xv.] GENERAL CHARACTERS. 245 



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 

 e 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. 82, 83, and 84, p. 248.) 



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. 



Special Characters of the Bones of the Arm and Fore- 

 arm in the different Groups. 



Order Primates. — In Man the humerus is long, slender, 

 and straight, with a large globular head. Neither the tube- 

 rosities, nor the deltoid and supinator ridges, are much 

 developed. The internal condyle is prominent, but there 

 is no supracondylar foramen as a normal condition. 



