THE ANTERIOR LIMBS. 105 



for ligamentous insertion — the internal salient and well circumscribed, the other 

 external, and excavated by a vertical fissure — in which passes a tendon. 3. In 

 front, three grooves for the gliding of tendons ; the external is the largest, 

 and vertical like the median ; the internal, the narrowest, is oblique downwards 

 and inwards. 4. Posteriorly, a strong transverse ridge which surmounts the 

 articular surface, and serves for the insertion of ligaments. 



Structure and development.— The radius is a very compact bone, and is 

 developed from three centres of ossification — one for the body, and two for the 

 extremities. 



2. Ulna (Figs. 65, 66). 



This is an elongated, asymmetrical bone, in the form of an inverted triangular 

 pyramid, applied against the posterior face of the radius, to which it is united 

 in adult Solipeds. It offers for description a middle portion and two extremities. 



Middle portion. — This has three faces wider above than below, and three 

 borders which become joined at the inferior extremity of the bone. The external 

 face is smooth and nearly plane. The internal is also smooth and shghtly 

 hollowed. The anterior is formed to correspond with the radius, and presents 

 pecuharities analogous to those of the posterior face of that bone. Thus, there 

 is found, in proceeding from above to below : 1. Two small diarthrodial facets.^ 

 2. A roughened surface. 3. A transverse groove for the formation of the radio- 

 ulnar arch. 4. A triangular surface, studded with rugosities, which occupie^the 

 remainder of the bone to its lower extremity. The lateral borders — externatand 

 internal — are sharp, and, like the anterior face, are in contact with the radius. 

 The posterior border is concave, rounded, and thicker than the other two. 



Extremities. — The superior extremity comprises all that portion which exceeds 

 the articular surface of the radius. It constitutes an enormous process — the 

 olecranon — flattened on both sides, and presenting : 1. An external face, slightly 

 convex. 2. An internal excavated face. 3. An anterior border, thin and sharp 

 superiorly, notched below to form the sigmoid cavity^ — an articular surface 

 concave from above downwards, rounded from one side to the other, which 

 corresponds with the humeral cavity, and is surmounted by a salient prolongation 

 named the bectk of the olecranon. 4. A concave and smooth posterior border. 5. 

 The summit — a kind of thick roughened tuberosity which terminates the uhia 

 above, and into which are inserted the extensor muscles of the forearm. 



At its inferior extremity, the ulna ends, towards the lower fourth of the 

 principal portion of the forearm, in an acute point, and sometimes by a small 

 knob (capitulum ulnce). It is not rare to see it prolonged, especially in the Ass 

 and Mule, to the inferior external tuberosity of the radius. This tuberosity then 

 appears to belong to it, at least in part ; and all that portion which is situated 

 behind its vertical groove might be justly considered as a dependency of the 

 uhia. 



Structure and development. — The ulna contains much compact tissue, even in 

 the region of the olecranon ; it is also very solid. It is an imperfect bone, 

 developed from two centres of ossification only, one of these being for the apex 

 of the olecranon. 



In the Ass (Fig. 66), the radius is more curved than in the Horse, and when 

 its anterior face is placed on a horizontal plane, the bone only rests on its upper 



' Tlv smaller siqmmrl cavity of Man. 

 * 'The qreater f^iijinoid en vity of Man. 



