THE MEMBRANOUS LABYRINTH. 757 



The scala tympani, 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 metnbrana tympani. Near 

 its commencement is the orifice of a small canal aqueductus cochleae,, which 

 extends downwards and inwards through the snbstance of the petrous part 

 of the temporal bone to near the jugular fossa, and transmits a small vein . 

 The surface of the spiral lamina which looks towards this scala is marked 

 with numerous transverse striae. The scala vestibuli is rather narrower than 

 the scala tympaui in the first turn of the cochlea ; it commences from the 

 cavity of the vestibule, and communicates, as already described, with the 

 scala tympani at the apex of the modiolus. 



TJie lining membrane of the osseous labyrinth. This is a thin membrane 

 (periosteum ?), which closely adheres to the whole inner surface of the 

 several parts of the labyrinthic cavity just described. It has no con- 

 tinuity with the lining membrane of the tympanum, being stretched 

 across the openings of the round and oval fenestrse. It is composed of 

 fibres of connective tissue. Its outer surface is rough, and adheres closely, 

 like periosteum, to the bone : the inner surface is pale and smooth, is 

 covered with a single layer of epithelium, like that of the arachnoid, and 

 secretes a thin, slightly albuminous or serous fluid. This secretion, known 

 as the liquor Cotunnii, or perilymph, separates the membranous from the 

 osseous labyrinth in the vestibule and semicircular canals, occupies the 

 cavities of the scala tympani and scala vestibuli in the cochlea, and is 

 continued into the aqueducts as far as the membrane lining these passages 

 remains pervious. 



THE MEMBRANOUS LABYRINTH. 



Within the osseous labyrinth, and separated from its lining membrane by 

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

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

 canals these structures have a general resemblance in form to the com- 

 plicated cavity in which they are contained. In the cochlea they complete 

 the septum between the scalse already mentioned, and enclose a third spiral 

 passage, the canalis membranacea, the existence of which has only been 

 discovered of late years. The liquid contained within the membranous 

 labyrinth is distinguished as endolymph. 



VESTIBULE. The membranous vestibule consists of two closely connected 

 sacs, and the parts by which they are united to the membranous semi- 

 circular canals and canal of the cochlea. 



The larger of the two sacs, the common sinus or utricle, is of an oblong 

 form and slightly flattened from without inwards. It is lodged in tho 

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

 elliptica. Opposite the crista vestibuli several small branches of the audi- 

 tory nerve enter from the foramina in the bone ; and here the walls of the 

 common sinus are thicker and more opaque than elsewhere. The extre- 

 mities of the membranous semicircular canals terminate in the cavity of 

 the common sinus. A small mass of calcareous particles, otoliths or oto* 

 conia, is lodged in the wall of the sac. These otoliths are crystals of 

 carbonate of lime, and are described as six-sided, and pointed at their extre- 

 mities. They are connected with the wall of the sac in a way not yet 

 clearly determined. 



The smaller vestibular vesicle, the saccule, is more nearly spherical than 

 the common sinus, but, like it, is somewhat flattened. The saccule is situated 



