THE HUMERUS 



115 



fuse to form the acromion, and join the spine at the twentieth year. This union 

 of the acromion and spine may be fitjrous, hence the acromion is often found 

 separate in macerated specimens. The cartilage along the vertebral border ossifies 

 from two centres: one in the middle, and one at the posterior inferior angle. A 

 thin scale may occasionally be detected at the tip of the glenoid fossa. 



Morphology. — It is impossible to comprehend the significance of the scapular 

 nviclei without considering briefly the morphology of the shoulder (pectoral) 

 girdle. 



In its most generalised form the shoulder girdle consists of cartilage, which is 

 disposed in three parts. Of these, a dorsal segment represents the scapula and a 

 ventral bar, reaching to the sternum, represents the coracoid. The meeting place 

 of the coracoid and sca})ula is the glenoid fossa. Anterior to the coracoid there is 



Fig. 122. — Ossificatiox of the Scapula. 



The Scapula at the third year, 

 showing the coracoid element. (Anterior view.) 



The Scapula at birth. (Anterior view.) 



a third piece, more or less parallel with the coracoid, named the pre-coracoid. The 

 human shoulder girdle is modified from the type form, mainly in the suppression 

 of the pre-coracoid and, in part, of the coracoid. The suppression is brought about 

 by the clavicle, which commences to ossify in the membrane overlying the pre- 

 coracoid ; it then invades and replaces the cartilage. 



The scapular end of the coracoid ossifies and becomes the coracoid process of 

 the scapula; the ventral end degenerates to form the costo-coracoid ligament, 

 which lies in the free border of the membrane of that name. The dorsal cartilage 

 ossifies and becomes the scapula; the large tract of cartilage on the vertebral border 

 of the young scapula represents the large suprascapular cartilage of batrachians. 

 The suprascapular notch indicates the line of union of scapular and coracoid 

 elements. In the embryo the notch is bridged over by cartilage, which may ossify 

 or become lijjament. 



THE HUMERUS 



The humerus is the longest bone of the upper limb. Its upper extremity 

 presents a hemispherical surface covered with cartilage, and known as the head. 

 The head articulates with the glenoid cavity of the scapula, and is directed upwards, 

 inwards, and backwards. Below the articular surface, the bone is rough and con- 

 stricted, constituting the anatomical neck. To the outer side i>f tlio head are two 

 tuberosities, separated by the bicipital groove. The greater tuberosity is the 



