76 ANATOMY IN A NUTSHELL. 



digitorum proximally, and the Pronator quadratus distally. The posterior 

 border commences at the apex of a triangular subcutaneous surface at the back 

 of the olecranon and continues to the back part of the styloid process. It is 

 well marked in the proximal three-fourths and gives attachment to the common 

 aponeurosis which has three muscles attached to it. viz.. the Extensor carpi 

 ulnaris, Flexor carpi ulnaris and Flexor profundus digitorum. This border 

 separates the internal and posterior surfaces. The outer or interosseous bor- 

 der, is divided above, extending to the anterior and posterior extremities of 

 the sigmoid cavity; embracing the triangular depression (the bicipital hollow). 

 In the trout part of this impression lodges the tubercle and tendon of the Bi- 

 ceps, when the arm is pronated, and the back part gives attachment to the 

 Supinator brevis. It separates the anterior and posterior surfaces. To its 

 distal four-fifths is attached the interosseous membrane. The anterior sur- 

 face is grooved in the proximal three-fourths of its extent, which is broader 

 than the distal portion, and gives attachment to the Flexor profundus digi- 

 torum. Its distal one-fourth gives attachment to the Pronator quadratus. 

 The pronator ridge, or oblique line, is directed distally and inward at the prox- 

 imal limit of the Pronator quadratus. 



The nutrient foramen is on this surface at the junction of the proximal one- 

 third with the middle one-third, and is directed towards the elbow. The nutri- 

 ent artery is a branch of the anterior interosseous. 



The internal surface is broad and concave proximally. narrow and convex 

 distally. The proximal three-fourths gives attachment to the Flexor profun- 

 dus digitorum, whereas the distal one-fourth is subcutaneous. The posterior 

 surface is broad and concave proximally, narrow and convex in the middle. 

 narrow, smooth, and round distally. It> oblique line runs from the posterior 

 extremity of the lesser cavity distally to the posterior border. The first part 

 of this line gives attachment to the Supinator brevis. The triangular surface 

 between this line and the elbow receives the insertion of the Anconeus. The 

 "Uter portion of this surface gives attachment to the Supinator brevis. Ex- 

 tensor ossis metacarpi pollicis, Extensor Longus pollicis, and Extensor indicis. 

 The inner portion is smooth, being covered with the Extensor carpi ulnaris. A 

 perpendicular ridge separates these two portions of this surface. 



The lower extremity is small ami consists of two parts, a head and styloid 

 process. ' m the articular surface they are separated by a groove which re- 

 ceive- the apex of the interarticular fibro-cartilage, thus separating the ulna 

 from the wrist-joint. The margin of the head is received in the sigmoid cavity 

 of the radius and the styloid process is a continuation of the posterior border 

 projecting from the inner and back part of the bone. Its apex gives attach- 

 ment to the internal lateral ligament; it> posterior surface is grooved for the 

 Extensor minimi digiti. 



Ossifk ITION. From three center-, one for the shaft about the eighth 

 week, one tor the distal extremity about the fourth year which joins the shaft 

 the eighteenth or twentieth year, and one tor the proximal extremity the 

 tenth year which join- the shaft the sixteenth or seventeenth year. 



Ai:t)< i lation.- It articulates with the humerus and radius. 



