THE ALIMENTARY CANAL. 245 



always a tympanic cavity which communicates with the air-spaces in 

 the neighbouring bones of the skull, and with the pharynx close 

 behind the posterior nares by means of the Eustachian tubes. 

 Towards the exterior the tympanic cavity is closed by a tympanic 

 membrane, to which the long rod-shaped auditory ossicle (columella), 

 corresponding to the stapes of Mammalia, is fastened. On the outer 

 side of the tympanic membrane there is a short external auditory 

 meatus, the opening of which is often surrounded by a circle of 

 larger feathers, and in the Owls is overlapped by a cutaneous valve 

 which is likewise beset with feathers, and constitutes a rudimentary 

 pinna. 



The olfactory organ has three pairs of turbinal bones in 

 the spacious nasal cavities, which are separated by an incomplete 

 septum (nares pervice). The two nasal apertures, except in Apteryx, 

 lie more or less near the root of the upper beak ; sometimes (Crows) 

 they are covered and protected by stiff hairs; in the Procellaridce 

 they are elongated into a tube and join one another. A so-called 

 nasal gland usually lies on the frontal bone, more rarely beneath the 

 nasal bone or at the inner corner of the eye ; it opens by a simple 

 duct into the nasal cavity. 



The sense of taste is connected with the soft base of the tongue 

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

 extent only in the Parrots. In most other cases it has a firmer cover- 

 ing, and in "many cases lends important aid in mastication. 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 avian digestive organs present a fairly uniform 

 structure; their peculiarities have relation to the power of flight. 

 The jaws are covered by a hard horny sheath and transformed into 

 the beak. True teeth are entirely absent, at least in living Birds 

 as opposed to the fossil Odontornithes (Ichthyornis, Hesperornis) ; 

 dental papillae were however discovered by Etienne Geoffroy St. Hilaire 

 in the jaws of the embryo Parrot. While the upper beak is formed 

 by the fused praemaxillse, 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 eyo 



