a, vestibule; b, cochlea; c, external semicircular canal; d, superior semicircular canal; e, posterior semicircular canal; f, scala tympani; g, scala vestibuli; h, fenestra cochleæ; i, fenestra vestibuli.
The labyrinth ([Fig. 173]) consists of a cavity, the vestibule (a), and connected with this a coiled tube, the cochlea (b), and the three semicircular canals (c, d, and e).
The cochlea (b) is situated in the promontory,—the rounded elevation of the petrous bone extending craniad from the fenestra cochleæ. It consists of a bony tube coiled spirally about a central column known as the modiolus. The coil has nearly the form of a snail’s shell, and is somewhat more lengthened in the cat than in man. A plate of bone, the lamina spiralis, incompletely divides the cavity of the canal lengthwise into two parts, known as scalæ (f and g). One of these opens at the fenestra cochleæ (h) into the tympanic cavity (or would so open but for the thin membrane covering the fenestra); this is known as the scala tympani (f). The other scala is connected with the vestibule (a) and is known as the scala vestibuli (g).
The vestibule (a) is a pyramidal cavity situated mediad of the caudal end of the cochlea. It is connected with the scala vestibuli of the cochlea, and with the three semicircular canals. The fenestra vestibuli (i) is an opening in its bony wall, filled in the recent state by the stapes. On the medial surface of the vestibule are a number of minute orifices through which the auditory nerve enters.
The semicircular canals (c, d, and e) are three curved tubes, each forming about two-thirds of a circle, and opening at both ends into the vestibule. They are imbedded in the petrous bone. The planes of the three tubes intersect nearly at right angles; from their position in man they have been named external (c), superior (d), and posterior (e). The external one (c) is nearly horizontal in position, and surrounds the curved fossa ([Fig. 25], f) for the stapedius muscle, caudad of the fenestra vestibuli. The superior ([Fig. 173], d) and posterior (e) are in nearly dorsoventral planes, but at right angles to each other. The superior (d) lies in the ridge forming the caudal boundary of the appendicular fossa. The posterior (e) is situated in a continuation caudad of the ridge which forms the dorsal boundary of the internal auditory meatus. One end of each canal is slightly enlarged to form an ampulla. The superior (d) and posterior (e) canals join at one end and enter the vestibule by a common opening. One end of the external canal (c) opens into the vestibule so close to one end of the posterior canal (e) that they appear to have a common opening, and this is commonly stated to be the case; according to Jayne, however, the openings are really separate. The two remaining ends (of the superior (d) and external (c) canals) also enter the vestibule separately, so that the three canals enter the vestibule by five openings.
III. THE OLFACTORY ORGAN. ORGANON OLFACTUS.
The olfactory organ consists of the olfactory mucous membrane, in the dorsocaudal part of the nasal cavity. The nasal cavity has been [described] in connection with the respiratory system.
IV. THE ORGAN OF TASTE. ORGANON GUSTUS.
The sense of taste is located in the mouth-cavity, especially in the mucosa of the tongue and the soft palate. The proper organs of taste are the so-called taste-buds, found on the papillæ of the tongue, the soft palate, etc. Their exact distribution in the cat has apparently not been worked out.