166 ELEMENTARY ANATOMY. [LESS. 



14. The RADIUS is a bone constantly present and distinctly 

 represented, wherever an arm exists, in all Vertebrates 

 above Fishes. In Fishes its exact representative cannot be 

 determined. The ossicles or cartilages, however, which may 

 be distally annexed, to the representatives of the humerus, 

 probably as a whole represent both the radius and the ulna. 



In the constancy of its development the radius shows its 

 generally greater importance as the main bone of the fore- 

 arm, when compared with the ulna, which latter may more 

 or less completely abort. 



In Birds, however, it is subordinate in development to the 

 ulna, which is the main bone of the fore-arm in them, e.g. 

 the Eagle. 



In its elongated figure the radius of man agrees with the 

 same bone as generally developed, but it may be shortened 

 and flattened even in some Birds, e.g. the Penguin, and very 

 much more so in Cetacea, e.g. the Dolphin, and still more 

 so in the extinct Ichthyosauria. 



Instead of being free as in man, the radius may anchylose 

 at each end with the corresponding extremities of the ulna, 

 as is the case with the Sirenia. 



It may be the solitary apparent representative of the bones 

 of the fore-arm, which are completely fused together, as in 

 the Frog and Camel. It may be all but the only bone of the 

 fore-arm, through the small development of the ulna, as is 

 the case in Bats and Ruminants. 



That crossed position of the radius with relation to the 

 ulna which is called pronation in man, is the constant and 

 only position of the bone in many, as e.g. the Dog, Elephant, 

 and hoofed beasts generally. Pronation and supination, 

 however, are not confined to man. They exist sometimes 

 very distinctly, as e.g. in the Apes and Sloths. 



No motion of the kind is possible in many forms in which 

 the radius does not cross the ulna, as in Bats and Birds, 

 where such a flexibility of the limb would be fatal to flight. 

 Pro- and supi-nation are also impossible in the Cetacea, 

 where there is no movable elbow-joint at all. 



But the limbs may permanently retain a position which 

 indeed is primitive and temporary in man ; that is to say, 

 there may be no crossing of the bones of the fore-arm, and 

 yet the whole dorsum (or extensor surface) of the limb may be 

 turned outwards somewhat in the position in which a man puts 

 his arms when, resting on his two palms, he stoops to drink 

 from a pool between them. Even thus the primitive position 



