THE MEMBRANOUS LABYRINTH OF THE EAR. 



927 



The membranous *> mi<-ir<-ular 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 canal-?. Their ampullae are thicker than the rest of the tubes, 

 and nearly fill the cavities in which they are contained. 



Numerous fibrous bands stretch across between the membranous and bony 

 labyrinths. These fibrous bands convey the blood-vessels and nervous fila- 

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

 canal. 



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

 consists of three layers. The outer layer is a loose and flocculent structure, 

 apparently composed of ordinary fibrous tissue, containing blood-vessels and 

 numerous pigment-cells analogous to those in the pigment-coat of the retina. The 

 middle layer, thicker and more transparent, bears some resemblance to the hyaloid 



/AMPULLyE 



DUCTUS 



ENDO-LYMPHATICU8 



CANALISREUNIENS 

 FIG. 551. The membranous labyrinth. (Enlarged.) 



membrane, but it presents on its internal surface numerous papilliform projections, 

 and on the addition of acetic acid presents an appearance of longitudinal fibrilla- 

 tion and elongated nuclei. The inner layer is formed of polygonal nucleated epi- 

 thelial cells, which secrete the endolvmph. 



The endolymph (liquor Scarpce) is a limpid serous fluid which fills the membra- 

 nous labyrinth ; in composition it closely resembles the perilymph. 



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

 talline grains of carbonate of lime, held together in a mesh of delicate fibrous 

 tissue, and contained in the walls of the utricle and saccule, opposite the distribu- 

 tion of nerves. A calcareous material is also, according to Bowman, sparingly 

 scattered in the cells lining the ampulla of each semicircular canal. 



The arteries of the labyrinth are the internal auditory, from the basilar; the 

 stylo-mastoid, from the posterior auricular ; and, occasionally, branches from the 

 occipital. The internal auditory divides at the bottom of the internal meatus into 

 two branches, cochlear and vestibular. 



The cochlear branch subdivides into from twelve to fourteen twigs, which 

 traverse the canals in the modiolus. and are distributed, in the form of a capillary 

 network, in the substance of the lamina spiralis. 



The vestibular branches accompany the nerves, and are distributed, in the form 

 of a minute capillary network, in the substance of the membranous labyrinth. 



The veins (auditory) of the vestibule and semicircular canals accompany the 

 arteries, and, receiving those of the cochlea at the base of the modiolus, terminate 

 in the superior petrosal sinus. 



The auditory uerve, the special nerve of the sense of hearing, divides, at the 



