THORACIC LIMBS 583 



The posterior surface (Fig. 448) is divided into two parts by a 

 groove, which is not quite vertical, but runs outward as well as down- 

 ward. The parts on each side of the groove are convex from side to 

 side, and also from above downward, and pass into the neck below. 

 They are covered by ligaments. 



The neck is most clearly marked on the front and on the sides. 

 It presents well-marked inner and outer borders dividing the anterior 

 surface from the posterior surface. The anterior surface is concave in 

 both directions, exhibiting a pit more or less distinct between the 

 head and the tubercle. The inner part of the posterior surface faces 

 backward, and the outer part backward and outward. The inner side 

 of the anterior surface is continued on the tubercle, the other parts 

 pass without a break into the shaft. The neck is pierced, especially 

 on its anterior surface, by a number of foramina for blood-vessels, and 

 is encircled by the orbicular ligament uniting the head with the ulna. 



The tubercle is a linear oval swelling near the inner anterior 

 border, between the neck above and the shaft below. The tendon of 

 the biceps muscle is attached along its inner third, which is sometimes 

 marked by a roughened depression. 



The Shaft is narrow and nearly cylindrical above, but wide and 

 flattened from before backward below. It presents on examination 

 three borders, the anterior, the posterior, and the internal, and three 

 surfaces, the anterior, the posterior, and the external. Of the three 

 borders, the anterior and internal are well denned ; the posterior is 

 but faintly marked. The anterior and posterior surfaces are easily 

 recognized ; the external, however, is less evident. 



The anterior border (Fig. 446), which may be also called the 

 external, begins on the outer side of the neck and passes straight 

 down to the front of the outer side of the lower end of the bone. At 

 the junction of its upper and middle thirds it is joined by a second 

 branch, known as the oblique line of the radius, which begins at the 

 outer part of the lower end of the tubercle and runs obliquely down- 

 ward and outward across the front aspect of the shaft. Just below the 

 point where the oblique line ends the border is marked by a rough 

 line from five to ten millimetres long, on which is inserted the pronator 

 teres muscle. On the very end of the border, where it passes into 

 the border of the lower extremity, is the small area of insertion of 

 the supinator longus muscle. The oblique line separates the origin of 



