BIRDS. 223 



epiglottis. The nasal and tracheal openings are two nar- 

 row slits, capable of being enlarged or closed at pleasure. 



The gullet, in some species, is furnished with a membra- 

 naceous enlargement or crop, and in all, towards the entry 

 to the stomach, there is a thickening of the walls, for the 

 reception of the zone of gastric glands (ventriculus succen- 

 turiatus). The stomach is either in the form of a gizzard, 

 the walls of which consist of powerful muscles and are 

 lined by a thick cuticle, or a membranaceous bag. Exter- 

 nally, there is but little difference between the large and 

 small intestines. At the commencement of the former, 

 there are usually two caeca, varying in dimensions in diffe- 

 rent species. In a few birds, however, they appear to be 

 wanting. In many birds, particularly among the waders, 

 there is an appendix attached to the small intestine, which 

 performs the office of a mucous gland. It is the remains 

 of the ductus vitello-intestinalis, or communication between 

 the yolk-bag and the small intestine of the cheek *. The 

 rectum terminates in the cloaca, which is of considerably 

 larger dimensions than the intestines, and receives the ori- 

 fices of the ureters, sexual organs, and the bursa Fabricii. 

 This last organ is in the form of an oval bag, filled with a 

 mucus secreted from its glandular walls. It opens into 

 the cloaca by a linear aperture. Its use is unknown. 



The liver of birds is larger in size, and more uniform 

 in shape, than in the quadrupeds. Its two lobes are nearly 

 of equal size. The gall-bladder is absent in a few birds. 

 The hepatic and cystic ducts unite in some species, and in 

 others, open separately into the intestine. 



The pancreas is situate between the folds of the duode- 



* See " An account of an Appendix to the small Intestines of Birds,' 

 by JAMES MACARTXEY, Esq. Phil. Trans, 1811, p. 257. 



