950 



THE SENSORY APPARATUSES. 



inferior is smaller, spherical in shape, and forms the sacculus ; it appeal's to be 

 perfectly closed, though in contact with the utriculus. 



The membranous vestibule is composed of two distinct layers— an external, 

 of connective tissue ; and an internal, epithelial, resting on an amorphous mem- 

 brane. At the expansion of the nerve-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 

 (otoliths). 



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



lining membrane of the laby- 

 F'?- -^H. rinth ; a vascular, with multi- 



tudes 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 Semicir- 

 cular Canals (Fig. 513). 



These are three thin tubes, 

 which correspond exactly with, 

 though they are of smaller 

 diameter than, the osseous semi- 

 circular 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-ampuUareus) ; 

 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. 



THE COCHLEA OPb;NF:D. TO SHOW THE ARRANGF.MENT 

 OF THE TWO RAMPS AND DISTRIBUTION OF THE 

 AUDITORY NERVE. 



«, Cochlea ; b, auditory nerve ; c, blood-vessel ; d, d', 

 vascular ramifications ; e, posterior part of facial 

 nerve turned upwards; /, intermediate nerve of 

 VVrisberg ; g, summit of tiie cochlea ; h, common 

 trunks of the petrosal nerves. 



3. The Membranous Cochlea (Fig. 514). 



The membranous cochlea is represented by two membranes, which complete 

 the lamina spiralis ; they continue the osseous laminte of the latter, and are 

 inserted into the external wall of the cochlea. 



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

 the ear — an inferior, or ti/mpanis scala ; a superior, or vestibular scala ; and a 

 middle, or auditive scala, in which the ore/an of Corti is lodged. The vestibular 

 scala is itself divided by the membrane of Reissner into two canals — the proper 



