584 MAMMALIAN ANATOMY 



insertion of the supinator brevis muscle above from the area of origin 

 of the radio-ulnar head of the flexor profundus digitorum below. 

 The rest of the anterior border marks the external limit of the same 

 flexor muscle and of the pronator quadratus, and separates them from 

 the extensor ossis metacarpi pollicis arising on the external and pos- 

 terior surfaces. 



The internal or interosseous border (Fig. 446) begins above at the 

 inner side of the tuberosity, near the origin of the oblique line, and 

 extends inward and downward to the lower fourth, where it divides 

 into two branches, which go to the anterior and posterior angles of 

 the inner side, limiting a small triangular internal surface on the lower 

 extremity. The internal border is rough at its upper part. It affords 

 attachment to the interosseous ligament, which comes from the external 

 border of the ulna. It separates the area of origin of the radio-uluar 

 head of the flexor profundus digitorum from that of the extensor ossis 

 metacarpi pollicis muscles. It also limits the inner boundary of the 

 origin of the pronator quadratus muscle. 



The posterior border (Fig. 448) is not well marked, except in the 

 middle of its course. It begins above on the inner side of the back of 



<Z2 



the neck, near the tubercle, and runs at first downward and obliquely 

 outward, then, at about the middle of the bone, when it has passed the 

 median line, it is continued straight down not far from the anterior 

 border to the middle vertical ridge on the back of the lower extremity. 

 The posterior border separates the area of origin of the supinator 

 brevis from that of the extensor ossis metacarpi pollicis. 



The anterior surface (Fig. 446) is distinct. It is divided by the 

 oblique line into two parts. The upper part is small, wide above and 

 pointed below ; it is flattened or slightly convex transversely and faces 

 forward and outward, affording an area of origin to the supinator brevis 

 muscle. The lower larger part is limited by the anterior and internal 

 borders. It is concave from above downward, and, in the upper 

 half, concave also from side to side ; in its lower half it is slightly 

 convex transversely. The radio-ulnar head of the flexor profundus 

 digitorum arises from the upper half, the pronator quadratus from the 

 lower half. On the anterior surface, at the junction of the middle 

 and upper thirds, near the internal border, is found the nutrient 

 vascular foramen. It is directed toward the proximal end of the bone. 



The external surface (Fig. 448) is bounded by the anterior and 



