THE HUMERUS, OR ARM BONE 



183 



articular surface (the lines of greatest pressure), and they are bound together by other bone 

 fibres, which usually correspond to the plane of the articulation (the lines of greatest tension). 

 This arch-like arrangement strengthens the head of the bone, and it is further strengthened by 

 the binding fibres. 



Development. From eight centres (Fig. 144), one for each of the following parts: The 

 shaft, head, each tuberosity, the radial head, the trochlear portion of the articular surface, and 

 each condyle. The nucleus for the shaft appears near the centre of the bone in the eighth 

 week, and soon extends toward the extremities. At birth the humerus is ossified nearly in its 

 whole length, the extremities remaining cartilaginous. During the first year, sometimes even 

 before birth, ossification commences in the head of the bone, and during the third year the centre 

 for the greater tuberosity makes its appearance, that for the lesser being small and not appearing 

 until the fifth year. By the sixth year the centres for the head and tuberosities have increased in 

 size and become joined, so as to form a single large epiphysis. 



Epiphyseal line. 



Epiphytes of head and } 

 tuberosities blend at 

 5th year, and unite 

 u-ith shaft at 20th 

 year. 



Unites with shaft \ ^m& 

 at 18th year. \ 



FIG. 143. Longitudinal section of head of left 

 humerus. 



FIG. 144. Plan of the development of the 

 humerus from eight centres. 



The lower end of the humerus is developed in the following manner: At the end of the second 

 year ossification commences in the capitellum, and from this point extends inward, so as to form 

 the chief part of the articular end of the bone, the centre for the inner part of the trochlea nbt 

 appearing until about the age of twelve. Ossification commences in the internal condyle about 

 the fifth year, and in the external one not until about the thirteenth or fourteenth year. About 

 the sixteenth or seventeenth year the outer condyle and both portions of the articulating surface 

 (which have already joined) unite with the shaft; at the eighteenth year the inner condyle becomes 

 joined; while the upper epiphysis, although the first formed, is not united until about the twen- 

 tieth year. 



Articulations. With the glenoid cavity of the scapula and with the ulna and radius. 



Attachment of Muscles. To twenfi/-four to the greater tuberosity, the Supraspinatus, 

 Infraspinatus, and Teres minor; to the lesser tuberosity, the Subscapularis; to the external 

 bicipital ridge, the f'ectoralis major; to the internal bicipital ridge, the Teres major; to the bicip- 

 ital groove, the Latissimus dorsi; to the shaft, the Deltoid, Coracobrachialis, Brachialis anticus, 

 external and internal heads of the Triceps; to the internal condyle, the Pronator teres, and com- 

 mon tendon of the Flexor carpi radialis, Palmaris longus, Flexor sublimis digitorum, and Flexor 

 carpi ulnaris; to the external, supracondylar ridge, the Brachioradialis and Extensor carpi radi- 



