|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
|behind with the semicircular ducts by five orifices. From its anterior wall is given off the ductus utriculosaccularis, which opens into the ductus endolymphaticus.|
The Saccule (sacculus).The saccule is the smaller of the two vestibular sacs; it is globular in form, and lies in the recessus sphæricus near the opening of the scala vestibuli of the cochlea. Its anterior part exhibits an oval thickening, the macula acustica sacculi, to which are distributed the saccular filaments of the acoustic nerve. Its cavity does not directly communicate with that of the utricle. From the posterior wall a canal, the ductus endolymphaticus, is given off; this duct is joined by the ductus utriculosaccularis, and then passes along the aquæductus vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior surface of the petrous portion of the temporal bone, where it is in contact with the dura mater. From the lower part of the saccule a short tube, the canalis reuniens of Hensen, passes downward and opens into the ductus cochlearis near its vestibular extremity (Fig. 924).
FIG. 924 The membranous labyrinth. (Enlarged.) (See enlarged image)
The Semicircular Ducts (ductus semicirculares; membranous semicircular canals), (Figs. 925, 926).The semicircular ducts are about one-fourth of 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. They open by five orifices into the utricle, one opening being common to the medial end of the superior and the upper end of the posterior duct. In the ampullæ 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 semicircular ducts are held in position by numerous fibrous bands which stretch across the space between them and the bony walls.|
Structure (Fig. 927).The walls of the utricle, saccule, and semicircular ducts consist of three layers. The outer layer is a loose and flocculent structure, apparently composed of ordinary fibrous tissue containing bloodvessels and some pigment-cells. The middle layer, thicker and more transparent, forms a homogeneous membrana propria, and presents on its internal surface, especially in the semicircular ducts, numerous papilliform projections, which, on the addition of acetic acid, exhibit an appearance of longitudinal fibrillation. The inner layer is formed of polygonal nucleated epithelial cells. In the maculæ of the utricle and saccule, and in the transverse septa of the ampullæ of the semicircular ducts, the middle coat is thickened and the epithelium is columnar, and consists of supporting cells and hair cells. The former are fusiform, and their deep ends are attached to the membrana propria, while their free extremities are united to form a thin cuticle. The hair cells are flask-shaped, and their deep, rounded ends do not reach the membrana propria, but lie between the supporting cells. The deep part of each contains a large nucleus, while its more superficial part is granular and pigmented. The free end is surmounted by a long, tapering, hair-like filament, which projects into the cavity. The