50 THE SURGICAL ANATOMY OF THE HORSE 



these surfaces give attachment to the arciform ligaments which assist in 

 binding the ulna to the radius. 



The olecranon process is a massive piece of bone, projecting upwards 

 and slightly backwards from the body. It has two surfaces, two edges, 

 and a summit. The outer surface is convex, whilst the inner one is con- 

 cave and smooth. Upon this inner surface the ulnar artery and vein lie, 

 where they are covered by the scapulo-ulnaris muscle. The posterior 

 edge is thick and rounded, and is continuous inferiorly with the posterior 

 edge of the body. The anterior edge is thinner and concave. It termi- 

 nates inferiorly in a projecting piece of bone called the " beak," which 

 enters the olecranon fossa when the elbow is extended. The beak over- 

 hangs a semicircular outcut which is articular. This is the sigmoid cavity, 

 and it articulates with the facet on the inferior extremity of the humerus, 

 which curves upwards from the inferior surface into the olecranon fossa. 

 The summit of the olecranon process is rough for the most part and 

 tuberous. It is not well protected, and forms the prominent point of the 

 elbow. To it is attached the tendon of insertion of the triceps extensor 

 cubiti muscle. 



Inferiorly the ulna extends to the lower third of the radius, where it 

 terminates in a point or in a small nodule. 



The bone ossifies from two centres, one of which is for the body and 

 the other for the olecranon process. 



Fracture of this bone occurs much more commonly than of the 

 radius or humerus. At times a small portion is broken off the summit ot 

 the olecranon process as the result of a blow. Occasionally the ulna is 

 separated from the radius. This occurs most frequently in young animals 

 in which ossification of the two bones is not very firm. In these cases 

 the arciform and the interosseous ligaments are ruptured. The beak of 

 the ulna may be snapped off through coming too forcibly into contact 

 with the floor of the olecranon fossa. By far the most common form of 

 fracture of the ulna takes place transversely, and extends through the 

 sigmoid cavity. The cause in these cases is undue extension of the elbow 



