OR OSSEOUS SYSTEM. 105 



back part of the skull presents an elevated transverse ridge, 

 and a broad surface of attachment for the muscles and liga- 

 ments of their heavy head and strong neck, the head being 

 sometimes employed in digging, as in the hog tribe, or to 

 support a strong instrument of defence, as in the rhino- 

 ceros, or being proportionally large and weighty, as in the 

 hippopotamus. The air is admitted from the frontal sinuses 

 over a large portion of the diploe in the babyrussae and 

 other animals of the hog tribe, to extend the external sur- 

 face without adding to the weight of the head, as we see to 

 a much greater extent in the huge head of the elephant. 

 All the processes of the cervical vertebrse are here more 

 strongly developed than in the long flexible neck of the ru- 

 minantia, and the spinous processes of the dorsal vertebrse 

 are lengthened and strong, and generally terminated by 

 round tubercles. The scapula is generally broader at its 

 vertebral margin, and the strong pelvic arch is more vertical 

 in its direction. The extremities are generally shorter and 

 more massive, and the separate bones more completely 

 formed than in the former groups of quadrupeds, the ulna 

 and the fibula being developed throughout, and four toes, 

 at least, generally reaching the ground on all the extremities. 

 As the kind of vegetable food varies much more here than 

 in the ruminating quadrupeds, there is a greater diversity 

 in the forms of the teeth, and of the jaws, and of many 

 other parts of the skeleton. 



The general forms and proportions of the bones 

 most common in the ordinary pachyderma are seen 

 in the massive skeleton of the rhinoceros ( Fig. 56, ) 

 where the head and neck are more lengthened than 

 in the proboscidian tribe, and the trunk is almost en- 



