The brachium or upper arm includes only a single bone, the humerus.
The humerus is a stout elongated bone, articulating by its large proximal head (fig. 77, 1) with the glenoid cavity of the scapula, and at its distal end by the trochlea with the bones of the fore-arm. The head passes on its inner side into an area roughened for the attachment of muscles and called the lesser tuberosity (fig. 77, 2); while in front it is divided by the shallow bicipital groove from a large roughened area, the greater tuberosity (fig. 77, 3), which is continued as a slight roughened ridge, extending about one-third of the way down the outer side of the shaft. This ridge, which in many animals is much more strongly developed than it is in the dog, is called the deltoid ridge. The trochlea (fig. 77, 5) at the distal end of the bone is a pulley-like surface, elevated at the sides and grooved in the middle. It articulates with the radius and ulna of the fore-arm. On each side of it are slight roughened projections, the internal and external condyles (fig. 77, 7). In the cat and many other animals there is a foramen, the ent-epicondylar foramen above the internal condyle, but in the dog this is not developed. Passing up the shaft from the external condyle is a slight ridge, the supinator or ectocondylar ridge; this is better developed in many mammals. Immediately above the trochlea in front and behind are the deep supra-trochlear fossae, which communicate with one another through the supra-trochlear foramen (fig. 77, 8). The posterior of these, the olecranon fossa, is much the deeper, and receives the olecranon process of the ulna when the arm is extended. The head and tuberosities of the humerus ossify from one centre, the shaft from a second, and the trochlea and condyles from a third.
The fore-arm or antibrachium contains two bones, the radius and ulna; they are immovably articulated with one another, but not fused. The pre-axial bone, the radius (fig. 77, B), which lies more or less in front of the ulna, is external to the ulna at its proximal end, and at its distal end is internal to that bone. It articulates with the external portion of the trochlea, while the ulna articulates with the internal portion. It is a straight bone with its distal end slightly larger than its proximal end. The proximal end articulates with the trochlea, the distal end with the bones of the carpus.
The postaxial bone, the ulna (fig. 77, C), has the proximal end much enlarged, forming the olecranon (fig. 77, 11), and tapers gradually to the distal end. Near its proximal end the ulna is marked by a deep sigmoid notch, which bears on its inner side a concave surface (fig. 77, 12) for articulation with the trochlea. The pointed proximal end of the sigmoid notch is called the coronoid process. Somewhat in front of and below the sigmoid notch is a smaller hollow (fig. 77, 13), with which the radius articulates.
Fig. 77. Bones of the left upper arm and fore-arm of a Dog
(Canis familiaris) × ½.
A, humerus (seen from the posterior side); B, radius, C, ulna, both seen
from the anterior side.