24 THE SUPERIOR EXTREMITY 



sheath. In these cases the lateral rotation carried out in 

 Kocher's method of reduction exerts a powerful twisting or 

 spiral strain upon the bone and ; if the adhesions are strong, 

 the humerus may be fractured spirally at the surgical neck. 

 In the old-fashioned method of reduction by means of the heel 

 in the axilla, the extending force is exerted along the long axis 

 of the shaft of the humerus, and there is therefore no danger of 

 spiral fracture. On the other hand, the upper ribs may be 

 fractured, unless the pressure of the heel is directed towards 

 the coracoid process and not against the chest wall. 



Congenital Elevation of the Shoulder. At an early period of 

 development, the scapula and clavicle occupy positions much nearer the head 

 end of the embryo than they do at birth, and as the scapula grows in size 

 it migrates caudally. Incomplete caudal migration of the scapula accounts 

 for the condition of congenital elevation of the shoulder described by Eulen- 

 burgh and Sprengel. It may be due to the presence of a bridge of bone 

 joining the vertebral border of the scapula to the lower cervical spines. 

 This bridge of bone, which may possess a false joint at either end, is homo- 

 logous with the cartilaginous supra-scapula found in some lower mammals. 



The upper border and medial (superior) angle of the scapula lie consider- 

 ably above the head of the first rib, and may be mistaken for an exostosis. 

 The muscles which elevate the scapula are all definitely shortened, and the 

 skin is drawn up into a fold, which extends from the mastoid process to the 

 acromion. The condition may be bilateral and is associated with cervical 

 ribs, or with faulty ossification of the cervical vertebrae, which may be fused 

 together leading to restriction of movement. 



THE BREAST AND AXILLA 



Development of the Mammary Gland. The connective 

 tissue of the mamma is mesodermal in origin, while the 

 glandular tissue which infiltrates it is derived from the ectoderm. 

 During the second month of embryonic life a distinct thickening 

 of the ectoderm forms the mammary ridge, which extends from 

 the axilla to the groin. Normally the thickening is limited 

 to that part of the ridge which subsequently overlies the thorax, 

 but occasionally it is not so limited, and then supernumerary 

 nipples or breasts may appear. 



During the third month the ectodermal nodule becomes 

 depressed or invaginated into the underlying connective tissue 

 so that a small pit is formed. In the fifth month solid ectodermal 

 buds, from fifteen to twenty in number, grow inwards, and 

 their stalks subsequently give rise to the epithelial lining of the 



