THE SKELETON 



67 



The bones of the antebrachium in many mammals are more 

 or less coalesced. In the Chiroptera and many of the Ungu- 

 lates the radius is enlarged at the expense of the ulna, whose 

 proximal third only remains. The primitive Ungulates of the 

 lower tertiary period possessed a complete 

 ulna as well as radius. The phylogeny of k 



the horse's limb illustrates the gradual de- 

 velopment of the antebrachium of the 

 Equidae (Fig. 36). Fossil remains reveal 

 the fact that mammals existed as early as 

 the triassic period, when the sedimentary 

 rock forming the triassic strata was laid 

 down. This probably occurred 80,000,000 

 years ago. 



According to paleontological investiga- 

 tions, the Ungulata probably arose from 

 the Condylarthra, a group of small five- 

 toed mammals of the lower Eocene, best 

 represented by the typical genus Phenacodus. 

 In this genus and its successor, Hyracother- 

 ium, the ulna and radius are well developed 

 and distinct. Orohippus, the descendant 



Fig 



. 35. — Medial 



of Hyracotherium, also shows a distinct °^ ^^^^^ Aspect of 



Right Radius. 



radius and ulna, but in the later forms of 

 the horse line the ulna gradually dimin- 



fc, Articulatory sur- 

 face for capitulum 

 of humerus; hd, head 



ishes in size and becomes more and more —the point of the 



arrow is on the ar- 

 ticulating surface for 

 the radial notch; nk, 

 neck; sc, articulatory 

 surface for scapho- 

 lunar; st, styloid proc- 

 ess; tb, tubercle; ul. 



coalesced with the radius, until in Equus 

 scarcely more than the proximal third 

 remains (Fig. 38). 



Carpus.— The carpus consists of seven 

 bones arranged in two rows (Fig. 37). Be- facet for ulna. 

 ginning on the pollex (thumb) side, the 

 scapholunar, triquetral, and pisiform compose the proximal 

 row, and the greater multangular, lesser multangular, capitate, 

 and hamate form the distal row. The scaphohinar, easily dis- 



