662 THE SENSES. 



blind extremity ; but at its base it communicates, by a narrow channel, 

 with the cavity of tlie sacculus. It is consequently an extension of the 

 sacculus, and a part of the membranous labyrinth ; while the scala ves- 

 tibuli is only an extension of the general cavity of the vestibule. The 

 ductus cochlearis may be considered as a tubular prolongation of the 

 sacculus, rolled upon itself in a spiral form, and maintained in position 

 by the bony and membranous partitions of the cochlea by which it is 

 enveloped. Like the rest of the membranous labyrinth, it is filled with 

 a watery fluid, and is bathed externally on both sides by the perilymph, 

 except where it is adherent to the wails of its bony cavity. 



Organ of Corti. The inner surface of the ductus cochlearis is lined 

 for the most part with a thin layer of pavement epithelium, except along 

 a longitudinal line situated at about the middle of the membrana basi- 

 laris. Here there is a continuous elevated ridge, four or five times 

 thicker than the epithelium elsewhere, following a spiral course, like 

 the rest of the cochlear structures, and consisting of enlarged and modi- 

 fied epithelium cells, with the terminal fibres of the auditory nerve. 

 This body is termed the organ of Corti, from the name of the observer 

 who first described it in 185 1. 1 It is justly considered as the most 

 remarkable and complicated structure in the internal ear, although in 

 its essential features it is analogous to the auditory spots in the sac- 

 culus and utricle. 



Fig. 213. 



DIAGRAMMATIC SECTION OP THE ORGAN OF CORTI, in profile; from the descrip- 

 tions of various authorities. 1. Membrana basilaris. 2,3. Internal and external fibres of the 

 arch. 4. Epithelium cells r.ear its inner and outer borders. 6, 5, 5, 5 Hair cells lying in con- 

 tact with the arch. Magnified 500 diameters. 



The organ of Corti rests upon the upper surface of the membrana 

 basilaris. Its framework consists of a series of elongated, rafter-like 

 bodies, arranged in two rows, internal and external. These bodies, 

 the internal and external "fibres of Corti," are separated from each other 

 at their base, where they rest upon the membrana basilaris, by a consid- 

 erable interval ; but they lean toward each other and lie in contact by 

 their upper extremities or heads, thus forming a roof-like or arched con- 

 nection, the " arch of Corti." Near the situation of the arch of Corti, the 

 epithelium cells lining the ductus cochlearis become modified in form, 

 gradually increasing in size and length. At the inner border of the 

 arch there is a single row of epithelium cells which are nearly as long 



Zeitschrift fur Wissenschaftliche Zoologie. Leipzig, 1851, Band III. p. 109. 



