104 THE EAR. 



structure is more open and spongy, and contains numerous small canals for 

 vessels and nerves, continuous with, but running at right angles to, the canals in 

 the modiolus. Winding round the modiolus, in the base of the spiral lamina, is a 

 small canal, named the spiral canal of the modiolus. 



The scala tympani (fig. Ill ; fig. 113, 3), the portion of the tube on the basal 

 side of the lamina spiralis, commences at the fenestra rotunda, where in the recent 

 state it is separated from the tympanum by the secondary membrane of the tympa- 

 num. Close to its commencement is the orifice of a small canal (aqueductus cochlece), 

 which extends downwards and inwards to the lower border of the petrous bone, 

 where it opens into a depression immediately in front of the jugular fossa, It 

 transmits a small vein which joins the inferior petrosal sinus. There is also a 

 communication along the aqueductus cochleae between the subarachnoid space and 

 the perilymph in the scala tympani. The scala vestibuU (figs. Ill, 113, 4) is rather 

 narrower than the scala tympani in the first turn of the cochlea, but in the 

 succeeding turns is larger ; it commences from the cavity of the vestibule, and com- 

 municates, as already described, with the scala tympani at the apex of the modiolus. 



THE MEMBRANOUS LABYRINTH. 



Within the osseous labyrinth, and separated in most parts from its lining mem- 

 brane by the perilymph, membranous structures exist in which the ultimate ramifi- 

 cations of the auditory nerve are spread. In the vestibule and semicircular canals 



Fig. 115. PLAN OF THE RIGHT MEMBRANOUS LABYRINTH 



VIEWED FROM THE MESIAL ASPECT. (E. A. S.) 



u, utricle, with its macula and the three semicircular 

 canals with their ampullae ; s, saccule ; s.e. saccus endo- 

 lymphaticus ; c.r. canalis reuniens ; c.c. canal of the cochlea. 



these structures have a general resemblance in 

 form to the complicated cavity in which they 

 are contained. They do not, however, lie loose 

 within the osseous cavity, but along the convex 

 border of the canals, and at the places of 

 entrance of the nerves into the vestibule and 



ampullae are fixed to its wall. In the cochlea the membranous structures complete 

 the septum between the scalas already mentioned, and enclose an intermediate 

 passage, the membranous canal of the cochlea. As before stated, the liquid contained 

 within the membranous labyrinth is distinguished as endolymph. 



The cavity which, contains the perilymph communicates through the sheath, of the auditory 

 nerve with both the subdural and subarachnoid spaces. 



Within the osseous vestibule are two membranous sacs, the one of which, termed 

 the utricle, is connected with the semicircular canals, whilst the other, the saccule, 

 is connected with the cochlea. These two sacs although in close contact do not 

 open directly into one another although they are in indirect communication, in a 

 manner presently to be explained. 



The larger of the two sacs, the common sinus or utricle (fig. 115, u ; fig. 116), 

 is of a very irregular oblong form, measuring in all G mm. to 7 mm. in length, and 

 averaging 5 mm. in breadth, slightly flattened from behind forwards. It is lodged 

 in the upper and back part of the vestibule, occupying the fovea hemi-elliptica and 

 the space immediately below this. The part which lies in the fovea is termed the 

 recessus utriculi (fig. 116, rec. utr.). This forms a distinct blind forward projection, 

 some 3 mm. in length, into which, opposite the crista vestibuli, several small 

 branches of the auditory nerve enter from the foramina in the bone ; and here the 



