276 THE ARM AND FORE- ARM. [CHAP. 



deltoid ridge is an extremely salient, compressed, and 

 everted tuberosity. 



In the fore-arm the two bones are nearly always dis- 

 tinct, though closely applied to each other. The breadth 

 of the upper end of the radius, and the amount of 

 rotation permitted upon the ulna, vary much in different 



genera. 



In the great order UNGULATA the humerus is stout and 

 rather short. The outer tuberosity is very large, and gene- 

 rally sends a strong curved process inwards, overhanging the 

 bicipital groove (not, however, in the Horse and Camel). 

 The deltoid ridge is usually not strongly marked, and placed 

 rather high on the bone ; but in the Rhinoceros it is a very 

 salient ridge. The lower end is always particularly straight 

 and flat on the inner side (see Fig. 89, p. 273), the condyle 

 forming no prominence, and there is never a supracondylar 

 oramen. The outer condyle and the ridge above it are 

 rather more developed. 



The radius is large at both ends, and superiorly extends 

 across the whole of the humeral trochlear surface (see Fig. 

 89). The ulna is a complete and distinct bone in the Pig, 

 Hippopotamus, Tapir, and Rhinoceros. In the Ruminants 

 it is more or less rudimentary and fixed behind the radius. 

 In the Camel the two bones become completely coalesced. 

 In the Horse the olecranon and proximal part of the shaft 

 alone remain, firmly ankylosed to the radius. 



In the PROBOSCIDEA the humerus is remarkable for the 

 great development of the supinator ridge. The ulna and 

 radius are quite distinct, and permanently crossed. The 

 upper end of the latter is small, while the ulna not only 

 contributes the principal part of the articular surface for the 

 humerus, but has its distal end actually larger than that of 

 the radius, a condition unique among Mammals. 



