THE INTERNAL EAR. 223 



semicircular canals open into it by 5 orifices. The wall of the utricle has 

 attached to it the 



Otoliths, or Ear-stones, a powder, consisting of crystals of carbonate of 

 calcium, -jJ^ ^ to ^^^ inch in diameter, held together by a mucoid 

 substance. 

 Macula Acoustica, See below, under Saccule. 



Describe the Membranous Semicircular Canals. They are 3 canals, 

 having the same shape as the osseous ones in which they are contained, being 

 fastened thereto by their convex curves, and elsewhere surrounded by peri- 

 lymph, except at the ampullae, where they fill the osseous canals. They open 

 into the utricle by 5 orifices. 



What is the Saccule ? It is a flask-shaped, membranous sac, about T \ 

 inch in diameter, attached at one point to the utricle, and also to the fovea 

 hemispherica of the vestibule, being elsewhere surrounded by perilymph, and 

 containing endolymph. It communicates with the utricle through the small 

 canal in the aqueductus vestibuli, and also with the membranous cochlea by 

 its neck, the 



Canalis Reuniens, about ^ inch long and T | 7 inch in diameter, is V. 

 shaped, its arms opening respectively into the saccule and the vestibular 

 end of the membranous cochlea. 



Macula Acoustica, is a dense, circular point, -^ inch in diameter, on the 

 wall of both the utricle and the saccule, formed by a thickening of the 

 connective tissue where the fibres of the vestibular nerve enter it. 



Describe the Membranous Cochlea. This portion of the membranous 

 labyrinth, also called the Ductus FIG. 



Cochlearis, commences in the lower 

 end of the osseous cochlea and send 

 at its summit in a closed extremity. 

 It is attached on one side to the 

 lamina spiralis, on the other side to 

 the outer wall of the osseous cochlea. 

 It is formed by the Membrana 

 Basilaris* below, the Membrane 

 of Reissner 4 on one side, and the 

 periosteum of the cochlear wall on 

 the outer side, its cross section being 

 of a triangular form. It is filled with endolymph, and contains the Organ 

 of Corti* covered by the Membrana Tectoria. 11 The 



Limbus Lamina Spiralis, is the soft structure on the edge of the lamina 

 spiralis, and lies in the membranous cochlea. It has two lips, an upper. 



