THE INTERNAL EAR OR LABYRINTH. 839 



and is named the utriculus ; it communicates with the semicircular canals, of 

 which it is a confluent. The inferior is smaller, spherical in shape, and 

 forms the sacculus ; it appears to be perfectly closed, though in contact with 

 the utriculus. 



The membranous vestibule is composed of two distinct layers : an 

 external, cellular, and an internal, epithelial, resting on an amorphous 

 membrane. At the expansion of the nervous filaments, the latter is absent, 

 and is replaced by a white calcareous substance (minute crystalline particles 

 of carbonate and phosphate of lime) which, in the domesticated animals, 

 appears as a powder, and is named the calcareous powder of the vestibule, ear 

 dust, or otoconites (otoliihs). 



(Some authorities give four layers : an external or serous, derived from 

 the lining membrane of the labyrinth ; a vascular, with multitudes of vessels ; 

 a nervous, formed by the expansion of the filaments of the vestibular nerve ; 

 and an internal serous membrane, which secretes the limpid fluid contained 

 in its interior. Spots of pigment are constantly found in the tissue of the 

 membranous labyrinth.) 



2. The Membranous Semicircular Canals. 



These are three thin tubes, which correspond exactly with, though they are 

 of smaller diameter than, the osseous semicircular canals ; they open into the 

 utriculus in the same manner as the latter do into the bony vestibule. Each 

 has one of its two extremities dilated into a sac or ampulla (sinus-ampullaceus) ; 

 for the two superior and external canals it is the anterior extremity, and 

 for the posterior canal the outer extremity. 



In structure they resemble the vestibular sacs. 



3. The Membranous Cochlea. 



The membranous cochlea is represented by two membranes, which 

 complete the lamina spiralis ; they continue the osseous laminae of the 

 latter, and are inserted into the external wall of the cochlea. 



They give rise to three cavities, or scales, in the interior of this portion 

 of the ear : an inferior, or tympanic scala ; a superior, or vestibular scala ; 

 and a middle or auditive scala, in which the organ of Corti is lodged. The 

 vestibular scale is itself divided by the membrane of Heissner into two canals 

 the proper vestibular scala, and Lowenberg's, or the collateral scala ; so that, 

 in reality, there are four cochlean scales. 



We do not, therefore, find in the cochlea, as in the other regions of the 

 labyrinth, a system of membranous cavities included in osseous cavities. 



The structure of the membranes that limit the auditive scala is not 

 perfectly known, and is still disputed by anatomists ; but connective, epi- 

 thelial, and nervous elements appear to form their base. 



With regard to the organ of Corti, it is a very curious and interesting 

 portion of the auditory scala, being formed by a series of solid and elastic 

 arches resting by their extremities on the membrane the basilar that sepa- 

 rates the auditory from the tympanic scala, their convexity being towards- 

 the superior, or membrane of Corti. These arches number about three 

 thousand in Man, and are composed of two portions or articles : an external 

 and an internal, united by a thickening in the vicinity of the membrane of 

 Corti. To these elastic arches are added conical or fusiform cells, whose 

 nature is not yet determined. 



i 



