150 ANATOMY OF THE WOOD RAT 



upon which is inserted the triceps lateralis, best developed 

 of course, in old adults. Upon the posterior or proximal as- 

 pect of the olecranon is inserted the triceps longus, and 

 directly distad upon the slightly defined anconeal process on 

 the medial side is the insertion of the anconeus, in definition 

 variable with age. Along the caudo-medial border distad 

 from the olecranon for one-third the length of the ulna the 

 flexor carpi ulnaris takes partial origin^ but only the proximal 

 portion of this is indicated by a slight ridge. Similarly situ- 

 ated upon the central half of the shaft is the ulnar portion 

 of the origin of the third head of the flexor digitorum 

 profundus. Upon the distal third of the medial border of 

 the shaft is a slight ridge, from which the pronator quad- 

 ratus takes origin. Just distad of the coronoid process upon 

 the medial border is located the short brachial ridge, upon 

 which is inserted the brachialis. If anything it is best 

 defined in Teonoma, although no difference was noted in this 

 muscle during dissection. Upon the lateral aspect of the 

 ulna is found the lateral concavity or fossa. Upon the caudal 

 border of this originates the extensor indicis, and from the 

 cranial border, the ulnar portion of the extensor metacarpi 

 pollicis. 



The distal termination of the ulna consists of the conical 

 styloid process, the caudo-lateral aspect of which is grooved 

 for the passage of tendons. Articulation is with the trique- 

 trum. 



Radius. In Homodontomys the radius averages 83.2 

 (81.8 to 85.3) per cent of the ulnar length; in Neotoma the- 

 average is 82.3 (81.5 to 82.8); and in Teonoma 82.5 (80.6 to 

 84.5) per cent. 



All except the distal portion of the radius is situated fairly 

 upon the cranial aspect of the forearm. It consists of a 

 shaft with an enlarged extremity at either end, the distal one 

 being much the larger. Upon its proximal extremity there 

 is a facet for articulation with the capitellum of the humerus^ 



