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's 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. [Illustration FIG. 77. BONES OF THE LEFT UPPER ARM AND FORE-ARM OF A DOG (_Canis familiaris_) × 1/2. A, humerus (seen from the posterior side); B, radius, C, ulna, both seen from the anterior side. 1. head. 2. lesser tuberosity. 3. greater tuberosity. 4. shaft of the humerus. 5. trochlea. 6. internal condyle. 7. external condyle. 8. supra-trochlear foramen. 9. proximal end of the radius. 10. shaft of the radius. 11. olecranon. 12. surface for articulation with the trochlea. 13. surface for articulation with the radius. 14. distal end of the ulna.]