848 



THE OKGANS OF SENSE. 



which is especially developed towards the concavity of the tube. The membranous 

 ampullae nearly fill the corresponding portions of the bony tubes, but the diameter 

 of the semicircular ducts is only about one-fourth of that of the osseous canals. 



Each semicircular duct consists of three layers, viz. : (a) an outer vascular 

 and partly pigmented fibrous stratum which fixes the duct to the bony wall; 



Perilymphatic space 



Macula acustica 



Cuticular wall 



Fibres of the ramus recessus utriculi 

 FIG. 722. VERTICAL SECTION OF THE WALL OF THE RECESSUS UTRICULI WITH THE MACULA ACUSTICA 



AND THE BUNDLES OF NERVE FIBRES. 



(5) an intermediate, transparent tunica propria, presenting a number of papilliform 

 elevations which project towards the lumen. The fibrous layer and tunica propria 

 are thinnest along the attached surface of the duct, and in this region also the 

 papilliform elevations are absent; (c) an internal epithelial layer, composed of 

 pavement cells. In the ampullae the tunica propria is much thickened, and projects 

 into the cavity as a transverse elevation, termed the septum transversum, which, 

 when seen from above, is somewhat fiddle-shaped; its most prominent part is 

 covered by acoustic epithelium forming the crista ampullaris, at each end of which 

 is a half-moon-shaped border of small columnar cells, the planum semilunatum. The 

 cells covering the crista ampullaris consist of supporting cells and hair cells, and 

 are similar in their arrangement to those in the maculae of the utricle and saccule ; 

 the hairs of the hair cells are, however, considerably longer, and project as far as 

 the middle of the ampullary lumen. In fresh specimens they appear to end free, 

 but in hardened preparations are seen to terminate in a soft, clear, dome -like 

 structure, the cupula terminalis, which is striated, the striae converging towards its 

 concavity. The nerves form arborisations around the bases of the hair cells. 



SCALA VESTIBULI 



Membrana vestibularis 



Membrana tectoria s 



Sulcus spiralis 

 internus 



Limbus laminae 

 spiralis 



Stria 

 vascularis 



i llL.j_V/__ Ligamentum 

 spirale 



/ Ma Sulcus spiralis 

 externus 



Crista basilaris 



Inner hair cell \ 



Outer hair cells y, ^ 



Membrana basilaris 

 SCALA TYMPANI 



FIG. 723. SECTION ACROSS THE DUCTUS COCHLEARIS (Retzius). 



Ductus Cochlearis. The ductus cochlearis (O.T. membranous cochlea or seal; 

 media) is closed at both of its extremities; the lower extremity occupies th< 

 recessus cochlearis of the vestibule and communicates with the saccule through th 

 ductus reuniens. It forms a spirally arranged canal inside the cochlea, and a 



