THE HUMERUS 



213 



Ossification (Figs. 210, 211). The humerus is ossi- 

 fied from eight centers, one for each of the following 

 parts: the body, the head, the greater tubercle, the 

 les.ser tubercle, the capitulum, the trochlea, and one 

 for each epicondyle. The center for the body appears 

 near the middle of the bone in the eighth week of fetal 

 life, and soon extends toward the extremities. At birth 

 the humerus is ossified in nearly its whole length, only 

 the extremities remaining cartilaginous. During the 

 first year, sometimes before birth, ossification commences 

 in the head of the bone, and during the third year the 

 center for the greater tubercle, and during the fifth that 

 for the lesser tubercle, make their appearance. By the 

 sixth year the centers for the head and tubercles have 

 joined, so as to form a single large epiphysis, which fuses 

 with the body about the twentieth year. The lower end 

 of the humerus is ossified as follows. At the end of 

 the second year ossification begins in the capitulum, 

 and extends medialward, to form the chief part of the 

 arucular end of the bone; the center for the medial part 

 of the trochlea appears about the age of twelve. Ossifi- 

 cation begins in the medial epicondyle about the fifth 

 year, and in the lateral about the thirteenth or four- 

 teenth year. About the sixteenth or seventeenth year, 

 the lateral epicondyle and both portions of the articu- 

 lating surface, having already joined, unite with the 

 body, and at the eighteenth year the medial epicon- 

 dyle becomes joined to it. 



Epiphysial line 



FIG. 209. Longitudinal section of head of 

 left humerus. 



Epiphyses of head and\ 

 tubercles blend at fifth I 

 year, and unite with > 

 body at twentieth I 

 year J 



Unites with body\ 

 at eighteenth year) 



FIG. 210. Plan of ossification of the humerus. 



FIG. 211. Epiphysial lines of humerus in a young 

 adult. Anterior aspect. The lines of attachment of 

 the articular capsules are in blue. 



