THORACIC LIMBS 579 



The posterior border is subcutaneous ; it begins on the olecranon 

 and ends at the styloid process. Its upper part is prominent, but it 

 becomes indistinct lower down. It separates the posterior and internal 

 surfaces. 



The anterior surface is marked above by two triangles, whereof 

 the inner is the roughened so-called ulnar tuberosity and the outer is 

 under the lesser sigmoid cavity and affords attachment to the supinator 

 brevis muscle. The rest of the surface is a transversely concave area 

 of origin for the flexor profundus digitoruni muscle, except below, 

 where it is rounded and offers attachment to the pronator quadratus. 



The posterior surface is crossed by an oblique ridge, above which 

 is a depressed area for the insertion of the anconeus muscle. Below 

 the ridge the surface is divided into two parts by a longitudinal line : 

 the narrow area near the posterior border is for the extensor carpi 

 ulnaris ; the broader area near the external border gives origin to the 

 extensores ossis metacarpi pollicis, secundi internodii pollicis, and 

 indicis. 



The internal surface is marked above by a deep depression behind 

 the coronoid process ; below this depression it is flattened and at the 

 lower end becomes rounded. Its upper three-fourths afford attach- 

 ment to the flexor profundus digitorum ; its lower fourth is sub- 

 cutaneous. 



The lower extremity of the ulna is a rounded head, from the 

 posterior side of which springs the small curved, obtusely pointed 

 styloid process. Between the styloicl process and the head, on the 

 posterior aspect, is a groove for the flexor carpi ulnaris muscle. The 

 transversely convex articular surface for the radius is more extensive 

 than on the ulna of the cat, owing to the greater degree of rotation 

 accorded the radius. The lower surface of the head is flattened for 

 articulation with an interarticular cartilage which separates the ulna 

 from the bones of the wrist. 



THE RADIUS. 



General Description. The radius is the shorter of the two bones 

 of the forearm. It lies on the outer and anterior sides of the limb, 

 but its position varies with the different movements of the hand. Its 

 upper proximal end always maintains the same relation with the upper 

 end of the ulna, lying to its outer side, but the lower end lies in front 



