686 



DISSECTION OF THE EAR. 



is named endolymph ; and in the wall of the sac is a small calcareous de- 

 posit (otolith, e) opposite the entrance of the nerve into it. 



The saccule (fig. 248, a) is a smaller and rounder cyst than the utricle, 

 and is placed in front of it in the hollow of the fovea hemispherica. It is 

 separate from the other ; but it is continuous, below, by a short and small 

 duct (c, ductus reuniens) with the canal of the cochlea. 



Like, the larger sac it has a translucent wall, in which is an otolith (b) 

 opposite the entrance of the nerve; it is also filled by endolymph, which is 

 continued into the canal of the cochlea through the ductus reuniens. 



Fig. 248. 



PETROUS BONE PARTLY REMOVED TO SHOW THE MEMBRANOUS LABYRINTH IN PLACE (Breschet). 

 o. Small sac. e. Its otolith. 



6. Its otolith. /. Ampullary enlargement on a semi- 



c. Ductus reuniens. circular tube. 



d. Large sac, or utricle. g. Semicircular tube. 



Structure of the sacs. The wall of the sacs of the membranous labyrinth 

 is translucent and firm ; but it is more opaque where the vessels and 

 nerves enter it. Three strata enter into its construction, together with 

 bloodvessels and nerves. 



The outer fibrous covering is loose and flocculent, is easily detached, 

 and contains irregular pigment cells with ramifying bloodvessels. The 

 middle stratum (tunica propria) is clear and tough, like the hyaloid mem- 

 brane in the interior of the eyeball : its inner surface is irregular, owing 

 to slight eminences ; and in the ampulla of the semicircular tube this layer 

 forms a transverse projection. The inner one is formed by a layer of 

 fiat toned nucleated cells, which are continued over the eminences in the 

 interior. In the region of the nerve, both in the ampulla of the semicir- 

 cular canal and in the sacs, the epithelium is said to become columnar, 

 and to have intermixed spindle-shaped nucleated cells (like the olfactorial), 

 which possess hair-like processes at their free ends. 



The small calcareous masses, or the otoliths, consist of minute, elon- 

 gated, six or eight-sided particles of carbonate of lime, which are pointed 

 at the ends, and are situate in the inner part of the wall of the utricle and 



