252 



THE SKELETON. 



Epiphyses of head and } 

 tuberosities blend at 

 5th year, and unite 

 with shaft at 20th 

 year. j 



Unites with shaft ) 

 at 18th year. \ 



JR 







nator muscles. The internal condyle, larger and more prominent than the exter- 

 nal, is directed a little backward : it gives attachment to the internal lateral liga- 

 ment, to the Pronator radii teres, and to a 

 tendon common to the origin of some of the 

 flexor muscles of the forearm. The ulnar- 

 nerve runs in a groove at the back of the 

 internal condyle, or between it and the 

 olecranon process. These eminences are 

 directly continuous above with the external 

 and internal borders i. e. the external and 

 internal condyloid ridges. The great 

 prominence of the inner one renders it 

 more liable to fracture. 



Structure. The extremities consist of 

 cancellous tissue, covered with a thin, com- 

 pact layer ; the shaft is composed of a 

 cylinder of compact tissue, thicker at the 

 centre than at the extremities, and hollowed 

 out by a large medullary canal, which ex- 

 tends along its whole length. 



Development. By seven, or occasionally 

 eight, centres (Fig. 199), one for the shaft, 

 one for the head, one for the tuberosities, 

 one for the radial head, one for the troch- 

 lear portion of the articular surface, and 

 one for 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 ex- 

 tremities remaining cartilaginous. At the 

 beginning of the second year ossification 

 commences in the head of the bone, and during the third year the centre for the 

 tuberosities makes its appearance, usually by a single ossific point, but sometimes, 

 according to Be'clard, by one for each tuberosity, 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 enlarged and become joined, so as to form a single 

 large epiphysis. 



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

 the end of the second year ossification commences in the radial portion of the 

 articular surface, 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 articular surface 

 not 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 sixteen or seventeen years the outer condyle and both 

 portions of the articulating surface (having already joined) unite with the shaft ; 

 at eighteen years the inner condyle becomes joined; whilst the upper epiphysis, 

 although the first formed, is not united until about the twentieth year. 



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

 radius. 



Attachment of Muscles. To twenty-four: to the greater tuberosity, the 

 Supraspinatus, Infraspinatus, and Teres minor ; to the lesser tuberosity, the 

 Subscapularis ; to the external bicipital ridge, the Pectoralis major ; to the internal 

 bicipital ridge, the Teres major; to the bicipital groove, the Latissimus dorsi ; to 

 the shaft, the Deltoid, Coraco-brachialis, Brachialis anticus, external and internal 

 heads of the Triceps ; to the internal condyle, the Pronator radii teres, and common 

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



FIG. 199. Plan of the development of the 

 humerus. By seven centres. 



