1144 



THE ORGANS OF SPECIAL SENSE 



terior wall is given off a canal, the ductus endolymphaticus. This duct is joined 

 by the ductus utriculosaccularis, and then passes along the aquaeductus vestibuli 

 and ends in a blind pouch on the posterior surface of the petrous portion of the tem- 

 poral bone, where it is in contact with the dura. From the lower part of the saccule 

 a short tube, the canalis reuniens of Hensen (ductus reuniens [Henseni]}, passes 

 downward and outward to open into the ductus cochlearis near its vestibular 

 extremity. 



The Membranous Semicircular Canals (ductus semicirculares) are about one-third 

 the diameter of the osseous canals, but in number, shape, and general form they 

 are precisely similar, and each presents at one end an ampulla (ampullae mem- 

 branaceae}. The canals open by five orifices into the utricle, one opening being 

 common to the inner end of the superior and the upper end of the posterior 

 canal. In the ampullae the wall 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 attached here and there to 

 the bone by numerous fibrous bands, the so-called ligaments (ligamenta laby- 

 rinthi canaliculorum). 



FIG. 852. Floor of scala media, showing the organ of Corti, etc. 



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

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

 nary fibrous tissue, containing bloodvessels and pigment cells. 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 thickened; 

 the epithelium is columnar, is increased in height, and passes into the neuroepithelium. The 

 neuroepithelium consists of supporting cells and hair cells. 



1. The supporting cells are long and fusiform, and contain an oval nucleus. Their deep ends 

 are attached to the membrana propria, while their free extremities are united to form a thin 

 cuticle. The protoplasm contains yellowish pigment granules. 



2. The hair cells are columnar, with bulged lower ends and free upper ends. The bulged 

 lower ends, each of which contains a spherical nucleus, do not reach higher than the middle of 

 the epithelial layer. Each free upper end is surmounted by a long, tapering filament. These 

 filaments constitute auditory hair, and they project into the cavity. Each filament is found to 

 consist of many fine hairs. The filaments of the auditory nerve enter these parts, and, having 

 pierced the outer and thickened middle layer, they lose their myelin sheaths, and their axones 

 divide into three or four branches at the larger and deeper ends of the hair cells. These branches 

 form a horizontal plexus (stratum plexiforme). 



Numerous small prismatic bodies termed statoliths, otoconia, or otoliths, and consisting of a 

 mass of minute crystalline grains of carbonate of lime, held together in a gelatinous substance, 

 are contained in the walls of the utricle and saccule opposite the distribution of the nerves. The 

 membrane is called the otolith membrane. A calcareous material is also, according to Bowman, 

 sparingly scattered in the cells lining the ampullae of the semicircular canals. The conical 

 thickening in the ampulla corresponds to the otolith membrane. 



