ORGANS OF HEARING. 95 



surrounded by a very loose osseous tissue, so that it may be readily 

 exposed for examination. The vestibule is very small, while on 

 the other hand the semicircular canals are large and vary in size ; 

 being slender, elevated and broad in the Rapacious and Passerine 

 birds, but depressed and thick in most of the Grallae, Palmipedes, 

 and Gallinae. The vertical canal from the depth of its arc is appa- 

 rent at once within the cranial cavity, without any previous prepara- 

 tion, and is directed inward and forward. The two others, which 

 are directed externally and posteriorly, cross each other completely ; 

 the external or horizontal canal being the smallest. In the direc- 

 tion forward there projects a slightly-curved osseous cone, with the 

 concavity directed downward, inward, and somewhat backward, 

 which represents the cochlea, although it is not spirally twisted like 

 that organ. The foramen rotundum, which is small and closed by 

 a kind of secondary tympanic membrane, is here met with, and serves 

 to bring the cochlea arid vestibule into functional communication 

 with each other. The foramen ovale is in Birds also round, but 

 larger than the f. rotundum, and is closed by the basi-opercular plate 

 of the columella. 



A mucous membrane is found lining the bony labyrinth ; the tubes 

 of the semicircular canals dilate into ampullae, the external of which 

 is the least developed, while the anterior and posterior are supported 

 by a cruciform septum, and it is upon the bulging prominence of this 

 septum that the auditory nerve expands. These dilatations are visi- 

 ble at once upon opening the cavity of the labyrinth. The cochlea 

 contains internally, as in the squamigerous Amphibia, two curved 

 cartilaginous lamellae of a triangular form, united by a delicate 

 membrane ; upon this lies another plicated and very highly vascu- 

 lar membrane, which gives off a sacciform expansion, called the 

 Utricle (lagcna), into the round terminal dilatation of the conical 

 cochlea. One of these cartilaginous lamellae (the vestibular) is 

 fringed with dento; * processes, the number of which differs in the 

 several genera. The other or tympanic cartilage is situated in the 

 direction backward, and faces the foramen rotundum. A branch 

 of the auditory nerve enters the cochlea upon the external side of 

 one of these cartilages, expands and perforates it by numerous fila- 

 ments, which are distributed upon the membrane connecting the two 

 cartilages, and analogous to that covering the lamina spiralis in the 

 higher animals ; a branch is also given to the utricle. In the 

 membranous sac of the vestibule, which is double, being divided by 

 a thin septum, pulverulent masses of crystallized phosphate of lime 



