THE ULNA 157 



On the lateral surface is the radial notch (lesser sigmoid cavity), an oblong 

 articular surface which articulates with, the circumference of the head of the radius, 

 the anterior and posterior margins of which afford attachment to the annular 

 ligament and the radial collateral ligament of the elbow-joint. In flexion of the 

 elbow the tip of the process is received into the coronoid fossa of the humerus. 



The body [corpus ulnae] or shaft throughout the greater part of its extent is 

 three-sided, but tapers toward the lower extremit}', where it becomes smooth and 

 rounded. It has three borders and three surfaces. Of the three borders, the 

 lateral, the interosseous crest, is best marked. In the middle three-fifths of the 

 shaft it is sharp and prominent, but becomes indistinct below; above it is contin- 

 ued by two lines which pass to the anterior and posterior extremities of the radial 

 notch and enclose a depressed triangular area (bicipital hollow), the fore part of 

 which lodges the tuberosity of the radius and the insertion of the biceps tendon 

 during pronation of the hand, while from the posterior part the supinator takes 

 origin. The interosseous crest separates the volar from the dorsal surface and 

 gives attachment by the lower four-fifths of its extent to the interosseous mem- 

 brane. The volar border is directly continuous with the medial edge of the rough 

 surface for the brachiahs and terminates inferiorly in front of the styloid process. 



Fig. 186. — Upper End of Ulna Showing Two Epiphyses. (E. Fawcett.) 



Beak centre 



Scale on summit of olecranon 



y Semilunar notch 



Throughout the greater part of its extent it is smooth and rounded, and affords 

 origin to the flexor digitorum profundus and the pronator quadratus. It separates 

 the volar from the medial surface. The dorsal border commences above at the 

 apex of the triangular subcutaneous area on the back of the olecranon, and takes a 

 sinuous course to the back part of the styloid process. The upper three-fourths 

 gives attachment to an aponeurosis common to three muscles, viz., the flexor and 

 extensor carpi ^ilnaris and the flexor digitorum profundus. This border separates 

 the medial from the dorsal surface. 



Surfaces. — The volar (or anterior) surface is grooved in the upper three- 

 fourths of its extent for the origin of the flexor digitorum profundus, narrow and 

 convex below, for the origin of the pronator quadratus. The upper limit of the 

 area for the latter muscle is sometimes indicated by an oblique line — the pronator 

 ridge. Near the junction of the upper and middle thirds of the anterior surface is 

 the nutrient foramen, directed upward toward the proximal end of the bone. It 

 transmits a branch of the volar interosseous artery. The medial surface, smooth 

 and rounded, gives attachment, on the upper two-thirds, to the flexor digitorum 

 profundus, whereas the lower third is subcutaneous. The dorsal (or posterior) 

 surface, directed laterally as well as backward, presents at its upper part the 

 oblique line of the ulna running from the posterior extremity of the radial notch 

 to the dorsal border. 



