HUMERUS. 207 



being formed of two separate nuclei, has its base formed by an extension into 

 it of the centre of ossification which belongs to the spine, the extent of which 

 varies in different cases. The two separate nuclei unite, and then join with the 

 extension carried in from the spine. These various epiphyses become joined 

 to the bone between the ages of twenty-two and twenty-five years. Sometimes 

 failure of union between the acromion process and spine occurs, the junction 

 being effected by fibrous tissue, or by an imperfect articulation; in some cases 

 of supposed fracture of the acromion with ligamentous union, it is probable 

 that the detached segment was never united to the rest of the bone. 



Articulations. With the humerus and clavicle. 



Attachment of Muscles. To the anterior surface, the Subscapularis ; posterior 

 surface, Supraspinatus, Infraspinatus; spine, Trapezius, Deltoid; superior border, 

 Omo-hyoid; vertebral border, Serratus Magnus, Levator Anguli Scapulae, 

 Ehomboideus Minor and Ehomboideus Major; axillary border, Triceps, Teres 

 Minor, Teres Major; glenoid cavity, long head of the Biceps; coracoid process, 

 short head of the Biceps, Coraco-brachialis, Pectoralis Minor; and to the in- 

 ferior angle occasionally a few fibres of the Latissimus Dorsi. 



THE HUMERUS. 



The Humerus is the longest and largest bone of the upper extremity; it pre- 

 sents for examination a shaft and two extremities. ,, 



The Upper Extremity is the largest part of the bone; it presents' a rounded 

 head joined to the shaft by a constricted part, called the neck, and two other 

 eminences, the greater and lesser tuberosities (Fig. 154). 



The head, nearly hemispherical in form, is directed upwards, inwards, and a 

 little backwards ; its surface is smooth, coated with cartilage in the recent state, 

 and articulates with the glenoid cavity of the scapula. The circumference of 

 its articular surface is slightly constricted, and is termed the anatomical neck, in 

 contradistinction to the constriction which exists below the tuberosities, and is 

 called the surgical neck, from its often being the seat of fracture. It should be 

 remembered, however, that fracture of the anatomical neck does sometimes, 

 though rarely, occur. 



The anatomical neck is obliquely directed, forming an obtuse angle with the 

 shaft. It is more distinctly marked in the lower half of its circumference than 

 in the upper half, where it presents a narrow groove, separating the head from 

 the tuberosities. Its circumference affords attachment to the capsular ligament, 

 and is perforated by numerous vascular foramina. 



The greater tuberosity is situated on the outer side of the head and lesser 

 tuberosity. Its upper surface is rounded and marked by three flat facets, sepa- 

 rated by two slight ridges. The anterior facet gives attachment to the tendon 

 of the Supraspinatus; the middle one to the Infraspinatus; the posterior facet, 

 and the shaft of the bone below it, to the Teres Minor. The outer surface of 

 the great tuberosity is convex, rough, and continuous with the outer side of the 

 shaft. 



The lesser tuberosity is more prominent, although smaller than the greater. It 

 is situated in front of the head, and is directed inwards and forwards. Its 

 summit presents a prominent facet for the insertion of the tendon of the Sub- 

 scapularis muscle. The tuberosities are separated from one another by a deep 

 groove, the bicipital groove, so called from its lodging the long tendon of the 

 Biceps muscle. It commences above between the two tuberosities, passes 

 obliquely downwards and a little inwards, and terminates at the junction of the 

 upper with the middle third of the bone. It is deep and narrow at its com- 

 mencement, and becomes shallowed and a little broader as it descends. In the 

 recent state it is covered with a thin layer of cartilage, lined by a prolongation 

 of the synovial membrane of the shoulder-joint, and receives part of the ten- 

 don of insertion of the Latissimus Dorsi about its centre. 



