824 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



cesses of the rods of Corti. The physical contact of the cells of Corti with 

 those of Deiters is so intimate if, indeed, their substance is not continuous 

 that impulses generated in the one can probably easily be communicated to 

 the other. 



The upper wall of the canalis cochlearis is made of a sheet of homogenous, 

 fibrillated connective tissue covered with flat cells, and stretches from the 

 limbus of the spiral lamina outward and upward to the side wall of the 

 cochlea. It is known as the membrane of Reissner. The periosteal con- 

 nective tissue of the bony wall of the cochlea is generally well developed 

 within the area enclosed between the membrane of Reissner and the membrana 

 basilaris ; it is particularly thick at the line of division between the scala media 

 and the scala tympani, where it forms a projecting ridge at the outer attach- 

 ment of the basilar membrane. This ridge is the spiral ligament ; an exten- 

 sion from it, gradually decreasing in thickness, reaches into both the vestibular 

 and the tympanic scala. 



A thick layer of both columnar and cuboidal epithelium lines the con- 

 nective tissue forming the outer wall of the canalis cochlearis. This epithe- 

 lium is peculiar in that the blood-vessels of the underlying connective tissue 

 penetrate between the epithelial cells themselves. The tectwial membrane 

 (membrana tedoria) is a sheet of radially-fibri Hated tissue, thin at its point of 

 attachment to the vestibular lip of the limbus, and becoming thicker and then 

 thinner again as it stretches out over the organ of Corti, reaching as far as the 

 most external row of hair-cells. It is said to lie in actual contact with the 

 rods of Corti and the free ends of the hair-cells, and it has been presumed to 

 serve as a damper for the vibrations imparted to the organ of Corti. 



Theory of Auditory Sensation. It may now be mentioned that the 

 generally-accepted theory of auditory sensation, as concerned with impulses 

 generated in the cochlea, supposes that the vibrations of the perilymph, the 

 endolymph, or of both are imparted to the basilar membrane. This membrane, 

 from its fibrillated structure, may perhaps rightly be regarded as a sheet of 

 parallel wires like those of a piano-board. As the wires of a piano have dif- 

 ferent rates of vibration according to their length, and respond sympathetically 

 to correspondingly different notes sounded in their neighborhood, so it has been 

 supposed that different radial fibres of the basilar membrane are set into sym- 

 pathetic vibration by different rates of vibration in the fluids bathing them. 

 These vibrations must be imparted to the structures in the organ of Corti, and 

 the irritation of the nerves connected with the cells of Corti is a natural 

 sequel. It may be repeated that, though the canal of the bony cochlea as a 

 whole diminishes in diameter from base to cupola, the canal of the mem- 

 branous cochlea, the scala media with its lower wall or basilar membrane, 

 increases in diameter. Thus the radial fibres of the basilar membrane are 

 longest near the apex of the cochlea. The radial width of the basilar mem- 

 brane, measured near the bottom, middle, and top, respectively, is given as 

 .21 millimeters, .34 millimeters, and .36 millimeters. The number of fibres 

 of the basilar membrane is said to be 24,000; the number of inner hair- 



