THE ULNA. 189 
a third centre for the small tuberosity begins to appear about the end of the third year, 
or may be delayed till the fourth or fifth year. These three centres are all blended by 
the seventh year, and form an epiphysis, which ultimately unites with the shaft about 
the age of twenty-five. It may be noticed that the superior end of the diaphysis is 
conical and pointed in the centre, over which the epiphysis fits as a cap, an arrangement 
which thus tends to prevent its displacement before union has occurred. The first centre 
to appear in the lower extremity is that for the capitellum about the second or third 
year. This extends inwards, and forms the outer half of the trochlear surface, the 
centre for the inner half not making its appearance till the eleventh or twelfth year. 
At birth. About 5 years. About 12 years. About 16 years. 
Fic. 133.—OssiFICATION OF THE HUMERUS. 
1. Appears early in 2nd month foetal life. 8. Centres for small tuberosity fuse with other centres 
2. For tuberosity, appears 2 to 3 years. ; about 7 years. 
3. For head, appears within first 6 months. 9, Appears about 11 or 12 years. 
4. For internal condyle, appears about 5 years. 10. Inferior epiphysis fuses with shaft about 16 to 17 years. 
5. For eapitellum, appears 2 to 3 years. 11. Superior epiphysis fuses with shaft about 25 years. 
6. Appears about 12 years. 12. Fuses with shaft about 17 to 18 years. 
7. Centres for head and great tuberosity, coalesce about 
5 years. 
Separate centres are developed in connexion with the epicondyles ; that for the external 
appears about the twelfth year, and rapidly coalescing with the centres for the capitellum and 
trochlea forms an epiphysis, which unites with the shaft about the sixteenth or seventeenth 
year. The centre for the internal epicondyle appears about the fifth year; it forms a 
separate epiphysis, which unites with the shaft about eighteen or nineteen. These two 
epiphyses at the lower end of the bone are separated by a down-growth of the shaft, 
which lies between the internal epicondyle and the trochlea, and forms part of the 
base and inner side of the latter process. 
The epicondylic process when present is developed from the diaphysis, and has 
been observed to be already well ossified by the third year. (Proc. Anat. Soe.” Journ. 
Anat. and Physiol., 1898.) 
THE ULNA. 
Of the two bones of the forearm, the ulna, which is placed internally, is the 
longer. It consists of a large superior extremity supporting the olecranon and 
coronoid processes; a shaft tapering from above downwards; and a small rounded 
inferior end called the head. 
Superior extremity.—The olecranon process (olecranon) lies in line with the 
shaft. Its posterior surface, more or less triangular in form, is smooth and 
