296 



HUMAN ANATOMY. 



Fig. 310. 



thinnest in the centre (five milUmetres), the Hne at that point crossing the bone 

 about three miUimetres below the tip of the prominent middle thecal tubercle. 



The need for an accurate conception of this epiphysis is emphasized by the 

 facts: (I) that it is more often separated than any other in the body, with the 

 possible exception of the lower epiphysis of the femur ; (2) that its line has more 

 than once been figured and described as a line of fracture on the basis of skia- 

 graphs. 



The cause of separation is almost always a fall on the pronated hand. The 

 carpal bones are carried against the posterior border of the radial epiphysis, the pro- 

 nator quadratus and other muscles fix the lower ends of the diaphyses of the radius 

 and ulna, and the epiphysis is forced backward. The anterior carpal ligament and 

 the tendons on the palmar surface of the wrist are put on the stretch and aid in the 

 displacement. The supinator longus is directly attached to the epiphysis and aids in 

 maintaining the deformity. 



The synovial membrane of the wrist-joint does not reach the level of the epi- 

 physeal line of either the radius or the ulna. That joint is, therefore, not frequently 

 involved. 



The thinness of the centre of the epiphysis would lead to the expectation that 

 fracture would often complicate the separation. This is not the case, however. 



Poland says that the epiphysis is more solid than the lower 

 end of the bone of the adult (which has, of course, become 

 cancellous in structure), and that it thus escapes the fracture, 

 comminution, and impaction which are so frequent in later 

 life. 



The radius is often the subject of rickets, and of both 

 syphilitic and tuberculous epiphysitis, especially at its lower 

 end, on account of the exceptional frequency of falls upon 

 the hand and strains of the epiphyseal joint. 



Subperiosteal sarcomata are rare. Central sarcomata 

 almost invariably attack the lower end of the bone (page 

 366). 



Landmarks. — The head of the bone may be felt at the 

 bottom of the dimple or depression just below the external 

 condyle and to the outer side of the olecranon. It lies be- 

 tween the outer border of the anconeus and the muscular 

 swell of the supinator longus and radial extensors of the car- 

 pus. It is covered by the external lateral and orbicular liga- 

 , - ments. It can readily be felt to move when the forearm is 



Lower end ofie^ift radius, pronated and supinated. Its presence in that position demon- 



showing epiphyseal line, r ,. ,^ . r i "^ i- f i i i r i 



dorsal aspect. strates that dislocation of the radius or of both bones of the 



forearm backward — the common dislocation at the elbow — 

 has not occurred. Its free rotation negatives the existence of a non-impacted 

 fracture of the radius. 



The upper edge of the head lies immediately below the elbow-joint. In full 

 supination the tubercle can be indistinctly felt a little below the lower edge of the 

 head. The upper half of the radial shaft cannot be felt, as it lies beneath the bellies 

 of the extensors and the supinator brevis. The lower half is almost subcutaneous 

 and can readily be palpated through or between the tendons and muscles. The 

 expanded lower extremity is partly subcutaneous (at the base of the styloid exter- 

 nally) and is readily felt. The styloid itself, the prominent tubercle at the radial 

 side of the groove for the extensor longus pollicis (middle thecal tubercle), and 

 the sharp tubercle at the base of the styloid can easily be recognized. The latter 

 is the inferior termination of the pronator crest of the diaphysis, marks the ex- 

 ternal termination of the epiphyseal line, and is on a level with the lower and outer 

 part of the pronator quadratus muscle. The posterior end of the middle thecal 

 tubercle is three millimetres above the epiphyseal line on the posterior aspect of the 

 bone. 



The styloid process of the radius is lower — i.e. , nearer the hand — than the styloid 

 process of the ulna. 



