*So8 



STRUCTURE OF THE COCHLEA. 



413 LABYRINTH AND AUDITORY NERVE. Scheme. The vestibule (fig. 591, III) 

 contain two separate sacs; one of them, the saccule s (round sac or S. henusDhsricus), com- 

 municates with the ductus cochlearis, Ce, of the cochlea; the other, the utricle, U (elliptical 

 sac, or sacculus hemiellipticus), communicates with the semicircular canals, U, U. 

 ' The cochlea consists of 2* turns of a tube disposed round a central column or modiolus. The 

 tube is divided into two compartments by a horizontal septum, partly osseous and partly 

 membranous, the lamina spiralis ossea and membranacea (hg. o9o ; fig. 591, I). The lower 

 compartment is the scala tympani, and is separated from the cavity of the tympanum by the 

 membrane of the fenestra rotunda. .-'<.., j -v *v *-u i < 



The upper compartment is the scala vestibuli, which communicates with the vestibule of 

 the labyrinth (fig 591, I). These two compartments communicate directly by a small opening 

 at the apex of the cochlea, a sickle-shaped edge ["hamulus"] of the lamina spiralis bounding 

 the helicotrema (fig. 577). The scala vestibuli is divided by Reissner's membrane (fig. 591, I), 



Fig. 591. 

 I, transverse section of a turn of the cochlea ; II, A, ampulla of a semicircular canal with the 

 crista acustiea ; a, p, auditory cells ; p, provided with a fine hair ; T, otoliths ; III, 

 scheme of the human labyrinth ; IV, scheme of a bird's labyrinth ; V, scheme of a fish's 

 labyrinth. 



which arises near the outer part of the lamina spiralis ossea, and runs obliquely outwards to 

 the wall of the cochlea so as to cut off a small triangular canal, the ductus or canalis cochlearis, 

 or scala media, O, whose floor is formed for the most part by the lamina spiralis membranacea, 

 and on which the end-organ of the auditory nerve Corti's organ is placed. The lower end 

 of the canalis cochlearis is blind, III, and divided towards the saccule, with which it com- 

 municates by means of the small canalis reunions, Cr (Hensen). The utricle (fig. 591, III, U) 

 communicates with the three semicircular canals, Cs, Cs each by means of an ampulla, 

 within which lie the terminations of the ampullary nerves, but as the posterior and the 

 superior canals unite, there is only one common ampulla for them. The membranous semi- 

 circular canals lie within the osseous canals, perilymph lying between the two. Perilymph also 

 fills the scala vestibuli and tympani, so that all the spaces within the labyrinth are filled by 

 Huid, while the spaces themselves are lined by short cylindrical epithelium. 



The system of spaces, filled by endolymph, is the only part containing the nervous end-organs 

 for hearing. All these spaces communicate with each other ; the semicircular canals directly 

 with the utricle, the ductus cochlearis with the saccule through the canalis reuniens ; and 

 lastly, the saccule and utricle through the "saccus endolymphaticus, " which springs by an 

 isolated limb from each sac; the limbs then unite, as in the letter Y> and puss through the 

 osseous aqueductus vestibuli to end blindly in the dura mater of the brain (fig. Ill, TLB6ttchcr, 

 Rctzius). The aqueductus cochlea? is another narrow passage, which begins in the scala tympani, 

 immediately in front of the fenestra rotunda, and opens close to the fossa jugularis. It forms 

 a direct means of communication between the perilymph of the cochlea and the subarachnoid 

 sj>ace. 



Semicircular Canals and Vestibular Sacs. The membranous semicircular canals do not fill 

 the corresjKmding osseous canals completely, but are separated from them by a pretty wide 

 space, which is filled with perilymph (fig. 592). At the concave margin they are fixed by con- 



