THE ULNA, OR ELBOW BONE 189 



terminates below at the back part of the styloid process; it is well marked in the 

 upper three-fourths, and gives origin to the aponeurosis common to the Flexor 

 carpi ulnaris, the Extensor carpi ulnaris, and the Flexor profundus digitorum 

 muscles; its lower fourth is smooth and rounded. This border separates the 

 internal from the posterior surface. 



The external or interosseous border (crista interossea) commences above by the 

 union of two lines, which converge one from each extremity of the lesser sigmoid 

 cavity, enclosing between them a triangular space for the origin of part of the 

 Supinator [brevjs], and terminates below at the middle of the lower extremity. 

 Its two middle fourths are very prominent; its lower fourth is smooth and rounded. 

 This border gives attachment to the interosseous membrane, and separates the 

 anterior from the posterior surface. 



The anterior or palmar surface (fades volaris) lies between the anterior and 

 external borders, is much broader above than below, is concave in the upper three- 

 fourths of its extent, and affords origin to the Plexor profundus digitorum; its 

 lower fourth, also concave, is covered by the Pronator quadratus. The lower 

 fourth is separated from the remaining portion of the bone by a prominent 

 ridge (jfronator ridge} directed obliquely from above downward and inward; 

 this ridge marks the extent of attachment of the Pronator quadratus. At the 

 junction .of the upper with the middle third of the bone is the nutrient foramen. 

 It opens into the nutrient canal, which is directed obliquely inward and upward 

 Iproximally). 



The posterior, dorsal, or extensor surface (fades dorsalis) is bounded internally 

 by the interosseous border, externally by the posterior border, and is directed 

 backward and outward; it is broad and concave above, somewhat narrower and 

 convex in the middle of its course, narrow, smooth, and rounded below. It pre- 

 sents, above, an oblique ridge, which runs from the posterior extremity of the 

 lesser sigmoid cavity, downward to the posterior border; the triangular surface 

 above this ridge receives the insertion of the Anconeus muscle, while the upper 

 part of the ridge itself affords origin to the Supinator. The surface of bone 

 below this is subdivided by a longitudinal ridge into two parts; the internal part 

 is smooth, and covered by the Extensor carpi ulnaris; the external portion, wider 

 and rougher, gives origin from above downward to part of the Supinator, the 

 Extensor ossis metacarpi pollicis, the Extensor longus pollicis, and the Extensor 

 indicis muscles. 



The internal surface (fades medialis) is broad and concave above, narrow and 

 convex below. It gives origin by its upper three-fourths to the Flexor profundus 

 digitorum muscle; its lower fourth is subcutaneous. The anterior and the inner 

 surfaces constitute the flexor surface. 



The Distal or Lower Extremity is of small size, and excluded from the 

 articulation of the wrist-joint. It presents for examination two eminences, the 

 outer and larger of which is a rounded, articular eminence, termed the head 

 (capitulum ulnae}, the inner, narrower and more projecting, is a nonarticular 

 eminence, the styloid process (processus styloideus). The head presents an 

 articular facet, part of which, of an oval form, is directed downward, and plays 

 on the upper surface of the triangular fibrocartilage which separates it from the 

 wrist-joint; the remaining portion, directed outward, is narrow, convex, and 

 received into the sigmoid cavity of the radius. The styloid process projects from 

 the inner and back part of the bone, and descends a little lower than the head, 

 terminating in a rounded summit, w r hich affords attachment to the internal lateral 

 ligament of the wrist. The head is separated from the styloid process by a 

 depression for the attachment of the triangular intra-articular fibrocartilage; and 

 behind, by a shallow groove for the passage of the tendon of the Extensor carpi 

 ulnaris. 



