OSTKOLOGY OF THE HORSE. 51 



OF THE ARM. 



The arm is composed of a single bone, the os brachii, which 

 corresponds to the human radius and ulna united together. 



ARM-BONE (OS BRACHII.) 



Form. — Cylindroid ; flattened before and behind ; slightly 

 curved forwards. 



Division. — Into radial and ulnar portions. 



The RADIAL PORTION consists of a body and superior and 

 inferior extremities. 



The body is long; prominent and smooth anteriorly ; slightly 

 excavated and roughened posteriorly, where it is pierced in a 

 direction downward by the medullary foramen : the former is 

 clothed by the extensor muscles of the leg and foot ; the latter by 

 the flexors of those parts. 



The superior extremity, like the inferior, expanded into greater 

 breadth than the body, presents an articulatory surface, divided, 

 by a gentle eminence running across the middle, into two concavi- 

 ties, of which the inner is broader and more circular than the 

 outer; the latter, indeed, has a superficial rising, taking the 

 direction which makes a similar but imperfect subdivision of it : 

 those concavities receive the condyles of the humerus. The exter- 

 nal lateral process is more prominent and sharper than the inter- 

 nal, and is surmounted in front by a small tubercular eminence : 

 to these processes the lateral ligaments are attached ; the eminence 

 serves also to deepen the outer cup of articulation. 



The inferior extremity is remarkable for the number and variety 

 of its articulatory surfaces, and for presenting a hiatus extero- 

 posteriorly, as if a piece of the bone had been chiselled out. It 

 possesses three articulatory surfaces. The largest, or inner one, 

 is quadrilateral in outline, sigmoid in superficies; the middle is 

 similar in form, but of smaller dimensions; the outer, or smallest, 

 is an ovoid convexity. The internal lateral process is the most 

 prominent; the external has a groove along it. The bone is also 

 grooved in front by the passage of the extensor tendons. 



The Ulnar Portion consists of body, projection, and articu- 

 latory surface. 



The body is the tapering triangular part which is firmly united 

 above, but more intimately below, with the radial portion ; be- 

 tween the two is an interval, admitting of the passage of a circulus 

 of blood-vessels. — The surface composing part of the humero- 

 brachial articulation, in form a semilunar concavity, is only in part 

 articulatory, the lower half presenting roughened inequalities for 

 ligamentous attachment : it is adapted to the smooth trochleal 



