106 



THE BONES. 



end and the middle of the body. It is also distinguished from that of the Horse 

 by the depth of a narrow groove, which passes through the rough lip above and 

 behind the inferior surface, superior to the small fossa 

 F'g- 66. that receives the third bone of the upper row of the 



carpus during flexion and extension movements. The 

 ulna of this animal is more developed than that of the 

 Horse, and its olecranon is proportionately shorter, 

 broader, and more hollowed on its internal face. The 

 inferior extremity (Fig. 66, 9) is nearly always formed 

 by a particular nucleus, which is also sometimes the case 

 in the Horse. 



In the Hinny, the bone of the forearm resembles 

 that of the Ass, with the exception of the smallness of 

 the lower end of the ulna. 



Differential Characters in the Forearm Bones of 

 OTHER Animals. 



The principal ditferential characters that they present are con- 

 nected with the relative dimensions of the two bones and their mode 

 of union. Regarding these, and as generally applicable, tlie follow- 

 ing principles may be laid down : — 



1 . The development of the ulna is in direct relation to the division 

 of the foot. — Mouodactylous animals — such as the Horse, Ass, and 

 Mule — have, in fact, only a rudimentary ulna. In the pentadacty- 

 lous animals, on the contrary — as Man, the Cat, Elephant, etc., — 

 this is a veritable long bone which equals, or even exceeds, the 

 radius in volume. 



2. The closeness of union between the radius and ulna is in 

 increased proportion as the animal exchmvely employs its inferior 

 extremity for standing and walking. — Thus, in Solipeds and Rumi- 

 nants, and Pachyderms in general, the two bones are consolidated, 

 or at least united by an interosseous ligament, and in so firm a 

 manner that they can only execute very obscure movements on 

 each other. The anterior limb of these animals is, indeed, only 

 used to support the body on the ground. In those, on the contrary, 

 which may employ it to dig up the soil, climb on trees, etc., or as 

 an organ of preliension, the radius and ulna are merely joined at 

 their extremities by an articulation, which permits them to move 

 upon one another with the greatest facility. Rodents, the 

 majority of the Caniivora, and the Quadrumana, are so provided ; 

 but it is in Man that the relative independence of the two bones 

 is carried to the highest degree. No animal can so easily execute 

 the movements of pronation and supination of tiie hand, which are 

 determined by the play of tlie two bones of the forearm on each other. 



To the indication of these fundamental characters, may be added 

 some details on a few particular and important points. 



A. Ox, Sheep, Goat. — The forearm of the Ox is short; 

 that of the Sheep and Goat is longer ; but in the three species, the 

 ulna— thicker than in the Horse— is a long bone developed from 

 three primary nuclei. It extends the whole length of tlie radius, 

 and concurs in forming the articular surface corresponding with 

 the carpal liones. 



The inferior articular surface is cut obliquely downwards and 

 inwards. There are two radio-ulnar arches — a superior and in- 

 ferior — united externally by a deep fissure. The union of tlie two 

 bones is more intimate than in the Horse; for ossification always 

 ends by invading that portion uf the interosseous ligament placed 

 above the superior vascular arcli (Fig. 71). 



forearm bones OF THE 



1, Diaphysis of the radius; 



2, bicipital tuberosity ; 



3, external and superior 

 tuberosity of the radius ; 



4, groove for the passage 

 ot the anterior extensor 

 of the phalanges ; 5, 

 surface for insertion of 

 branch of the perforans 

 tendon ; 6, body of the 

 ulna interrupted at its 

 lower third, but more 

 complete than usual ; 7, 

 summit of the olecranon ; 

 8, beak of ditto ; 9, in- 

 ferior extremity of ditto ; 

 10, radio-ulnar arch ; 11, 

 crest above the inferior 

 articular surface of the 

 radius, behind. 



