1054 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



Two small rounded bodies termed otoconia, each consisting of a mass of minute crystalline 

 grains of carbonate of lime, held together in a mesh of gelatinous tissue, are suspended in 

 the endolymph in contact wish the free ends of the hairs projecting from the maculae. Accord- 

 ing to Bowman, a calcareoutmaterial is also sparingly scattered in the cells lining the ampullae 

 of the semicircular ducts. 



Connective tissue binding 

 duct to periosteum 



Semicircular duct 



-Fibrous band uniting 

 free surface of duct 

 to periosteum 



emicircular 

 canal 



Periosteum 

 Fio. 927. Transverse section of a human semicircular canal and duct (after Riidinger) . 



The Ductus Cochlearis (membranous cochlea; scala media). The ductus cochlearis 

 consists of a spirally arranged tube enclosed in the bony canal of the cochlea and 

 lying along its outer wall. 



As already stated, the osseous spiral lamina extends only part of the distance 

 between the modiolus and the outer wall of the cochlea, while the basilar membrane 

 stretches from its free edge to the outer wall of the cochlea, and completes the roof 

 of the scala tympani. A second and more delicate membrane, the vestibular mem- 

 brane (Reissneri) extends from the thickened periosteum covering the osseous 

 spiral lamina to the outer wall of the cochlea, where it is attached at some little 

 distance above the outer edge of the basilar membrane. A canal is thus shut off 

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

 cochlearis or scala media (Fig. 928). It is triangular on transverse section, its roof 

 being formed by the vestibular membrane, its outer wall by the periosteum lining 

 the bony canal, and its floor by the membrana basilaris and the outer part of the 

 lamina spiralis ossea. Its extremities are closed; the upper is termed the lagena 

 and is attached to the cupula at the upper part of the helicotrema; the lower is 

 lodged in the recessus cochlearis of the vestibule. Near the lower end the ductus 

 cochlearis is brought into continuity with the saccule by a narrow, short canal, 

 the canalis reuniens of Hensen (Fig. 924). On the membrana basilaris is situated 

 the spiral organ of Corti. The vestibular membrane is thin and homogeneous, 

 and is covered on its upper and under surfaces by a layer of epithelium. The 

 periosteum, forming the outer wall of the ductus cochlearis, is greatly thickened 

 and altered in character, and is called the spiral ligament. It projects inward below 

 as a triangular prominence, the basilar crest, which gives attachment to the outer 

 edge of the basilar membrane; immediately above the crest is a concavity, the 



