376 THE ANIMAL KINGDOM. 



Accipitrine birds and in the Parrots where it is thick and fleshy. Its an- 

 terior extremity maybe forked (Humming Birds, Trochilidae), or terminate 

 in a brush of delicate papillae (Trichoglossinae among Parrots) : and it is of 

 great length and extensile in Woodpeckers (Picidae) and Humming Birds. 

 It has a horny surface and is often covered by recurved papillae. There 

 are numerous glands in and around the tongue as well as palatal glands, 

 and an especially large gland opening at the angle of the mouth 1 . In the 

 oesophagus the circular muscle layer is external to the longitudinal, thus 

 reversing the relations usual in Vertebrata. The oesophagus itself is of 

 uniform calibre in omnivorous, frugivorous and insectivorous birds. In birds 

 which swallow large masses of food at once, e.g. fish-eating birds, its calibre 

 is large : and in birds of prey, and especially grain-eating birds, it is pro- 

 vided with a dilatation or crop in which the food is stored. In grain-eating 

 birds such as Gallinae, Pigeons, &c., the walls of the crop are generally said 

 to be glandular, and the secretion of the glands to act upon the food (but 

 see p. 53), whilst in other birds the walls are non-glandular. The stomach is 

 divisible in all birds into a glandular stomach or proventriculus, and a 

 muscular stomach and gizzard, connected by a narrow tube or lower oeso- 

 phagus. The proventriculus varies much in size : its glands are simple or 

 compound and are disposed in various ways. The gizzard is saccular, or 

 more or less angular in outline. Its walls show two tendinous spots 

 between which radiate muscle fibres. The degree to which these fibres are 

 developed varies much with the food, least in flesh- and fruit-eating birds, 

 most of all in grain-eating birds and Lamellirostres. The character of the 

 secretion poured out by the glands varies in the same direction : and it 

 either forms a tenacious coat, or a thick horny coat, which is continually 

 worn away by the action of the food and of the stones swallowed with it to 

 assist in trituration, and as continually secreted anew. The two apertures, 

 one leading into the gizzard the other from it, are close together. The 

 pyloric aperture is often guarded by a raised fold to prevent stones, &c. 

 passing on into the intestine : and, in a few birds, there is a small so-called 

 ' pyloric stomach ' intervening between the gizzard and the pylorus, e. g. in 

 the Heron. The duodenum is, as a rule, wide and long. The length of the 

 small intestine varies much, as does also the mode In which it is coiled, a 

 point which has been utilised for classificatory purposes. The large intes- 

 tine is usually straight and short : in the Ostrich alone it is of great length 

 and disposed in coils. It opens into the anterior end of the cloaca, which 

 is large in Ratitae and Accipitrine birds, but in others usually small. The 

 urinary and genital ducts ordinarily open separately on its dorsal wall, but 

 there may be a more or less distinct urogenital section of the cloaca. The 



1 The edible bird's nest produced by a Swift is composed of a substance closely akin to 

 mucin, secreted by two glands lying one on either side of the tongue. These glands are said to 

 enlarge at the nesting season. See Green, Nature, xxxiv. 1886 : Journal of Physiol. vi. 1885. 



