THE SKELETON. 





of the ulna, and distally with the scapholunar and radius. 

 It bears but two processes, the tubercle and the siyloid pro- 

 cess. The tubercle (Fig. 34) is a slight 

 knot near the proximal end on the ulnar 

 side. Proximal from the tubercle is the 

 neck which supports the head bearing a 

 concave crown for articulation with the 

 capitellum of the humerus. The styloid 

 process projects from the distal end par- 

 allel with the process of the same name 

 on the ulna. 



The bones of the antebrachium in 

 many mammals are more or less coal- 

 esced. In the Chiroptera and many of 

 the Ungulates the radius is enlarged at 

 the expense of the ulna, whose proximal 

 third only remains. The primitive Un- 

 gulates of the lower tertiary period pos- 

 sessed a complete ulna as well as radius. 

 The phylogeny of the horse's limb illus- 

 trates the gradual development of the 

 antebrachium of the Equidae (Fig. 35). 

 Fossil remains reveal the fact that mam- 

 mals existed as early as the triassic pe- 

 riod, when the sedimentary rock forming 

 the triassic strata w r as laid down. This 

 probably occurred 10,000,000 years ago. 



According to palseontological investi- 

 gations, the Ungulata arose from the 

 Condylarthra, a group of small five-toed 

 mammals of the lower Eocene, best rep- 

 resented by the typical genus Phenaco- 

 dus. In this genus and its successor, Hyracotherinm, the 

 ulna and radius are well developed and distinct. Orohip- 



FIG. 34. - - MESAL 

 OR INNER AS- 

 PECT OF RIGHT 

 RADIUS. 



fc, Articulatory sur- 

 face for capitel- 

 lum of humerus; 

 hd, head, - - the 

 point of the ar- 

 row is on the ar- 

 ticulating surface 

 for the lesser sig- 

 moid cavity; nk, 

 neck; sc, articu- 

 latory surface for 

 scapholunar ; st, 

 styloid process ; 

 tb, tubercle ; ul, 

 facet for ulna. 



