THE INTERNAL EAR. 



863 



and present at one end an ampullary enlargement. They open by five orifices into 

 the utricle, one opening being common to two canals. In the ampullae the vail 

 is thickened, and projects into the cavity as a fiddle-shaped, transversely placed 

 elevation, the septum transversum, in which the nerves end. 



The utricle, saccule, and membranous canals are held in position by numerous 

 fibrous bands which stretch across the space between them and the bony walls. 



Structure. The walls of the utricle, saccule, and semicircular canals consist 

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

 composed of ordinary fibrous tissue, containing blood-vessels and 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 membrane, but it 

 presents on its internal surface, especially in the semicircular canals, numerous 

 papilliform projections, and, on the addition of acetic acid, presents an appearance 

 of longitudinal fibrillation and elongated nuclei. The inner layer is formed of 

 polygonal nucleated epithelial cells. In the maculae of the utricle and saccule, 

 and in the transverse septa of the ampullae of the canals, the middle coat is thick- 

 ened and the epithelium is columnar, and consists of supporting cells and hair- 

 cells, the free ends of the latter being surmounted by a long, tapering filament 

 (auditory hair) which projects into the cavity. The filaments of the auditory 

 nerve enter these parts, and, having pierced the outer and thickened middle layer, 

 they lose their medullary sheath, and their axis-cylinders ramify between the hair- 

 cells. 



Two small rounded bodies termed otoliths, and consisting of a mass of minute 

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



FIG. 463. Floor of scala media, showing the organ of Corti, etc. 



tissue, are contained in the walls of the utricle and saccule opposite the distribution 

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

 scattered in the cells lining the ampullae of the semicircular canals. 



The membranous cochlea, ductus cochlearis, or scala media consists of a 

 spirally arranged tube enclosed in the bony canal of the cochlea and lying along 

 its outer wall. The manner in which it is formed will now be described. 



The osseous spiral lamina, as above stated, extends only part of the distance 

 between the modiolus and the outer bony wall of the cochlea, A membrane the 

 membrana basilaris, stretches from its free edge to the outer wall of the cochlea, 

 arid completes the roof of the scala tympani. A second and more delicate mem- 

 brane, the membrane of Reissner, extends from the thickened periosteum covering 

 the lamina spiralis ossea to the outer wall of the cochlea, to which it is att 

 at some little distance above the membrana basilaris. A canal is thus shut 

 between the scala tympani below and the scala vestibuli above ; this is the mem 

 branous canal of the cochlea, ductus cochlearis, or scala media. 

 on transverse section, its roof being formed by the membrane of Reissner, i 

 outer wall by the periosteum which lines the bony canal, and its 1 



