158 



ANATOMY OF VERTEBRATES. 



smallness of the objects wliicli constitute the food of this spe- 



cies. 



Besides deglutition, the oesophagus is frequently concerned in 

 reo-urgitation ; and in the Birds in which this phenomenon occurs, 

 the muscular coat of the gullet, like that in Ruminants, is well de- 

 veloped. The Raj)tores, for example, habitually regurgitate the 

 bones, feathers, and other indigestible parts of their prey, which, 

 in the language of Falconry, are called ^ castings.' I have ob- 

 served a Toucan to regurgitate partially digested food, and after 

 submitting it to a rude kind of mastication by its enormous beak, 

 ao^ain to swallow it. 



The oesophagus possesses an external cellular covering, a mus- 

 cular coat, an internal vascular tunic, and a cuticular lining. The 

 muscular coat consists of two layers of fibres ; in the external 

 stratum they are transverse, fig. 81, a, in the internal longitu- 

 dinal, ib. h. The mucous coat is generally disposed in longi- 

 tudinal folds, rarely connected by transverse folds ; still more 

 rarely villous, as in the Ostrich.^ 



In those Birds which are omnivorous, as the Toucans and 

 Hornbills, in the frugivorous and insectivorous Birds, and in 

 most of the Grallatores, which find their food in tolerable abun- 

 dance and take it in small quantities without any considerable 

 intermission, it passes at once to the stomach to be there suc- 

 cessively digested, and the gullet presents no partial dilatations 

 to serve as a temporary reservoir or macerating receptacle. But 

 „g in the larger Raptorial 



Birds, as the Eagles and 

 Vultures, which gorge them- 

 selves at uncertain intervals 

 from the carcases of bulky 

 prey, the oesophagus does not 

 preserve a uniform width, 

 but undergoes a lateral di- 

 latation anterior to the fur- 

 culum at the lower part of 

 the neck. This pouch is 

 termed the ingluvies or 

 crop, fig. 78, 6. 



In those Birds, again, the 

 food of which is exclusively 

 of the vegetable kind, as 



Digestive canal of an Eagle. graiUS ancl SCeClS, ancl 01 



' XX-. vol. i. p. 125, prep. no. 458. 



