282 TJie Back of tJie Fore-arm 



obliquely across the back of the wrist, where its tendon occupies a 

 deep and solitary groove on the ulnar side of that for the radial 

 extensors of the wrist. Its oblique tendon, which crosses the radial 

 artery just as it is entering the root of the first interosseous space, is an 

 important and conspicuous landmark. The extensor inditis arises 

 from the ulna ; its tendon passes with the tendons of the extensor com- 

 munis and is inserted with the outermost of them (v. p. 287). 



All the muscles of the deep layer are supplied by the posterior 

 interosseous nerve. 



Epiphyses of ulna. 



The ulna begins to ossify at about the eighth week of foetal life, and 

 at birth ossification has extended from the shaft through the coronoid 

 and olecranon processes, with the exception of a shallow cap at the top 

 of the olecranon, which is still cartilaginous, and which does not begin 

 to ossify until the tenth year. This unimportant epiphysis joins the 

 shaft at puberty, but the lower end of the bone, which begins to ossify 

 in the fourth year, does not join until manhood. 



Inflammation of the bursa which is placed between the skin and the 

 olecranon process constitutes ' miner's elbow ' ; I have seen it greatly 

 enlarged in a bill-poster, as the result of constant friction against the 

 walls and hoardings. 



Epiphyses of radius. 



The radius begins to ossify at about the eighth week, and at birth 

 only its ends are cartilaginous. The lower epiphysis begins to ossify 

 in the second year and joins at manhood ; the upper epiphysis ossifies 

 in the fifth year and joins at puberty. 



Fracture may occur in any part of the fore-arm, but the most 



common site is at the lower end of the radius Colles's fracture. At 

 first view it seems unlikely that the stronger bone should oftenest break, 

 and that the fracture should be through the strongest part of that bone. 

 The explanation is simple : a man is falling and he puts out his hand 

 to break the shock. The hand being in the position of pronation, the 



