THE RADIUS. 193 
inferior surface of the head of the ulna does not articulate with the carpus, but rests on the 
upper surface of the interposed triangular fibro-cartilage. The wha is superficial throughout its 
entire extent. Superiorly the olecranon process can be readily recognised, particularly when the 
elbow is bent, as in this position the olecranon is withdrawn from the olecranon fossa of the 
humerus in which it rests when the joint is extended. Below this the subcutaneous triangular 
area on the back of the olecranon can be easily determined, and from it the posterior border of 
the bone can readily be traced along the line of the “ whar furrow” to the styloid process below. 
With the hand supine this latter process can be felt to the inner side and slightly behind the 
wrist. When the hand is pronated, the lower end of the radius rolls round the lower extremity 
of the ulna, and the posterior surface of the head of the latter bone now forms a well-marked 
projection on the back of the wrist in line with the cleft between the little and ring fingers. 
Architecture.—The weakest parts of the bone are the constricted portion “of the great 
sigmoid cavity, and the shaft in its lower third, the bone being most liable to fracture at these 
points. On section the medullary cavity is seen to extend upwards as high as the base of the 
coronoid process; inferiorly it reaches as low as the upper end of the lower fifth of the bone. The 
walls of the shaft, which are formed of dense bone, are much thicker posteriorly than anteriorly. 
Above they are continuous with the front of the coronoid process and the back of the olecranon, 
where they are composed of layers of looser texture, which, however, gradually become thinner as 
the points of these processes are reached. Interiorly they ervadually t taper until the head and 
styloid process are reached, round which they form a thin ‘shell; considerably thickened, however, 
in the region of the groove for the extensor carpi ulnaris muscle. The bulk of the upper 
extremity is formed of loose cellular bone, arranged in a series of arcades, stretching from the 
anterior to the posterior wall over the upper end of the medullary canal. Above the constricted 
part of the great sigmoid cavity the bone displays a different structure; here it is formed of 
spongy bone, of closer te xture, arranged generally m lines radial to the articular surface. At the 
point of constriction of the great sigmoid cavity, the layer unmediately subjacent is much denser 
and more compact. 
The lower fifth of the bone is formed of loose spongy bone, the fibres of which have a general 
longitudinal arrangement ; towards its extremity the meshes become smaller. 
Variations. —Cases of partial or complete absence of the wna through congenital defect have 
been recorded. Rosenmiiller has described a case in which the olecranon was separated from the 
upper end of the bone, resembling thus in some respects the patella. In powerfully developed 
hones there is a tendenc -y to the formation of a sharp projecting crest corresponding to the inser- 
tion of the triceps. 
Ossification.—The ulna is ossified from one primary and two or more secondary 
centres. The centre for the shaft appears 
early in the second month of foetal life. 
At birth the shaft and a considerable part Appears about 10.years 
of the upper extremity, including the 2 
coronoid process, are ossified, as well as part 
of the lower extremity. The olecranon 
process and the inferior surface of the head 
and the styloid process are cartilaginous. 
About ten years of age a secondary centre 
appears in the cartilage at the upper end 
of the bone, and forms an_ epiphysis 
which unites with the shaft about sixteen. 
A seale-like centre appears in the cartilage 
of the head about the sixth year, from 
which the under surface of that part of the 
bone is developed, and by the extension of 
which the styloid process is also ossified ; 
this epiphysis does not unite with the shaft 
till the twentieth or twenty-third year. 
Independent centres for the styloid process 
and for the extreme edge of the olecranon 
have also been described. The student ‘ 7 ; 
: 3 Fuses with shaft 20-23 years 
peu. here be warned that the epiphysial At Birth. About 12 years. About 16 year 
line between the shaft and superior or ; 
olecranon epiphysis does not correspond 
to the constricted part of the great sigmoid cavity, but lies considerably above it. 
Fuses with shaft about 16 years 
Appears early in 2nd month of foetal life 
Appears about 6 years 
Fic. 136.—THE OSSIFICATION OF THE ULNA. 
THE RADIUS. 
The radius, or outer bone of the forearm, is shorter than the ulna, with which 
it is united on the inner side. Superiorly it articulates with the humerus, and 
13 
