THOEACIC LIMBS 571 



Vexity of the inner aspect of the bone, but in the lower third it becomes 

 prominent and sharp. It forms the inner boundary of the area of 

 attachment of the radio-ulnar head of the flexor profundus digitorum 

 and pronator quadratus muscles. On it, or just anterior to it, at a 

 distance of about ten millimetres below the base of the coronoid process, 

 is the nutrient foramen of the shaft, which pierces the compact tissue 

 obliquely toward the upper extremity. The anterior border separates 

 the anterior and internal surfaces ; in walking it faces inward. 



The posterior border (Fig. 441) begins above as the prolongation 

 of the posterior surface of the olecranon, and is continued without 

 interruption to the back of the lower extremity. It is everywhere 

 distinct, but is more rounded above than below. It separates the areas 

 of attachment of the ulnar heads of the flexor carpi ulnaris and 

 flexor profundus digitorum muscles from the area of attachment of 

 the extensor indicis. The posterior border is between the internal 

 and posterior surfaces ; it faces backward and outward. 



The external or interosseous border (Figs. 438, 440) arises above 

 from the corouoid process, and, passing down on the front and outer 

 side, ends on the outer side of the lower extremity. Its smooth and 

 rounded upper fourth separates the flexor profundus digitorum and 

 extensor ossis metacarpi pollicis muscles. Its lower three-fourths are 

 prominent, sharp, and, in the upper part, rough, to give attachment 

 to the interosseous membrane connecting the ulna with the radius. 

 This part of the border and the iuterosseous membrane separate the 

 flexor profundus digitorum and pronator quadratus from the extensor 

 ossis metacarpi pollicis muscles. The external border faces outward 

 and forward and divides the anterior from the posterior surface. 



The anterior surface (Fig. 438) lies between the external and 

 anterior borders. Its upper fourth is slightly narrower than the 

 remaining portion and is convex transversely; sometimes just below 

 the coronoid process it is elevated in the middle line almost to a ridge. 

 Below the upper fourth, the surface becomes suddenly wider, owing 

 to the increased prominence of the external border ; it gradually be- 

 comes less convex, and in the lower third is quite flat and twisted 

 inward. The surface faces inward and forward. Its upper part 

 affords an area for the origin of the flexor profundus digitorum, 

 and the lower part gives attachment to the ulnar end of the pronator 

 quadratus. 



