MAMMALIA PACHYDKKMATA. 459 



tlie head, with its huge tusks and grinders, is of 

 enormous weight. Had this ponderous head been 

 suspended at the end of a neck of such length as to 

 admit of its being carried to the ground, as is the 

 case in grazing animals, it would have destroyed 

 the balance of the body, and would have required 

 greater force to raise and retain it in a horizontal 

 position than could have been given by any degree 

 of muscular power. Nature has accordingly aban- 

 doned this form of structure, and has at once cur- 

 tailed the neck, bringing the head close to the 

 trunk of the body, and supporting it by means of 

 short, but powerful muscles, which are not im- 

 planted in any particular point of the skull, as they 

 are in other quadrupeds, where the occipital bone 

 forms a crest or ridge for that purpose ; but the 

 general surface of the cranium has been enlarged 

 by an immense expansion given to its interior cel- 

 lular structure, and thus the muscles are attached 

 to a considerable extent of bone, instead of being 

 affixed to a single process, which would have in- 

 curred great risk of being broken off by their action. 

 These large cells are constructed with a view to 

 combine strength wdth lightness ; the plates which 

 form their sides being disposed in a radiated 

 manner towards the circumference, and arranged 

 with great regularity ; and the cells themselves, 

 instead of containing marrow, are filled with air, 

 by means of communications with the Eustachian 

 tubes, which open into the nostrils; thus a great 

 extent of surface is given to the skull, without any 

 addition to its weight. The ligamentum nucha? 

 also comes in aid of the muscular power, being here 

 of vast size and strength. 



