808 



AVKS. 



acini of the cells proceed. In the Albatross 

 and Penguin we have traced two or three 

 distinct ducts leading from this gland to the 

 nose. 



Organ of Hearing. The structure of the 

 organs of hearing in Birds resembles most closely 

 that in the higher Reptiles, especially the 

 Crocodile. There is no concha, or projecting 



Fig. 141. 



Organ of hearing. Owl. 



auricle in this class, for collecting and con- 

 densing the rays of sound ; but to compen- 

 sate for this deficiency, the labyrinth, and 

 especially the semicircular canals, are of large 

 size in proportion to the cranium. In those 

 Birds, however, which enjoy the locomotive 

 or visual faculties in a less perfect degree 

 than in the rest of the class, there is found 

 a peculiar arrangement of the feathers around 

 the external meatus auditorius, which serves 

 in some degree the office of an external ear. 

 The Ostrich and Bustard (d,fig. 155) are so 

 provided, and these birds can raise the auditory 

 circle of plumes to catch distinctly any distant 

 sound that may alarm them. The Owls, again, 

 are furnished with a large crescentic mem- 

 branous flap, or valve ; and the membrana 

 tympani is situated ^at the bottom of a cavity 

 (a, fig. 141), the lining membrane of which 

 is disposed in folds analogous to those of the 

 human auricle. The opercular flap is largely 

 developed in our common Barn-owl (Strix 

 flammea). This species is also remarkable in 

 having the membrana tympani attached ex- 

 clusively to the bony meatus (b,Jig. 141), and 

 not to the tympanic bone or os quadratum. 



The bony framework of the membrana tym- 

 pani is sunk below the surface of the head, 

 and rarely projects so far from the tympanum 

 as to deserve the name of a meatus or canal : 

 it is deficient anteriorly, where it is bounded 

 by the tympanic bone, to which, with the ex- 

 ception above mentioned, the membrana tym- 

 pani is attached for a greater or less extent of 

 its anterior circumference. 



The drum of the ear (c, Jig. 141) is more 

 or less of an oval shape ; it has the same 

 structure as in Mammals, but is extremely 

 delicate ; it is convex externally, as in the 

 Reptiles, not concave, as in most Mammals. 



The cavity of the tympanum is widest at its 

 outer part, and very irregular in the rest of its 

 extent. It communicates by the usual fora- 



mina with the internal ear, and is connected 

 with the fauces by means of the Eustachian 

 tube. It also communicates by three other 

 apertures with the cells of the bones of the 

 cranium. " These," Macartney observes, " are 

 widened into something like canals, where 

 the holes open into them. The largest of 

 the foramina is in the back of the tympa- 

 num, and leads to the posterior cells, and 

 communicates above the foramen magnum 

 with the cellular canal of the other side. The 

 second opening is placed at the anterior part 

 of the tympanum, and conducts to the cells on 

 the lower and anterior part of the cranium. 

 The third foramen is continued amongst the 

 cells which surround the labyrinth. Thus 

 each tympanum has a communication with 

 the interior of all parts of the cranium, and 

 with each other, from which they might be 

 reckoned as making only one cavity. The 

 end of the tympanic bone, also, where it 

 contributes to form the parietes of the tym- 

 panum, has a foramen by which it derives its 

 supply of air. The auditory cells of the cra- 

 nium of birds are analogous to the mastoid of 

 the human subject ; but from their extent they 

 multiply sound much more. They are of the 

 greatest magnitude in the nocturnal birds of 

 prey; the Night-jar (Caprimulgus) has them 

 also very large : they diminish in size in other 

 birds, in which the posterior canals have no 

 direct communication with each other; they 

 are little observable in the Struthious Birds, 

 and are wanting in the Parrots, but in their 

 place the cavity of the tympanum is enlarged 

 posteriorly." 



The Eustachian tube (e, e, Jig. 141) is very 

 large in birds; it is an osseous canal, and ter- 

 minates by a small aperture close to the one 

 of the other side, within the fissure of the 

 posterior nares. In the Swan the Eustachian 

 passages, after having reached the base of 

 the skull, pass forwards for about half an 

 inch and then unite to form one common 

 tube, which gradually expands until it termi- 

 nates just behind the posterior apertures of the 

 nose. 



The foramina, which lead from the tym- 

 panum into the labyrinth, are situated within 

 a fossa. They do not merit the distinctions of 

 foramen ovale and foramen rotundum, being 

 both oval, and only' separated by a small bony 

 process. 



The ossicula auditus are supplied by a sin- 

 gle bone, analogous to the stapes, and some 

 cartilaginous processes representing the rudi- 

 ments of a malleus and incus. The ossiculum 

 consists of a stalk or pedicle, crowned by an 

 oval plate, which is applied to the foramen 

 that leads into the vestibule of the labyrinth. 

 At the other extremity it is united to two or 

 three cartilaginous processes, which form a tri- 

 angle that is attached to the membrana tym- 

 pani. 



The elongated stapes, or tympanic ossicle, is 

 moved by one muscle (f, fig- 141), which 

 comes from the occiput and penetrating the 

 cavity, is affixed to the triangle that is con- 

 nected to the membrana tympani. This muscle, 



