1052 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 



cates 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 sphaericus 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 aqu?eductus 

 vestibuli and ends in a blind pouch (saccus endolymphaticus) on the posterior sur- 

 face 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 reunions 

 of Hensen, passes downward and opens into the ductus cochlearis near its vestibular 

 extremity (Fig. 924). 



I 



. . . CANALIS 

 AMPULLA// REUNIEN3 



, 'DUCTUS 

 ENDOLYMPHATICUS 

 FIG. 924. The membranous labyrinth. (Enlarged.) 



The Semicircular Ducts (ductus semicircular es; 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 ampullae the wall is thickened, and projects 

 into the cavity as a fiddle-shaped, transversely placed elevation, the septum trans- 

 versum, 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 maculae of the utricle and saccule, and in the trans- 

 verse septa of the ampullae of the semicircular ducts, the middle coat is thickened and the epi- 

 thelium 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 



