MEMBRANOUS LABYRINTH. 



641 



It has the same general form as the vestibule and semicircular canals, in which it 

 is inclosed ; but is considerably smaller, and separated from their lining membrane 

 by the perilymph. 



The vestibular portion consists of two sacs, the utricle and the saccule. 



The utricle is the larger of the two, of an oblong form, compressed laterally, 

 and occupies the upper and back part of the vestibule, lying in contact with the 



Fig. 318. The Membranous Labyrinth detached. (Enlarged.) 



OfoJitfts 



lien tirougA thf u'a 



tf. to Cochlea 



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

 the wall of this sac ; and its cavity communicates, behind, with the membranous 

 semicircular canals by five orifices. 



The saccule 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 similar ; 

 they are hollow, and open by five orifices into the utricle, one 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. 



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

 ments distributed to the utricle, 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 con- 

 sists of three layers. The outer layer is a loose and flocculent tissue, containing 

 bloodvessels and numerous pigment-cells, analogous to those in the choroid. The 

 middle layer, thicker and more transparent, bears some resemblance 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. 

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