840 TEE APPARATUS OF THE SENSES. 



FLUIDS OF THE LABYRINTH. 



These liquids are of two kinds : one is contained in the membranous 

 labyrinth, the other in the osseous labyrinth. 



The fluid of the membranous labyrinth, or endo-lymph of Breschet, is con- 

 tained in the sacs and tubes constituting the membranous vestibule and 

 semicircular canals. It is limpid and fluid like water. The fluid of the 

 osseous labyrinth, or peri-lymph of Breschet, fills the two scalae of the cochlea, 

 and bathes the external surface of the vestibule and membranous semi- 

 circular canals, which it separates from the corresponding walls of the 

 osseous labyrinth. 



DISTRIBUTION AND TERMINATION OF THE AUDITORY NERVE IN THE MEM- 

 BRANOUS LABYRINTH. 



This nerve (the portio mollis of the seventh pair) divides, as we have 

 said, into two branches : a cochlear and a vestibular. 



The cochlear branch, the largest, reaches the base of the cochlea, 

 where it breaks up into a large number of fasciculi, one portion of which 

 expands over the first turn of the lamina spiralis, the other on the second, 

 and a third on the third ; the latter ramifications penetrate to the auditory 

 scala, and terminate above or below on the organ of Corti. 



The vestibular branch divides into three portions, whose terminal fila- 

 ments ramify in the wall of the sacculus, utriculus, and the ampullae at the 

 extremities of the three semicircular canals. 



The precise manner in which these filaments of the auditory nerve 

 terminate is doubtful. (Breschet says they communicate and form a series 

 of minute arches. Some of the filaments of the other nerves pass into the 

 sac, and come into contact with the otoconies or ear-dust in its interior.) 



(The membrane of the labyrinth is supplied with blood-vessels by a 

 branch of the basilar artery, which passes with the auditory nerve to the 

 bottom of the meatus, and divides into twigs corresponding with the nerve 

 divisions ; its ultimate ramifications terminating, in the form of a fine net- 

 work, on the membranous labyrinth and the spiral lamina of the -cochlea. 

 The blood is returned by the auditory vein, which enters the superior 

 petrosal sinus.) 



ARTICLE II. MIDDLE EAR OR TYMPANUM. 



Excavated in the substance of the tuberous portion of the temporal bone, 

 on the limit of the petrous and mastoid sections, but chiefly in the latter, 

 the middle ear constitutes an irregular cavity, which we may consider as 

 composed of two ivalls and a circumference. 



The external wall is principally constituted by the membrane of the tym- 

 panum. The internal wall, formed by the petrous bone, offers two openings 

 the fenestra ovalis and fenestra rotunda, the one situated behind the other, 

 and separated by a small eminence named the promontory. The circumference 

 is occupied for nearly the whole of its extent by the mastoid cells, large 

 open cavities in the tympanum. 



Internally, the tympanum contains a chain of small bones named the 

 malleus, incus, os orbicular -e, and stapes ; these bones form the medium of 

 communication between the tympanum and the fenestra ovalis from 

 one wall to the other of the cavity of the tympanum. 



This cavity is lined by a fine mucous membrane, which is continuous with 



