150 ANATOMY OF VERTEBRATES. 



are hollow. The structure is the same in the lower mandible, 

 ib. 7/z, but the fibres composing the network are in general 

 stronger than those of the upper mandible. 



The air is admitted to the interior of the upper mandible from 

 a cavity, ib. b, situated anterior to the orbit, which communicates 

 at its posterior part with the air-cell continued into the orbit, and 

 at its anterior part mth the maxillary cavity. The nasal cavity 

 is closed at every part except at its external and internal apertures 

 by the pituitary membrane, and has no communication with the 

 interior of the mandible.^ 



The horny sheath of the mandibles in the Hornbills and Toucans 

 is so thin that it often becomes irregularly notched at the edge 

 from use. The Hornbills have, besides, upon their enormous 

 beak, horn-like prominences of the same structure and of different 

 forms, the use of which is not known. 



The Trogons, Touracos, Buccos, &c., exhibit forms of the bill 

 which are intermediate to that of the large but feeble bill of the 

 Toucans, and the short, but hard, strong, and broad bill of the 

 Parrot-tribe, which is also hooked, so as to assist in climbing, like 

 a third foot, fig. 30. 



The short, conical, and vaulted beak of the Rasores, fig. 72, 

 serves to pick up with due rapidity the vegetable seeds and 

 grains which constitute their food, as well 

 as small insects, as ants, &c., with which the 

 young are frequently nourished. The tooth- 

 billed pigeon of the Samoan Isles has the 

 lower mandible deeply cleft into three points 

 near the top, and the upper mandible hooked, 

 ^ , ,,, . , , the better for seizino; fruit and denudino- 



Beak of Gumeu-iuwl. O ^ » 



palm-nuts and other strongly coated kinds. 

 The bills of the small Insessorial or Passerine birds present 

 every gradation of the conical form, from the broad-based cone of 

 the Hawfinch to the almost filamentous cone of the Humming- 

 bird, fig. 63, and each of these forms influences the habits of 

 the species in the same manner as in the larger birds. The 

 short and strong-billed Insessores live on seeds and grains ; 

 those with a long and slender bill on insects or vegetable juices. 

 If the slender bill be short, flat, and the gape very wide, as 

 in Swallows, the bird takes the insects while on the wing; if 

 the bill be elono;ated and endowed with sufficient streno-th, 

 as in the Hoopoes, it serves to penetrate the soil and pick out 



