54 HUMAN ANATOMY. 



does not enter into the formation of the wrist-joint, being ex- 

 cluded by the interarticular fibro-cartilage. 



The shaft is prismatic above, smooth and rounded below, and 

 presents : 



Anterior surface, gives attachment to the deep flexors and 

 pronator quadratus ; 



Nutrient foramen on anterior surface, directed upward to- 

 ward the elbow- joint; 



Posterior surface marked above by an oblique line for part 

 of supinator brevis, above which is smooth triangular surface for 

 anconeus muscle, and the lower third for extensor muscles of the 

 thumb ; 



External border, sharp in middle two-thirds, for attachment 

 of interosseus membrane. 



The upper extremity is large and irregular, and presents : 



Olecranon process (head of elbow), projects upward and 

 forward, its apex being received into the olecranon fossa of the 

 humerus in extension of the forearm ; its upper border has rough 

 impression for the triceps muscle ; its lateral borders are grooved 

 for external and internal lateral ligaments; 



Coronoid process, smaller than olecranon, projects forward 

 from anterior surface, being received into coronoid fossa of hu- 

 merus in flexion. 



Its upper surface forms part of the great sigmoid cavity. 



Its under surface has rough impression for insertion of 

 brachialis anticus, and has, at its junction with the shaft, the 

 tubercle of the ulna for the oblique ligament. 



Its outer surface is the lesser sigmoid cavity. 



Its inner surface gives attachment to the internal lateral 

 ligament, and the flexor digitorum sublimis, flexor profundus 

 digitorum, and one head of pronator radii teres. 



Greater sigmoid cavity is a large, semi-lunar depression 

 between the olecranon and coronoid processes, divided into two 

 unequal lateral parts by an elevated ridge. It is continuous on 

 the outer side with the lesser sigmoid cavity and articulates with 

 the trochlear surface of the humerus. 



Lesser sigmoid cavity is an oval, concave, articular depres- 

 sion, external to the coronoid process, for articulation with the 

 head of the radius. Its prominent extremities give attachment 

 to the orbicular ligament. 



The lower extremity is small and cylindrical and presents : 



Head, an external, rounded, articular process, for the tri- 

 angular fibro-cartilage below and the sigmoid cavity of the radius 

 . externally; 



