CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 301 



opens into the vestibule by a distinct aperture, after dilating into an 

 ampulla clooe to the fenestra ovaiis (/ 3 ). Its other end joins, as 

 before said, with the adjacent part of the posterior vertical semi- 

 circular canal the two opening into the vestibule by a common 

 aperture. 



Within this osseous labyrinth is the true essential part of the 

 organ of hearing, namely, the MEMBRANOUS LABYRINTH (Fig. 137), 

 which is a very complex, closed sac, corresponding generally in figure 

 with the osseous labyrinth within which it floats. It floats because 

 the osseous labyrinth encloses a fluid called the perilymph, which 

 fluid surrounds the membranous labyrinth, which itself encloses 

 another fluid called the endolymph. Both these fluids are slightly 

 albuminous, and the former, the perilymph, is secreted by a delicate 

 membrane of connective tissue, with a layer of epithelium, which 

 lines the osseous labyrinth and has no communication with the 

 lining of the tympanum (being cut oif from that cavity by the 

 membranes which close the fenestra)) though more or less continued 

 into the aqueducts. 



The membranous labyrinth consists, like the osseous labyrinth, of 

 three divisions ; 1, that of the vestibule ; 2, that of the cochlea ; 

 and 3, that of the semicircular canals. 



That part of the membranous labyrinth which is enclosed in the 

 vestibule, consists of two sacs connected by a narrow bent tube 

 (which extends into the aqueductus vestibuli) and containing within 

 them small crystals of carbonate of lime, called otoliths or otoconia. 

 The more posterior of these sacs is called the UTRICLE, and it is into 

 this that the membranous semicircular canals open by four apertures, 

 corresponding with those of the osseous semicircular canals. The 

 other sac is termed the SACCULE, and a delicate tube proceeding 

 from it extends to and connects it with that part of the membranous 

 labyrinth which extends into the cochlea. 



The lamina spiralis of the cochlea has its free edge connected with 

 the opposite wall of the spiral tube by a membrane which completes 

 the separation of the two scalae except at their summit, where they 

 communicate by a small opening called the helicotrema. The mem- 

 brane which thus completes the partition between the scala3 is 

 called the basilar membrane. Another delicate membrane, called the 

 membrane of Reissner, proceeds obliquely upwards from the lamina 

 spiralis to the outer wall of the tube diverging in its course from 

 the basilar membrane, and so cutting off a triangular canal from 

 the region above the lamina spiralis. This triangular canal is 

 called the canalis membranacea (or scala media), and it is connected 

 by a minute tube (the canalis reuniens), with the saccule of which it 

 is a continuation forming as it does the second or COCHLEAR PART of 

 the membranous labyrinth, and being filled with endolymph. Thus 

 the canalis membranacea ascends the cochlear spiral between the 

 two scala3 the scala tympani ascending from the fenestra rotunda 

 to the apex, and there communicating with the descending scala 

 vestibuli, which ends in the vestibule. 



