HEAKING. 797 



internal side of the inner rod is a single row, and on the external side of the 

 outer rods are three rows of elongated ciliated cells. Extending across the 

 top of the organ of Corti, from the inner hair-cells to the external wall of 

 the canal, is a very delicate structure called the reticular membrane. The 

 auditory nerve-filaments probably terminate in the ciliated cells, being 

 intimately connected with the cilia. 



703. The osseous labyrinth is lined by a fibro-serous membrane which 

 secretes a watery fluid called the perilymph. The perilymph fills the scalse 

 of the cochlea, and surrounds the duetus cochlearis and the membranous 

 portions of the labyrinth, which are situated in the vestibule and semicircu- 

 lar canal. 



704. The membranous labyrinth is a closed sac consisting of the semi- 

 circular canals, a vestibular portion, and the duetus cochlearis of the cochlea. 

 The semicircular canals are of the same form as the osseous canals, and are 

 contained within them. The vestibular portion consists of an expanded 

 body, the utricle, and a smaller body, the saceule. The utricle is situated at 

 the fovea hemielliptica ; the semicircular canals open on its internal surface. 

 The saceule lies at the fovea hemispherica ; it is connected with the duetus 

 cochlearis by the duetus reuniens. In the walls of the saceule and utricle 

 are two calcareous bodies called the otoliths. The walls of the ampulla, 

 according to Bowman, also contain some grains of a similar substance. The 

 walls of the membranous labyrinth consist of a fibrous tissue, lined by pave- 

 ment nucleated epithelial cells, having a structureless basement-membrane. 

 These epithelial cells are much modified at the place of entrance of the 

 fibres of the auditory nerve. The vestibular branches of the auditory nerve 

 are distributed to the ampullae, utricle, and saceule. (Fig. 192.) In the 

 utricle and the saceule the fibres terminate in oval plates, called the maculae 

 acusticce, which are more or less colored by the deposition of yellow pigment. 

 In the ampullae the fibres terminate in elevations called the cristce acusticce. 

 After the nerve-filament pierces the membranous wall at these points, the 

 axis-cylinder alone penetrates the basement-membrane; it then forms a 

 plexus of delicate nerve-fibres with nuclei, and finally terminates in fusiform 

 epithelial cells which have terminal cilia called the auditory hairs. (Fig. 

 193.) These ciliated cells are supported by columnar epithelium. 



The membranous labyrinth is lined by polygonal nucleated epithelium, 

 which secretes the endolymph which fills the sac.] 



The Acoustic Apparatus. 



705. Waves of sound can and do reach the endolymph of the laby- 

 rinth by direct conduction through the skull. Since, however, sonorous 

 vibrations are transmitted with great difficulty from the air to solids and 

 liquids, and most sounds come to us through the air, some special apparatus 

 is required to transfer the aerial vibrations to the liquids of the internal ear. 

 This apparatus is supplied by the tympanum and its appendages. 



706. The concha. The use' of this, as far as hearing is concerned, is to 

 collect the waves of sound coming in various directions, and to direct them 

 on to the membrana tympani. In ourselves of moderate service only, in 

 many animals it is of great importance. 



707. The membrana tympani. It is a characteristic property of 

 stretched membranes that they are readily thrown into vibration by aerial 

 waves of sound. The membrana tympani, from its peculiar conformation, 

 being funnel-shaped with a depressed centre surrounded by sides gently con- 

 vex outward, is peculiarly susceptible to sonorous vibrations, and is most 

 readily thrown into corresponding movements when waves of sound reach it 



