THE ORGANS OF SPECIAL SENSE 



581 



Ampullar 



(the canalis communis), there are five openings from the vestibule 

 into the semicircular canals (Fig. 385). The bony labyrinth is lined 

 with periosteum, covered by a single layer of endothelial cells. 



The Vestibule and the Semicircular Canals. — In the vestibule 

 the membranous labyrinth is subdivided into two chambers, the sac- 

 cule and the utricle, which are connected by the utriculo- saccular duct. 

 From the latter is given off the endolymphatic duct which communi- 

 cates, through the aqueduct of the vestibule, with a subdural lymph 

 space, the endolymplialic sac. The saccule opens by means of the 

 ductus reuniens into the cochlea, 

 while the utricle opens into the 

 ampulke of the semicircular 

 canals. The saccule and utricle 

 only partly fill the vestibule, 

 the remaining space, crossed by 

 fibrous bands and lined wdth 

 endothelium, constituting the 

 perilymphatic space. 



Saccule and Utricle. — The 

 walls of the saccule and of the 

 utricle consist of fine fibro- 

 elastic tissue supporting a thin 

 basement membrane, upon 



which rests a single layer of low epithelial cells. In the 

 of each chamber is an area of special nerve distribution, 

 macula acustica. Here the epithelium changes to high columnar and 

 consists of two kinds of cells, sustentacular and neuro-epithelial. 

 The sustentacular cells are long, irregular, nucleated cylinders, narrow 

 in the middle, widened at each end, the outer end being frequently 

 split and resting upon the basement membrane. The neuro-epithelial 

 cells or "hair cells" are short cylinders which extend only about half- 

 way through the epithehum. The basal end of the cell is the larger 

 and contains the oval nucleus. The surface of the cell is provided 

 with a cuticular margin from which project several long hair-like 

 processes, the auditory hairs. Small crystals of calcium carbonate are 

 found on the surfaces of the hair cells. These are known as otoliths 

 and are embedded in a soft substance, the otolithic membrane. The 

 hair cells are the neuro-epithelial end-organs of the vestibular divi- 

 sion of the auditory nerve and are, therefore, closely associated with 

 the nerve fibres. The latter on piercing the basement membrane 



.Ampulla 



Fig. 3S5. — The Bony Labyrinth. 



(Heitzmann.) 



X3- 



wall 

 the 



