STRUCTURE OF THE LABYRINTH. 



8 97 



STRUCTURE OF THE LABYRINTH AND THE TERMINATIONS 

 OF THE AUDITORY NERVE. 



The vestibule of the labyrinth (Fig. 325, III) possesses two separate sacs: 

 one, the saccule (sacculus, or S. hemisphcericus, S) communicates with the cochlear 

 duct (Cc) of the cochlea; the other, the utricle (utriculus or sacculus hemiellipticus, 

 U) communicates with the semicircular canals (Cs, Cs) . The interior of the cochlea, 

 which consists of 2^ spiral turns, is divided into two compartments by a horizontal 

 septum (lamina spiralis ossea et membranacea) , which is bony internally and 

 membranous externally (Fig. 325, I). The lower compartment is the scala 

 tympani ; it is separated from the tympanic cavity by the membrane of the fenestra 

 rotunda. The upper compartment is the scala vestibuli, which leads into the 

 vestibule of the labyrinth (Fig. 314). These two passages are in direct com- 

 munication with each other through a small opening (helicotrema) at the apex 

 of the cochlea. A smaller space is separated from the upper passage by the 

 obliquely placed membrane of Reissner (Fig. 325, I), which bridges over the lower 

 outer angle. This space (ductus or canalis cochlearis, Cc) is bounded below 

 chiefly by the lamina spiralis membranacea, on which the organ of Corti, the 

 end-organ of the cochlear nerve, is placed. The lower end of the cochlear canal 

 (III) is blind and faces the saccule, with which it is united by the fine canalis 

 reuniens (Cr). The three semicircular canals (Cs, Cs) communicate with the 



FIG. 325. I, Cross-section of the cochlea; II, A, ampulla with the crista acustica; a p, a hair-cell and its bristle; 

 T, otoliths; III, diagrammatic representation of the human labyrinth; IV, diagrammatic representation of 

 the labyrinth of a bird; V, diagrammatic representation of the labyrinth of a fish. 



utricle (Fig. 325, III, U) . Each begins in an ampulla, within which lie the termina- 

 tions of the ampullar nerves; while at the other extremity they have only two 

 openings, as the posterior and superior canals unite so as to form one common 

 canal. A membranous lining continues from the utricle through the semicircular 

 canals. The limpid perilymph, which is present also in both cochlear passages, 

 and the viscid endolymph fill the entire cavity of the labyrinth. All of these 

 compartments are lined by low, cylindrical epithelium. 



Only those portions of the system of cavities that are filled with endolymph 

 contain the nervous end-organs. The cochlea, the semicircular canals, and the 

 ampullae belong to the organs of hearing. After extirpation of the cochlea 

 on each side there is still a distinct reaction to coarse sounds. The cavities of the 

 labyrinth are all in communication with each other, the semicircular canals directly 

 with the utricle, the cochlear duct with the saccule through the canalis reuniens. 

 Finally, the saccule and the utricle communicate through the endolymphatic duct, 

 which arises as an isolated branch from each sac. The canal thus formed passes 

 through the osseous aqueduct of the vestibule, to end beneath the dura in an 

 endolymphatic sac on the posterior aspect of the petrous portion of the temporal 



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