HUMERUS. 129 



THE HUMERUS. 



The Humerus (fig. 87) is the longest and largest bone of the upper extremity ; 

 it presents for examination a shaft and two extremities. 



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

 head, a constriction around the base of the head, the neck, and two other emi- 

 nences, the greater and lesser tuberosities. 



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, separated 

 by two slight ridges; the most anterior facet gives attachment to the tendon of 

 the Supra-spinatus ; the middle one, to the Infra-spinatus ; the posterior one, 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 Subscapularis 

 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 commencement, and becomes 

 shallow 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 tendon of insertion of the Latissimus dorsi 

 about its centre. 



The Shaft of the humerus is almost cylindrical in the upper half of its extent ; 

 prismatic and flattened below, it presents three borders and three surfaces for 

 examination. 



The anterior border runs from the front of the great tuberosity above, to the 

 coronoid depression below, separating the internal from the external surface. Its 

 upper part is very prominent and rough; forms the outer lip of the bicipital 

 groove, and serves for the attachment of the tendon of the Pectoralis major. 

 About its centre is seen the rough deltoid impression ; below, it is smooth and 

 rounded, affording attachment to the Brachialis anticus. 



The external border runs from the back part of the greater tuberosity to 

 the external condyle, and separates the external from the posterior surface. It is 

 rounded and indistinctly marked in its upper half, serving for the attachment of 

 the external head of the Triceps muscle ; its centre is traversed by a broad but 

 shallow oblique depression, the musculo-spiral groove.; its lower part is marked 

 by a prominent rough margin, a little curved from behind forwards, which 

 presents an anterior lip for the attachment of the Supinator longus, above the 

 Extensor carpi radialis longior below; a posterior lip for the Triceps, and an 

 interstice for the attachment of the external intermuscular apoueurosis. 

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