THE RADIUS 91 



and at its lower part divides into two ridges, which include 

 the margins of the sigmoid cavity, analogous to the division 

 of a like border of the ulna. 



The anterior surface is grooved longitudinally for the flexor 

 longus pollicis muscle; at the lower end to the extent of about 

 a quarter of the surface is an impression for the pronator 

 quadratus. A medullary foramen is above the middle of this 

 surface passing upward. 



The posterior surface shows at the junction of the upper 

 and middle thirds the posterior oblique line, running downward 

 and outward to the posterior margin; below this, the entire 

 width of the surface, is attached the extensor ossis metacarpi 

 pollicis, and below that, by a long, narrow impression con- 

 tiguous to the interosseous margin, the extensor primi internodii 

 pollicis (extensor brevis pollicis). 



The external surface is convex, and marked near the middle 

 by an impression for the pronator radii teres; above this, on 

 the area between the anterior and posterior oblique lines, 

 is inserted the supinator brevis. 



The lower extremity of the radius, broad and quadrilateral, 

 presents a carpal articular surface and an ulnar articular 

 surface. The former is divided by a line into a quadrilateral 

 inner part for the semilunar, and a triangular outer part for 

 the scaphoid. The articular surface for the ulna or sigmoid 

 cavity is at right angles to the inferior surface, and concave 

 from before backward. To the smooth border between these 

 two articular surfaces is attached the base of the triangular 

 fibrocartilage. Externally the styloid process projects downward. 

 Anteriorly a transverse ridge forms the lowest limit of the 

 pronator quadratus impression, which is continued into a 

 vertical ridge external to that impression; between this ridge 

 and the scaphoid facet is a triangular area for a strong band 

 of the anterior ligament. The external and posterior aspects 

 are marked by the following grooves from without inward; 

 a flat groove for the extensor ossis metacarpi pollicis and 

 extensor primi internodii or extensor brevis pollicis (next 

 descends the styloid process); a broad groove, subdivided 

 by a slight ridge, for the extensor carpi radialis longior and 

 brevior; an oblique narrow groove, directed downward and 

 outward, bounded externally by a tubercle, for the extensor 

 secundi internodii pollicis (extensor longus pollicis); a broad 



