200 THE MEMBRANOUS LABYRINTH. [BOOK in. 



three semicircular canals. These arc disposed in the three planes of 

 space. One (Fig. 176, H.S. C.) lies in a horixontal plane, and is called 

 the horizontal or, since its hoop is directed outwards, the external 

 semicircular canal. The other two lie in two vertical planes at 

 right angles to the above. One lying in a vertical plane more or 

 less parallel to the median plane of the head has its hoop directed 

 backwards and hence is called the posterior canal (P.S. C.) ; the 

 other lying in a vertical place more or less parallel to the trans- 

 verse plane of the head has its hoop directed upwards, and hence 

 is called the superior canal (A.S.C.). The three planes in 

 which the three canals lie are placed with great exactitude at 

 right angles to each other ; they do not however coincide with the 

 three planes of the head (sagittal or horizontal, median, and 

 frontal or transverse) but make angles with these. 



Each canal, at one of the ends by which it opens into the 

 utricle, is dilated into a flask-shaped swelling, the ampulla, (Figs. 

 175, 176), but at the other end does not shew any such marked 

 swelling. The two ends of the two vertical canals, superior and 

 posterior, which do not bear ampullae, join together into a common 

 canal (Fig. 175 X.) before they open into the utricle, but both ends 

 of the horizontal canal are separate. Hence the canals, taken 

 together, open into the utricle by five openings, three of which are 

 marked by ampullae, two are not. 



The saccule, though lying close to the utricle, is wholly distinct 

 from it and indeed is separated from it to a certain extent by a 

 bony partition stretching inwards from the bony envelope ; the 

 cavity of the one has no direct communication with that of the 

 other. An indirect communication is however supplied by the 

 duct us endolymphaticus (Figs. 175 d.e. 176 a.v.) which formed by 

 the union of a narrow tube, springing from the utricle with a wider 

 one leading from the saccule, runs inwards towards the median 

 line, in a canal hollowed out of the petrosal bone, and ends in a 

 flattened sac, sacc.us endolymphaticus (Fig. 162 s.e.), placed in the 

 cavity of the skull and supported between two layers of the dura 

 mater. Through this hollow tube the cavity of the utricle is made 

 continuous with that of the saccule. 



From the saccule there starts also another narrow tube, the 

 ranalis reuniens (Fig. 176 c.), which opens into the canalis coch- 

 learis, or cochlear division of the membranous labyrinth ; by this 

 the cavity of the vestibular division of the sac is made continuous 

 with that of the cochlear division. 



822. The wall of the membranous labyrinth consists through- 

 out of an epithelium, modified in certain places into auditory 

 epithelium ( 808) and of a connective tissue or dermis, on which 

 this epithelium rests. Around this dermis is developed the 

 lymphatic cavity, lined with lymphatic epithelium ( 809) and 

 filled with perilymph, the outer wall of the cavity being furnished 

 by connective tissue continuous with the periosteum of the bony 



