270 THE ARM AND FORE- A KM. [CHAP. 



very modified and adaptive position (as will be explained 

 more fully in the chapter on the comparison of the fore and 

 hind limbs), usually crossing each other in the fore-arm, the 

 radius in front of the ulna, so that the preaxial bone 

 (radius), though external (in the ordinary position of the 

 limb) at the upper end, is internal at the lower end; and 

 the hand being mainly fixed to the radius, also has its pre- 

 axial border internal. In the large majority of Mammals 

 the bones are fixed in this position ; but in some few, as 

 in Man, a free movement of crossing and uncrossing, or 

 pronation and supination, as it is termed, is allowed between 

 them, so that they can be placed in their primitive parallel 

 condition, when the hand (which moves with the radius) is 

 said to be supine, or they may be crossed, when the hand is 

 said to be prone. 



In most Mammals which walk on four limbs, and in which 

 the hand is permanently prone, the ulna is much reduced in 

 size, and the radius increased, especially at the upper end ; 

 and the articular surface of the latter, instead of being con- 

 fined to the external side of the trochlea of the humerus, 

 extends all across its anterior surface, and the two bones, 

 instead of being external and internal, are anterior and 

 posterior (see Figs. 87, 88, and 89). 



The ulna is always characterised by a conspicuous, more 

 or less compressed prolongation,, extending upwards beyond 

 the excavated humeral articular surface (sigmoid notch), and 

 serving as the point of attachment to the extensor muscles 

 of the fore-arm, called the olecranon or anconeal process. 



Each of the bones of the fore-arm has commonly a 

 principal centre of ossification for the shaft, and an epiphysis 

 at either end. 



