THE UPPER EXTREMITIES. 177 



border forms a guide in tying the brachial artery, as this 

 artery runs along its inner side. 



The brachialis anticus rises from the lower half of the 

 outer and inner surfaces of the humerus and is inserted 

 into the coronoid process of the ulna, thus covering and 

 proiecting the elbow-joint anteriorly. It is a flexor of 

 the forearm. 



Another smaller muscle on the anterior arm, which 

 also aids in flexion, is the coraco-brachialis , which ex- 

 tends from the coracoid process of the scapula to the 

 middle of the inner surface of the humerus. 



Extending the entire length of the posterior surface 

 of the humerus is the triceps, similar to the quadriceps 

 extensor in the thigh and direct antagonist to the bi- 

 ceps and brachialis anticus muscles. It rises by a long 

 head from below the glenoid fossa, by the external head 

 from the upper third of the posterior surface of the 

 humerus, and by the internal head from the middle 

 and lower thirds of the posterior surface. It is inserted 

 in the olecranon process of the ulna and serves to extend 

 the forearm and arm. 



The humerus is more often fractured by muscular 

 action than any other bone. Usually the fracture oc- 

 curs in the lower half of the bone and sometimes the 

 musculo-spiral nerve is involved. There is a great ten- 

 dency to non-union, probably due to interposition of 

 soft parts. Sometimes the break is across and down 

 between the condyles, T-fracture. Involvement of the 

 elbow-joint is more serious than fracture of the humerus 

 alone. Sarcoma of the humerus does occur and may 

 require the removal of the clavicle and scapula as well 

 as of the arm bone itself. In amputation of the hum- 

 erus in children a long skin flap is left to allow for growth 

 of the bone, as it is liable to grow again. 



The Ulna. In the forearm there are two bones, the ulna 



and the radius, of which the former is the longer. The 



ulna is on the inner side of the forearm and its upper end 



forms the greater part of the articulation with the humerus, 



12 



