BIRDS. 335 



of the orbits the lacrymal bones, which, in the parrots, unite with 

 the process (p. 331) behind the orbits to form a ring. In the 

 other birds the orbit is not closed below; the malar bone lies, as we 

 have already stated, much lower, and does not contribute to bound 

 the orbit. 



The jaws of birds have no teeth, and are covered with a horny 

 fibrous investment. This is harder in the birds of prey, and in 

 those that feed on hard fruits or nuts, as the parrots, the cross-bills 

 (Loxice) &c., or perforate the bark of trees with their bill, as the 

 woodpeckers. The hindermost part of the bill has a softer invest- 

 ment, which is named the cere (ceroma). When this part on the 

 bill is not concealed under the feathers of the head, but is large 

 and separated from the anterior hard part by a transverse fold, the 

 bill is named rostrum cerigerum. On the other hand, the bill 

 has a softer covering and greater sensibility in birds which have to 

 seek their food in mud or at the bottom of water, as in the ducks. 



Birds have salivary glands, which vary in number and develop- 

 ment in the different orders. They lie partly on each side under the 

 tongue, partly behind the anterior margin of the under jaw, and 

 also in the angle of the mouth under the malar bone. In the 

 woodpeckers, the hindmost submaxillary gland extends as far back 

 as the occiput; it has long excretory ducts, which terminate in a 

 common aperture at the forepart of the mouth *. 



The oesophagus has internally on the mucous membrane longi- 

 tudinal folds. The muscular tunic, which covers the mucous mem- 

 brane, consists for the most part of circular or transverse fibres; 

 these form an outer layer, whilst the inner, less developed, is formed 

 by longitudinal fibres. In many birds the oesophagus has an ex- 

 pansion which is sometimes not sharply distinguished from the re- 

 maining part, but in others, especially in the gallinaceous birds and 

 parrots, is sacciform. The name of crop (ingluvies) has been given 

 to this part ; it lies usually on the right side of the neck in front of 

 or within ikefurcula. In the crop the food undergoes a softening, 

 and is penetrated by the fluid secreted by the numerous mucous 

 glands of the part. In the pigeons two such sacs are found, one on 



1 CUVIEB Lep. cTAnat. comp. in. pp. 220 222; MECKEL'S System der vergl. Anat. 

 iv. s. 403 406, s. 465, 466; J. MUELLER De glandular, secern, structura penitiori, 

 Lipsiae, 1830, fol. pp. 58 60, Tab. vi. figs. 6 8. 



