444 Vertehrata. 



eye ; at tlie posterior, a small laclirynial gland. As to the auditory 

 organ, the membranous labyrinth, especially as regai-ds the 

 cochlea, is very like that of the Crocodilia. There is a short 

 external auditory meatus, at the base of which lies the tympanum 

 (c/., Eeptilia) ; its opening is covered by regularly-arranged feathers 

 (a pinna, i.e., a movable flap of skin covering the opening, only 

 occurs in the Owls. In the tympanic-cavity there is an ear-bone, 

 like that of Reptiles : it consists of a long rod, expanding at one 

 end into a disc, which fits into the fenestra ovalis ; whilst by the 

 other end, which is provided with two or three cartilaginous processes, 

 it is attached to the tympanum. A fenestra rotunda is present. The 

 Eustachian tubes, which are partially enclosed in the wall of 

 the skull (sphenoid bone), are united with each other, and open 

 into the mouth by a single aperture. 



Alimentary canal. There are no traces of teeth* in any 

 living Birds, but in Arch^opteryx and the Odontornithes, there were 

 simple conical teeth, placed in sockets on the edges of the jaws as 

 in the Crocodiles ; in some of the Odontornithes, the sockets become 

 confluent, so as to form a continuous groove in each jaw, a conditioji 

 which may also be observed in some Mammalia. The roof of the 

 mouth is usually provided with backwardly directed spiny processes. 

 The tongue is generally flattened, narrow, stifE, and hard, and 

 covered with a thick, hard cuticle, which is especially well-developed 

 at the anterior, usually pointed, end ; occasionally it is thick and soft, 

 as in the Parrots and the Flamingo ; not infrequently it is rough or 

 prickly. The cesophagus is of considerable length and fairly 

 wide. In many, but by no means in all Birds, it widens out at the 

 base of the neck to form a crop, in some a simple expansion of the 

 cesophagus, from which it is not sharply marked off; in others, a 

 more definite sac opening into the cesophagus. It usually serves 

 merely as a storage for food, but in Pigeons, it has another function : 

 here, during the breeding season (in cock as well as in hen), the 

 stratified epithelium lining it becomes thickened, the superficial cells 

 filled with oil globules break away, and form a crumby liquid with 

 which the fledglings are fed. The stomach of Birds is divided 

 into two, usually rather sharply deflned, chambers, a glandular and a 

 muscular portion. The glandular stomach is a short tube which 

 appears as a direct, somewhat widened continuation of the oesophagus; 

 embedded in its walls are numerous glands of two kinds : (1) large, 

 close-set glands secreting a digestive fluid, either distributed over 

 the whole wall, or limited to deflnite regions ; and (2) quite small 

 tubular glands, which secrete a mucous covering for the whole inner 

 surface of this region. Lower down, at its junction with the next 



* The " teeth " along the edge of the beak in, for mstanee. Merganser (Mergus), 

 are dentiform processes of the edge of the beak, and are thus horny structures. 



