DIGESTIVE ORGANS. 223 



The oesophagus varies in length with the length of the neck of the 

 animal, being short in the ichthyopsida, longer in the reptiles, and 

 reaching its extreme in the birds. In some its internal lining epithelium 

 is smooth, but more commonly it bears longitudinal folds, while in the 

 chelonians it is provided with cornified papillae .pointing backward. 

 Outside of the epithelium its walls contain muscles, those at the 

 cephalic end being striped and these may extend back, in some in- 

 stances, even on to the stomach. They are apparently derivatives 

 of the pharyngeal region. Usually the oesophagus is of the same di- 

 ameter throughout, but frequently in birds it has a marked dilatation, 

 the ingluvies or crop. This may be an expansion of one side of the 

 tube, or, as in pigeons, it may consist of a median and a pair of lateral 

 chambers. The extreme of development of the crop occurs in Opis- 

 thocomns, w T here the organ is extremely muscular and has numerous 

 longitudinal folds. 



The crop, which is usually supported by the furcula, may be either 

 a reservoir for food, or it may be a glandular organ, its secretions 

 serving to moisten the food or even to initiate its digestion. In the 

 pigeons at the breeding season the secretion is a milky fluid and is 

 used in feeding the young. 



THE STOMACH. 



The stomach is apparently a new acquisition in the vertebrates, 

 possibly arising as a place for the storage of food. This view is sup- 

 ported by several facts. In the embryo vertebrate and in the adult of 

 Amphioxus the duct from the liver immediately follows the pharynx, 

 opening just behind the last gill cleft; while the innervation from the 

 tenth nerve shows that both stomach and oesophagus are parts of the 

 pharynx greatly drawn out (fig. 209). 



The pylorus, which limits the stomach behind, is a fold of the 

 lining mucous membrane projecting into the interior and reinforced 

 by a circular (sphincter) muscle, which by its contraction, closes the 

 tube so that no food can pass from the stomach until it is properly 

 acted upon by the gastric fluids. The anterior end of the stomach is 

 not so well marked. Usually it is differentiated from the oesophagus 

 by its greater diameter, but in some of the fishes (fig. 227, a) there 

 is no distinction in size. The stomach lies in the ccelom and hence is 

 covered externally by the serous membrane (peritoneum), but the 



