MEMBRANOUS LABYRINTH. 



707 



fovea semi-elliptica. Numerous filaments of the auditory nerve are distributed 

 on the wall of this sac ; and its cavity communicates behind with the mem- 

 branous semicircular canals by five orifices. 



The saccuk is the smaller of the two vestibular sacs : it is globular in form, 

 lies in the fovea ^hemispherica, near the opening of the vestibular scala of the 

 cochlea, and receives numerous nervous filaments, which enter from the bottom 

 of the depression in which it is contained. Its cavity is apparently distinct 

 from that of the utricle. 



The membranous semicircular canals are about one-third the diameter of the 

 osseous canals, but in number, shape, and general form they are precisely simi- 

 lar; they are hollow, and open by five orifices into the utricle, one opening 

 being common to two canals. Their ampullae are thicker than the rest of the 

 tubes, and nearly fill the cavities in which they are contained. 



Fig. 383, The Membranous Labyrinth detached. (Enlarged.) 



tern t&rcu 



X. to Co 



The membranous labyrinth is held in its position by the numerous nervous 

 filaments distributed to the utricle, to the saccule, and to the ampulla of each 

 canal. These nerves enter the vestibule through the minute apertures on its 

 inner wall. 



Structure. The wall of the membranous labyrinth is semi-transparent, and 

 consists of three layers. The outer layer is a loose and flocculent tissue, con- 

 taining bloodvessels and numerous pigment-cells, analogous to those in the 

 choroid. The middle layer, thicker and more transparent, bears some resem- 

 blance to the hyaloid membrane, but it presents in parts marks of longitudinal 

 fibrillation and elongated nuclei on the addition of acetic acid. The inner layer 

 is formed of polygonal nucleated epithelial cells, which secrete the endolymph. 



The endolymph (liquor Scarpss) is a limpid serous fluid, which fills the mem- 

 branous labyrinth; in composition, it closely resernbles the perilymph. 



The otoliths are two small rounded bodies, consisting of a mass of minute 

 crystalline grains of carbonate of lime, held together in a mesh of delicate 

 fibrous tissue, and contained in the wall of the utricle and saccule, opposite the 

 distribution of the nerves. A calcareous material is also, according to Bow- 



