THE INTERNAL EAR. 373 



to the otolith membrane. The otoliths, also called otoconia, are minute 

 crystals, usually hexagonal in form, with slightly rounded angles, and from 

 91 1 /j. in length. They are composed of calcium carbonate with an organic 

 basis. 



On reaching the maculae the nerve-fibres form a subepithelial plexus, 

 from which fine bundles of fibres pass towards the free surface. The fibres 

 usually lose their medullary substance in passing through the basement mem- 



Trabeculae, ___. _ 



Membranous 

 canal 



Peri lymphatic 

 space' 



Trabecul* 



Bony wall 



FIG. 421. Transverse section of superior semicircular canal, showing relations of membranous to bone 



tube. X 35- 



brane and enter the epithelium as naked axis-cylinders. Passing between 

 the sustentacular cells to about the middle of the epithelium, they break up 

 into fine fibrillae, which embrace the deeper ends of the hair-cells and give 

 off fine threads that pass, as free axis-cylinders, between the cells to higher 

 levels. 



The crista acustica and the adjoining planum semilunatum are 

 covered with neuroepithelium similar to that of the maculae. The hairs of 

 the hair-cells, however, are longer and converge to and are embedded within 

 a peculiar dome-like structure, known as the cupola, which probably does 

 not exist during life, but is an artefact formed by coagulation of the fluid in 

 which the ends of the hairs are bathed. Otoliths are usually not present in 

 the cristae acusticae. 



The Cochlear Duct. The membranous cochlea or ductus cochlearis 

 lies within the bony cochlea and like it includes from two and one-half to 

 two and three-quarter turns, named respectively the basal, middle, and apical, 

 the latter being three fourths of a turn at the apex of the cochlea. The 

 tapering tube of the bony cochlea, winding spirally around the modiolus, is 

 subdivided into three compartments by the osseous spiral lamina and two 

 membranes, namely, the membranous spiral lamina and Reissner's mem- 

 brane. The membranous spiral lamina or basilar membrane extends from 



