The bone is preformed in cartilage. A centre for the shaft appears in the sixth to seventh week, and at birth the ends are cartilaginous, but in about a fifth of the total cases a small centre is present in the articular head at birth.
The upper is the growing end, and three centres, for the head, greater, and lesser tuberosities, appear in that order between the first and fourth or fifth year, quickly fusing with each other to make the upper epiphysis, so that this is present as a single mass in the fifth or sixth year. It does not unite with the shaft till between the twentieth and twenty-fifth years.
The lower end has four special centres, three of which are fused to form the lower epiphysis, while the fourth, for the internal condyle, is separated from these by an extension downwards of the shaft: this extension forms the bone between the condyle and trochlea (the part in relation with the ulnar nerve behind), so that the lower margin of the bone may be said to be made from five centres, of which four are epiphysial. The areas formed by these centres are shown in Fig. 64.
The centre for the capitulum comes first, in the second to third year, foUowed by that for the inner condyle in the fifth to eighth year: the trochlear centre appears at about eleven, and the outer condylar nucleus about a year later. Thus these centres for the lower end are all present a few years before puberty. After puberty the extension of the ossification of the shaft takes place, and the three centres of the lower epiphysis fuse with each other and with the shaft about the age of seventeen, followed by junction of the inner condyle with the shaft within a year.
(See Fig. 68 for the appearance of the cartilaginous lower end and its relation to the capsule).