12 
of the shaft rapidly subsides into the almost flattened outer or radial surface. The 
opposite surface at the proximal end of the shaft is concave, but becomes slightly 
convex, transversely, down the rest of the shaft. The borders bounding the flattened 
shaft, of which the radial one is formed chiefly by the pectoral ridge, describe very 
slight sigmoid curves. The pectoral ridge is long and low, much less developed than 
in birds of flight. A short ridge (PI. II. fig. 9,q), abruptly rising from the end of 
the radial border, extends to the middle of the distal anconal side of the radial condyle. 
This (fig. 10, h) is long and narrow, extending further proximad by half its length than 
the ulnar condyle (ib. 7) does. The ulnar condyle is shorter and thicker: both project 
towards the palmar aspect, and do not terminate the bone distally. The broad articular 
surface of the ulna is adapted to both condyles, but covers only the distal half of the radial 
one, the radius gliding upon the upper half. The breadth of both condyles is 5 lines. 
The anconal surface (fig. 9) is divided into two grooves by three ridges, the ulnar one 
(4) being the longest, the radial one (/) the shortest; and the ulnar anconal groove is 
consequently larger than the radial one. All the three anconal ridges project distally 
beyond the condyles. There is a well-marked flat surface above the ulnar condyle for 
the attachment of a strong lateral ligament. 
The shaft of the humerus contains a narrow medullary cavity (fig. 9). The orifice 
of the medullary artery is close to the anconal border, about an inch below the ulnar 
edge. 
The radius (Pl. I, 54) and ulna (55), connected only at their extremities, have oppo- 
site curves, leaving a wide interosseous space. The radius is but half the thickness of 
the ulna, and is more bent, <A tuberosity projects on the radial side just below the 
thickened proximal articular end: below this the shaft is slightly contracted ; it then 
expands and becomes compressed, with the radial border more convex than the ulnar 
one is concave, The anconal surface is longitudinally channelled near the radial edge, 
deepening towards the wrist, where there is a second but short groove ulnad of the 
longer one. 
The olecranon is short, obtuse, with a longitudinal groove on each side of its base, 
narrowing the bone between the process and the articular surface of the ulna. A sesa- 
moid in the biceps flexor tendon plays in the palmar groove ; it is the homotype of the 
patella: two other sesamoids in the triceps extensor tendon play in the two grooves in 
the anconal part of the distal end of the humerus: these answer homotypally to the 
fibular extension which plays in the popliteal groove of the outer femoral condyle. The 
shaft of the ulna, though compressed, is at the proximal half three-sided, the narrowest 
side being toward the radius, and contracting to a sharp border at the distal half. There 
are no quill-pits. The distal articular surface is convex from the radial to the ulnar 
side, but slightly concave transversely, and is accordingly somewhat trochlear. A 
tuberosity projects above it on the outer side, near the radius. The length of the anti- 
brachium is 2 inches 4 lines. 
