406 THE VERTEBRATE SKELETON. 



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 supi- 

 nator 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 ehd with the bones of the carpus. 



The post-axial 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 



