DIGESTIVE ORGANS. 223 
The cesophagus 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 papille 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 cesophagus 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- 
thocomus, where 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 cesophagus 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 cesophagus 
by its greater diameter, but in some of the fishes (fig. 227, a) there 
is no distinction in size. The stomach lies in the coelom and hence is 
covered externally by the serous membrane (peritoneum), but the 
