706 ORGANS OF SENSE. 



nerve-tubules are distributed. The osseous part of the spiral lamina extends 

 about half way across the diameter of the spiral canal ; it is called the osseous 

 zone. It commences in the vestibule between the tympanic and vestibular 

 opening of the cochlea, and, gradually becoming narrower in its course, termi- 

 nates in a projecting hook, the hamular process, just where the expansion of 

 the infundibulum commences. The lamina spiralis consists of two thin lamellae 

 of bone, between which are numerous canals for the passage of nervous fila- 

 ments, which open chiefly on the lower or tympanic surface. At the point 

 where the osseous lamina is attached to the modiolus, and following its wind- 

 ings, is a small canal, called by Rosenthal the canalis spiralis modioli. In the 

 recent state, the osseous zone is continued to the opposite wall of the canal by 

 a membranous and muscular layer (membranous zone), so as to form a complete 

 partition in the tube of the cochlea. Two passages, or scalae, are thus formed, 

 by the division of the canal of the cochlea into two. One, the scala tympani, 

 is closed below by the membrane of the fenestra rotunda; the other, the scala 

 vestibuli, communicates, by an oval aperture, with the vestibule. Near the ter- 

 mination of the scala vestibuli, close by the fenestra rotunda, is the orifice of 

 the aquaeductus cochlege. The scalae communicate, at the opening of the cochlea, 

 by an opening common to both, the helicotrema, which exists in consequence of 

 the deficiency of the lamina spiralis in the last half coil of the canal. 



In structure, the membranous zone is a transparent glassy lamina, presenting 

 near its centre a number of minute transverse lines, which radiate outwards, 

 and give it a fibrous appearance ; and at its circumference, where connected 

 with the outer wall of the spiral canal, it is composed of a semi-transparent 

 structure, which is described by Todd and Bowman as a muscle (the Cochlearis), 

 and by Kolliker as connective tissue. 



The vestibular surface of the osseous portion of the lamina spiralis is covered 

 for about the outer fifth of its surface with a thin layer, resembling cartilage in 

 texture. It is described as the denticulate lamina (Todd and Bowman), from its 

 presenting a series of wedge-shaped teeth which form its free margin, and which 

 project into the vestibular scala. 



The inner surface of the osseous labyrinth is lined by an exceedingly thin 

 fibro-serous membrane, analogous to a periosteum, from its close adhesion to 

 the inner surface of these cavities, and performing the office of a serous mem- 

 brane by its free surface. It lines the vestibule, and from this cavity is con- 

 tinued into the semicircular canals and the scala vestibuli of the cochlea, and 

 through the helicotrema into the scala tympani. Two delicate tubular pro- 

 cesses are prolonged along the aquaaducts of the vestibule and cochlea, to the 

 inner surface of the dura mater. This membrane is continued across the fenestra 

 ovalis and rotunda, and consequently has no communication with the lining 

 membrane of the tympanum. Its attached surface is rough and fibrous, and 

 closely adherent to the bone ; its free surface is smooth and pale, covered with 

 a layer of epithelium, and secretes a thin, limpid fluid, the aqua labyrinthi (peri- 

 lymph of Blainville, liquor Cotunnii.} In the vestibule and semicircular canals, 

 it separates the osseous from the membranous labyrinth ; but in the cochlea it 

 lines the two surfaces of the bony lamina spiralis ; and being continued from 

 its free margin across the canal to its outer wall, forms the lamina spiralis 

 xnem^ranacea, serving to complete the separation between the two scalae. 



The Membranous Labyrinth (Fig. 383) is a closed membranous sac, containing 

 fluid. The ramifications of the auditory nerve are distributed upon the wall of 

 the sac. It has the same general form as the vestibule and semicircular canals, 

 in which it is inclosed; but is considerably smaller, and separated from their 

 lining membrane by the perilymph. 



The vestibular portion consists of two sacs, the utricle and the saccule. 



The utricle is the larger of the two, of an oblong form, compressed laterally, 

 and occupies the upper and back part of the vestibule, lying in contact with the 



