ULNA. 135 



radius, being the largest and longest of the two. Its upper extremity, of great 

 thickness and strength, forms a large part of the articulation of the elbow-joint ; 

 it diminishes in size from above downwards, its lower extremity being very 

 small, and excluded from the wrist-joint by the interposition of an interarticular 

 fibre-cartilage. It is divisible into a shaft, and two extremities. 



The Upper Extremity, the strongest part of the bone, presents for examination 

 two large curved processes, the Olecranon process, and the Coronoid process ; and 

 two concave articular cavities, the greater and lesser Sigmoid cavities. 



The Olecranon Process (w*.**^, elbow; xpdvov, head) is a large, thick, curved 

 eminence, situated at the upper and back part of the ulna. It rises somewhat 

 higher than the coronoid, is curved forwards at the summit so as to present a pro- 

 minent tip, its base being contracted where it joins the shaft. This is the 

 narrowest part of the upper end of the ulna, and, consequently, the most usual 

 seat of fracture. Its posterior surface, directed backwards, is of a triangular form, 

 smooth, subcutaneous, and covered by a bursa. Its upper surface, directed 

 upwards, is of a quadrilateral form, marked behind by a rough impression for 

 the attachment of the Triceps muscle ; and, in front, near the margin, by a slight 

 transverse groove for the attachment of part of the posterior ligament of the 

 elbow-joint. Its anterior surface is smooth, concave, covered with cartilage in 

 the recent state, and forms the upper and back part of the great sigmoid cavity. 

 The lateral borders present a continuation of the same groove that was seen on 

 the margin of the superior surface ; they serve for the attachment of ligaments, 

 viz., the back part of the interior lateral ligament internally ; the posterior liga- 

 ment externally. The olecranon process, in its structure as well as in its position 

 and use, resembles the Patella in the lower limb ; and, like it, sometimes exists 

 as a separate piece, not united to the rest of the bone. 1 



The Coronoid Process (xopwTj, a crow's beak ; ? i'6oj, form) is a rough triangular 

 eminence of bone which projects horizontally forwards from the upper and front 

 part of the ulna, forming the lower part of the great sigmoid cavity. Its base is 

 continuous with the shaft, and of considerable strength, so much so, that fracture 

 of it is an accident of rare occurrence. Its apex is pointed, slightly curved upwards, 

 and received into the coronoid depression of the humerus in flexion of the forearm. 

 Its upper surface is smooth, concave, and forms the lower part of the great sigmoid 

 cavity. The under surface is concave, and marked internally by a rough impression 

 for the insertion of the Brachialis anticus. At the junction of this surface with 

 the shaft is a rough eminence, the tubercle of the ulna, for the attachment of the 

 oblique ligament. Its outer surface presents a narrow, oblong, articular depression, 

 the lesser sigmoid cavity. The inner surface, by its prominent free margin, serves 

 for the attachment of part of the internal lateral ligament. At the front part of 

 this surface is a small rounded eminence for the attachment of one head of the 

 Flexor digitorum sublimis. Behind the eminence, a depression for part of the origin 

 of the Flexor profundus digitorum ; and, descending from the eminence, a ridge, 

 which gives attachment to one head of the Pronator radii teres. 



The Greater Sigmoid Cavity (aiyna, fZ6oj, form), so called from its resemblance 

 to the Greek letter s, is a semilunar depression of large size, situated between the 

 olecranon and coronoid processes, and serving for .articulation with the trochlear 

 surface of the humerus. About the middle of either lateral border of this cavity 

 is a notch which contracts it somewhat, and serves to indicate the junction of the 

 two processes of which it is formed. The cavity is concave from above downwards, 

 and divided into two lateral parts by a smooth elevated ridge, which runs from 

 the summit of the olecranon to the tip of the coronoid process. Of these two 

 portions, the internal is the largest ; it is slightly concave transversely, the external 

 portion being nearly plane from side to side. 



1 Professor Owen regards the olecranon to be homologous not with the patella, but with an 

 extension of the upper end of the fibula above the knee-joint, which is met with in the Ornitho- 

 rynchus, Echidna, and some other animals. (OWEN, " On the Nature of Limbs.") 



