THE ULNA 89 



before backward, with its long axis in (lie same direction, 

 for the head of the radius. 



The shaft, or body, tapers from above, is three-sided in its 

 upper three-fourths, slender and cylindrical in its lower fourth. 

 The upper three-fourths is convex backward; it is also convex 

 externally above and internally below. The anterior border 

 passes from the inner edge of the coronoid to the front of 

 the styloid; it is thick and rounded, and gives attachment to 

 the flexor profundus digitorum, and in the lower fourth to the 

 pronator quadra tus. 



The posterior border begins below the olecranon, and runs 

 with a sinuous curve to the back of the styloid. It is ill defined 

 below and subcutaneous throughout, and affords attachment 

 to an aponeurosis common to three muscles the flexor carpi 

 ulnaris, extensor carpi ulnaris, and flexor profundus. The 

 external or intcrosseous border is a sharp edge in the middle 

 three-fifths of the shaft. Below it is faintly marked. The 

 upper fifth is continued by two lines passing to the extremities 

 of the small sigmoid notch; the posterior line is the prominent 

 supinator ridge, for the supinator brevis muscle. 



The anterior surface is concave above, and gives origin to 

 the flexor profundus digitorum; the lower third is marked 

 off by the oblique pronator ridge, directed downward and up- 

 ward, limiting above the origin of the pronator quadratus. 

 Above the middle is a medullary foramen directed upward. 



The internal surface is smooth, and gives attachment to 

 the flexor profundus digitorum muscle; it is subcutaneous 

 in the lower third. 



The posterior surface looks outward and backward; an oblique 

 line descending from the supinator ridge to the posterior border 

 at the junction of its upper and middle thirds marks off a 

 triangular area for the anconeus muscle. The ridge itself 

 gives attachment to the supinator brevis. Below this is a 

 longitudinal ridge dividing the surface into a smooth inner 

 portion covered by the extensor carpi ulnaris, and an outer 

 part giving attachment from above downward to the extensor 

 ossis metacarpi pollicis, extensor secundi internodii pollicis, 

 or extensor longus pollicis, and extensor indicis. 



The inferior extremity presents a rounded head; from its 

 inner and back part the styloid process projects downward, 

 giving attachment to the internal lateral ligament and to the 



