438 AVES. 



which is rich in papillae. The tongue is soft throughout its 

 whole extent only in the jDarrots. In most other cases it has a 

 firmer covering, and in many cases lends important aid in mas- 

 tication. In general the tongue as well as the beak may be 

 regarded as a tactile organ. In rare cases (snipe, duck) the beak 

 is the seat of a finer tactile sensibility, owing to the possession 

 of a soft skin rich in nerves and in the end-corpuscles of Vater. 

 Alimentary canal. In spite of great differences in the mode 

 of nourishment the avine digestive organs present a fairly uni- 

 form structure ; their peculiarities have relation to the power 

 of flight. The jaws are covered by a hard horny sheath {rham- 

 photheca) and transformed into the beak. The rhamphotheca 

 is often composed of several pieces * (compound). True teeth 

 are entirely absent, at least in living birds as opposed to some 

 fossil forms {Ichthyornis, Hes'perornis, Archaeopteryx). While 

 the uj)per beak is formed by the fused praemaxillae, the maxillae 

 and the nasal bones, the lower corresponds to the two rami of 

 the lower jaw, the fused extremities of which are known as the 

 myxa. The lower edge reaching from the angle of the chin to 

 the extremity is termed the gonys, the edge of the upper beak 

 is the culmen, the region between the eye and the base of the 

 beak which is covered by the cere (ceroma) is the lore. The form 

 and development of the beak vary extremely according to the 

 special mode of subsistence (Fig. 242). 



The tongue, which is always moveable, lies on the floor of the 

 buccal cavity. It consists of the horny or fleshy covering of two 

 cartilages attached to the anterior end of the hyoid bone, and 

 serves for deglutition, and frequently for seizing food. The 

 buccal cavity, which in the j^elicans is dilated into a large 

 gular sac supported by the rami of the lower jaw, receives the 

 secretion of a number of small salivary glands (sublingual, sub- 

 maxillary and parotid ; in the woodpecker the sublingual glands 

 are large). There is no velum palati. The muscular, longi- 

 tudinally folded oesoi^hagus, the length of which in general 

 depends on that of the neck, frequently possesses — especially 

 in the birds of prey, but also in the granivorous birds — a crop- 

 like dilatation, in which the food is softened (Fig. 243). In the 

 pigeons the croj) bears two small round accessory sacs. 



The lower end of the oesophagus is dilated into a glandular 



♦ Lounberg, Arkiv for Zoologi, 1, 1904, p. 479. 



