the greater part of the bone is formed, so that at birth only the acromion, 

 coracoid, vertebral border and lower angle, and glenoid cavity are cartilaginous. This 

 is one of the primary centres for the bone, forming the mass of its dorsal or " scapular " 

 element : the other primary division of the bone is the ventral or " coracoid " element, 

 represented, in part at any rate, by the coracoid process and upper part of the glenoid 

 cavity in the human skeleton. This second primary centre is delayed till after birth, 

 and then appears within the first year in the thick part of the process, while another 

 centre appears later, in the eighth to tenth year, for the base and the upper part of 

 glenoid : these are consolidated with the main bone shortly after puberty. 



About this time or a little later secondary epiphysial centres make their appearance 

 two for the acromion, one for the angle, one for the vertebral border, and one for the 

 rest of the glenoid fossa : additional small centres may appear on the trapezoid ridge 

 of the coracoid and at its tip. 



All these coalesce with the bone between twenty and twenty-five. 



