THE MEMBRANOUS LABYRINTH. 105 



wall of the utricle is thickened, the thickening having a concave surface towards 

 the interior of the utricle, and being covered by auditory epithelium (macula 

 acusttca utriculi}. A small mass of calcareous particles (otoliths or oioconia) lies 



Fig. 116. OTOLITHS. (From Schwalbe.) 



within the sac, attached to the macula. These 

 otoliths are crystals of carbonate of lime, rhombic, 

 octahedral, or six-sided, often pointed at their ex- 

 tremities (fig. 118). 



The ends of all the membranous semicircular 

 canals open into the utricle in the situations shown 

 in the diagrams, and a fine canal (canalis utriculo- 

 saccularis) passes from the antero-mesial wall of the 



utricle, which joins with another one from the saccule to form the ducius endo- 

 lymphaticus (fig, 11G, d.e) (see below). 



The ampulla; of the superior and external canals open into the roof of the recessus 

 utriculi. The part of the utricle where the ampulla of the posterior semicircular canal opens 

 is termed by GL Retzius si nits posterior, and the conjoined limbs of the superior and external 

 canals form the .v/'^w.s- Kiijin-tor of that author. In all vertebrates below mammals the utricle 

 has a second macula acustica in its lower part near the sinus posterior {macula, ncgleeta of 

 Retzius). 



The smaller vesicle, the saccule (fig. 115, s ; fig. 117), is an irregularly oval 

 vesicle about 3 mm. long and nearly 2 mm. broad, and is somewhat flattened from 

 within out. The saccule is situated in the lower and fore part of the cavity of the 

 osseous vestibule, close to the opening from the scala vestibuli of the cochlea, and 

 is received into the hollow of the fovea hemispherica, from the bottom of which 

 many branches of nerve enter it, and here there is a similar broad and concave 

 macula in its wall, which is covered by a small mass of otoliths. The upper end of 

 the saccule bends round towards the recessus utriculi, with which it comes in 

 contact, without direct communication (fig. 117). 



The saccule gradually narrows below into a short funnel-shaped duct (1 mm. 

 long, 0*5 mm. wide), the canalis reuniens of Hensen (fig. 115, c. r. ; fig. 117, 19), 

 which passes downwards and outwards to open directly into the epithelial canal of 

 the cochlea a short distance from its blind lower extremity. There is also, as 

 already mentioned, a canal, lined with epithelium, which passes from the posterior 

 wall of the saccule along the aqueductus vestibuli to end blindly in a dilated 

 extremity (saccus endo-lymphaticus, fig. 115, s.e.) on the posterior surface of the 

 petrous bone just below the orifice of the aqueduct and lying in the tissue of the 

 dura mater (Cotugno). This canal is joined near its origin by a minute tube from 

 the utricle (fig. 117, 7), so that in this way the cavity of the saccule is brought into 

 communication with that of the utricle (Boettcher). 



The membranous semicircular canals are from one-third to one-fifth the 

 diameter of the osseous tubes in which they are lodged, and are dilated into ampullae 

 within the ampullary enlargements of those tubes. In section they are oval or some- 

 what elliptical (fig. 119). The ampullae measure from '2 mm. to 2*5 mm. in length ; 

 they are thicker and less translucent than the rest of the canals, and nearly fill 

 their bony cases, the (membranous) ampullae being nearly three times the diameter 

 of the canals. That part of each which is towards the concavity of the semicircle 

 of the canal is free ; whilst the opposite portion is fixed to the wall of the bony 

 canal ; in the ampulla this part is flattened and receives branches of nerves and 

 blood-vessels, and on its inner surface is a transverse projection (septum transversum) 

 which partly divides the cavity into two, and broadens out somewhat at either end. 

 The most prominent part of the septum, which is surmounted by the auditory 



