MEMBEANOUS LABYKINTH. 



847 



_ Wall of 

 ^T bony canal 



anteriorly an oval, whitish thickening, the macula acustica sacculi. This has a 

 breadth of about 1'5 mm., and to it the saccular fibres of the acoustic nerve are 

 distributed. The superior extremity of the saccule is directed upwards and back- 

 wards, and forms the sinus utricularis sacculi, which abuts against, but does not fuse 

 with, the wall of the utricle. From the lower part of the saccule a short canal, 

 the ductus reuniens (Henseni), opens into the ductus cochlearis, a short distance 

 in front of its vestibular extremity. A second small channel, the ductus 

 endolymphaticus, is continued from the posterior part of the saccule, and, passing 

 between the utricle and the medial wall of the vestibule, is joined by a small canal, 

 the ductus utriculosaccularis, which arises from the medial side of the utricle. It 

 then enters and traverses the aquaeductus vestibuli and ends, under the dura mater 

 on the posterior surface of the petrous part of the temporal bone, in a dilated 

 blind extremity, termed the saccus endolymphaticus ; this, according to Riidinger, is 

 perforated by minute foramina, through which the endolymph may pass into the 

 meningeal lymphatics. 



The vestibule also contains the vestibular extremity of the ductus cochlearis, 

 which lies immediately below the saccule in the recessus cochlearis. 



The walls of the utricle and saccule are composed of connective tissue which 

 blends with the periosteal lining of the vestibule. It is modified medially to form 

 a homogeneous membrana propria, which is covered with a layer of pavement 

 epithelium and is thickened at the maculae acusticae. Towards the periphery 

 of the maculae the epithelium is cubical, while on them it is columnar. 



The structure of the maculae in the utricle and saccule is practically the 

 same ; two kinds of cells are found, viz., (a) supporting cells, and (&) hair cells. The 

 supporting cells are some- 

 what fusiform, each con- 

 taming, >n ear its middle, a 

 nucleus. Their branched, 

 deep extremities are at- 

 tached to the membrana 

 propria; their free ends 

 lie between the hair cells 

 and form a thin inner 

 limiting cuticle. The 

 hair cells are flask-shaped 

 and do not reach the 

 membrana propria, but 

 end in rounded extremi- 

 ties which lie between 

 the supporting cells. 

 Each contains, at its 

 deepest part, a large 

 nucleus, the rest of the 

 cell being granular and 

 pigmented. From the 

 free end of each there 

 projects a stiff, hair-like 

 process, which, on the 

 application of reagents, 



splits into several finer filaments. The nerve-fibres pierce the membrana propria, 

 and ramify around the deep extremities of the hair cells (Fig. 722). A collection 

 of small, rhombic crystals of carbonate of lime, termed otoconia, adheres to each 

 of the maculae. 



Ductus Semicirculares. The semicircular ducts are * elliptical on transverse 

 section (Fig. 721), and are attached to the walls of the bony canals. The convex 

 wall of each duct is fixed to the periosteal lining of the canal, whilst the opposite 

 part is free, except that it is connected by irregular ligamentous bands, which pass 

 through the perilymphatic space to the bony wall. Like the bony canals, each 

 of the semicircular ducts is dilated at one extremity into a membranous ampulla, 



Lumen of semi- 

 ~v circular duct 



Periosteum 



FIG. 721. TRANSVERSE. SECTION OF HUMAN SEMICIRCULAR 

 CANAL AND SEMICIRCULAR DUCT (Riiclinger). 



