2 JAMES ROLLIN SLONAKER 
shape found in man, is flattened (figs. 13 and 14). <A cross- 
section of the shaft does not therefore approach that of a circle, 
but an ellipse, whose dimensions are 2.8 mm. by 1.5mm. The 
greater diameter lies in the plane of movement of the forearm, 
thus providing increased strength in the required plane. The 
length of the radius is 13mm. We thus have a ratio of the aver- 
age diameter of the shaft to the length of the bone of approxi- 
mately 1:6. In man the ratio is about 1:17 and in the dog 
1:15. This will give a fair idea of the comparative strength of 
this bone. Another modification from the human is seen in the 
proximal articular surface, the fovea capituli radii. Instead of 
being a shallow socket, which allows rotation of this bone on its 
axis, 1t is a deep spherical depression which is well extended 
proximally on the extensor side by a strong process. This alone 
would make rotation of the forearm difficult. The fovea capituli 
radu articulates with the capitulum humeri described above. 
The distal end of the radius is widened in the same plane as that 
of the manus to 6.2mm. ‘This forms a strong hinge-joint which 
would permit little lateral movement of the hand. It articulates 
at the wrist with the os naviculare manus, the os lunatum, and 
the radial sesamoid. 
The ulna shows a still greater modification than the radius. 
It is 18 mm. long and is extended well beyond the elbow-joint 
by the proximal elongation of the olecranon. The length from 
the distal end to the center of articulation at the elbow is 11 mm. 
From this articulation to the tip of the olecranon is 7 mm. 
Since the extensor muscles are attached to the greatly expanded 
end of the olecranon, we thus have a strong lever, the ratio of 
whose arms is as 11:7, or about 1.4:1. In the dog this ratio 
is approximately 7:1 and in man 14:1. This modification re- 
sults in great increase in strength at a loss of speed. In order to 
withstand the breaking strain at the elbow-joint, the ulna is 
greatly strengthened or reinforced on the flexor side by a broad 
keel-like outgrowth, approximately 10 mm. in length and 3.5 
mm. wide at its greatest width. This is broadest at the joint 
and gradually grows less in either direction (fig. 13). The incisura 
semilunaris (7s) which articulates with the trochlea humeri (fr) 
