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The saccule, which is somewhat oval, hes in front of the utricle, 

 and occupies the fovea spherica, where it is contiguous to the open- 

 ing leading into the scala vestibuli of the cochlea. Through the 

 openings of the fovea spherica it receives filaments of the auditory 

 nerve, and this portion of the saccule being thickened is known 

 as the macula acustica sacculi. Inferiorly the saccule is connected 

 with a small canal, called the canalis or ductus reuniens of Hensen, 

 which opens into the canal of the cochlea, or ductus cochlearis, 

 not far from its closed vestibular end. From the posterior part 

 of the saccule a minute canal, called the ductus sacculi, passes off, 

 which is soon joined by the ductus utricull, and so the ductus 

 endolymphaticus is formed. This latter duct traverses the 

 aqueductus vestibuli, and, having reached the posterior surface of 

 the petrous part of the temporal bone, it ends in a small blind 

 dilatation, called the sacculus endolymphaticus, which lies beneath 

 the dura mater. The saccule and utricle are thus indirectly con- 

 nected by means of the ductus sacculus and ductus utriculus ; 

 and the saccule communicates with the ductus cochlearis by means 

 of the canalis or ductus reuniens of Hensen. 



Semicircular Canals. — ^The membranous semicircular canals cor- 

 respond in outline to the osseous semicircular canals, within which 

 they lie ; they form about two-thirds of a circle ; and each presents 

 an ampulla at one extremity, which is situated within the ampulla 

 of the osseous canal. They are elliptical in transverse section, 

 and open into the utricle by five orifices, the non-ampullary ends of 

 the superior and posterior canals being imited, so that these two 

 ends open by a common orifice. The convex wall of each canal is 

 attached to the periosteal lining of the osseous canal, whilst the 

 concave wall is practically free from the osseous wall, and is bathed 

 by the perilymph. These canals contain endolymph. 



Structure. — ^The walls of the utricle, saccule, and membranous 

 semicircular canals consist of three layers, namely, (i) an outer or 

 fibrous layer, which is vascular ; (2) a middle layer, or membrana 

 propria, which is translucent ; and (3) an inner or epithelial layer. 

 In each ampulla the middle layer, or membrana propria, projects 

 into the cavity of the canal from the peripheral wall, this projecting 

 part being known as the septum transversum. It partially divides 

 the interior of the ampulla into two compartments, and its free 

 margin, which is covered by the auditory epithelium, is called the 

 crista acustica. The epithelial layer consists of a single stratum 

 of squamous cells, except in those regions to which the filaments 

 of the auditory nerve are distributed. These regions are as follows : 

 (i) the macula acustica utriculi ; (2) the macula acustica sacculi ; 

 and (3) the crista acustica of each ampulla. 



The macula acustica utriculi is the thickened part of the antero- 

 inferior wall of the recessus utriculi, and is lined with auditory 

 epithelium. The macula is cohered by calcareous particles, called 

 otoliths or otoconia, which consist of crystals of calcium carbonate. 

 The macula acustica sacculi i>^ the thickened part of the anterior 



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