238 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



PHARYNX 



The pharynx is the division of the alimentary canal intervening 

 between the cavity of the mouth and the oesophagus and is character- 

 ized by being at once alimentary and respiratory. From its walls are 

 developed the gill clefts and lungs as well as a number of derivatives 

 of these, and it also receives the internal openings of the nasal pas- 

 sages. Hence it is best described in connexion with the respiratory 

 system. 



THE (ESOPHAGUS 



That part of the digestive tract between the pharynx and the 

 entrance of the bile duct (fig. 233) develops into oesophagus, stom- 

 ach and that part of the intestine known as the duodenum. Stomach 

 and duodenum are separated by the pyloric valve described below, 

 but it is difficult to draw a clear line between oesophagus and stomach. 

 In general it may be said that the oesophagus is the tract immediately 

 succeeding the pharynx, lying in front of the body cavity and thus 

 lacking a serous coat; that it is smaller than the stomach, and that 

 there are no digestive glands in its walls ; but all of these statements 

 have exceptions. 



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 epithe- 

 lium is smooth, but more commonly it bears longitudinal folds, while 

 in the chelonians it is provided with cornified papillae pointing back- 

 ward. 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 

 diameter throughout, but frequently in birds it has a marked dilata- 

 tion, 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 

 Opisthocomus, where the organ is extremely muscular and has numer- 

 ous 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. 



