100 AVES. 



It is in the Parrot-tribe alone that the horny mandibles perform the 

 functions of a proper instrument of mastication ; for, as a rule, they 

 are adapted only for seizing and retaining the food within their grasp.. 

 The peculiar degree of mobility of which the superior maxillary arch 

 is capable in Birds, has been already alluded to in speaking of their 

 skeleton. The jaws are moved by several muscles (see Muscular 

 System) analogous to those of the Mammalia. 



In place of the velum palati, we find near to the opening of the 

 posterior nares within the faucial cavity^ tubercular elevations of epi- 

 thelium, or dentated ridges similar to those upon the root of the tongue 

 and the margins of the glottis, arid behind these are situated glan- 

 dular follicles, which have been regarded by some as analogous to 

 the amygdalae or tonsils. These follicles are frequently aggregated 

 into a thick glandular layer, provided with numerous excretory open- 

 ings, as in the Birds of Prey, while in others of the class Aves e. g. 

 _the Cormorant (Carbo) they are altogether wanting. 



The Oesophagus is always very muscular, slightly plicated inter- 

 nally, and expands in many Birds usually below its middle and in 

 front into a sacciform pouch called the crop, craw, or ingluvies. 

 This is commonly of a thin membranous texture, provided internally 

 with small mucous glands, and when distended with food hangs down 

 in the neck in front and between the branches of the furcula, and, as 

 in the Gallinae, is there supported by a special muscle. The crop is 

 found in all the Diurnal birds of Prey, but it is only feebly indicated 

 in the nocturnal Rapaces or Owls ; it is of large size in the Parrots, 

 particularly so in the Gallinag and Pigeons, and more rarely in the 

 Grallae and Palmipedes, as in the Flamingo (Phaenicopterus). The 

 craw is extremely large in the Pigeons, and during the period 'of 

 incubation obtains in both sexes a remarkable degree of vascularity, 

 its lining membrane becoming, at the same time, developed into a 

 network of folds and cells, which form two large laminiform layerSj 

 that secrete and pour out into the crop a creamy or lacteal fluid ; 

 with this alone, discharged from the crop of the parents, the young 

 are at first fed, but at a later period of their existence it is mingled 

 with grain, that has been macerated in the ingluvial secretion. 

 The craw is generally wanting in the Struthionidse, with the excep- 

 tion of the Cassowary, and also in the Scansores and Passeres, 

 where the oesophagus is frequently much expanded and even wider 

 than the proventriculus. In rare instances, as in the Pelican, there 

 occurs an additional expansion of the gullet which is situated between 

 the rami of the lower jaw, and being provided with muscular arches, 



