SKELETON AND TEETH 



81 



hand is put; if the capacity of rotation is re- 

 tained, the upper end, or head, of the radius is 

 small, circular or disk-like, covering little of the 

 humeral trochlea, but when the head of the radius 

 is broadened to cover the whole width of the 

 humerus, then all power of rota- 

 tion is lost. (Cf . Figs. 28 and 29.) 

 As a rule, the radius broadens 

 downward and covers two-thirds 

 or more of the breadth of the 

 wrist-bones. 



The ulna is longer than the 

 radius, its upper end being ex- 

 tended into a heavy process, the 

 olecranon, or anconeal process, 

 into which is inserted the tendon 

 of the great triceps muscle, the 

 contraction of which straightens 

 the arm ; this process is the bony 

 projection at the back of the el- 

 bow-joint. Below the olecranon 

 is a semicircular articular con- 

 cavity, which embraces the hume- 

 ral trochlea and its upper angle fits into the 

 '•»«'"WM anconeal fossa of the humerus. The ulna con- 

 tracts and grows more slender downwards and 

 its lower end covers but one of the wrist-bones. 

 Fig. 31.— Left fore- While in the more primitive mammals, and in 



arm bones of the ^^^^^ ^j^j^j^ ^^^^^^^ ^j^g p^^^j. ^f rotating the 



terrestris). R., ra- hand, the ulua has nearly or quite the same 

 heTdofradi^^ h'' thickuess as the radius, it is often much more 

 sigmoid notch of slender and in the more highly speciaUzed of 

 cranon. N.B. This the hoofcd auimals, such as the horses, camels 

 figm-eisonamuch and truc ruminants, the radius carries the en- 

 Fig. 30. tire weight and the ulna has become very slen- 



FiG. 30. — Coos- 

 sified bones of 

 left fore-arm of 

 Horse, front 

 side. For most 

 of its length, 

 the ulna is con- 

 cealed by the 

 radius. 



