348 THE SENSES 



the membrana tympani, the interesting structures within the middle 

 ear, the Eustachian tube leading from the latter to the pharynx, and the 

 endolymph of the inner ear. Here, as elsewhere, the reader is referred 

 to adequate descriptions of these mechanisms to be found in anatomical 

 works. (Plate IX.) 



THE ORGAN OF CORTI is the proper afferent end-organ of the audi- 

 tory nerve, all the rest of the ear being but a mechanical means of securing 

 the proper stimulation of this minute and obscurely intricate structure. 

 We shall note here only a few of the very numerous details of this organ 

 those which seem most important. Erected on the membrana basilaris 

 (which extends the length of the spiral of the cochlea) are two rows of 

 chitinous pillars called the rods of Corti. These are connected above, 

 while the tunnel of Corti runs between them below. The inner row is 

 made up of about 7000 of these pillars; the outer has about 4600 of them. 

 From the summit of this arch extends inward and outward a thin mem- 

 brane (also of chitinous material), the membrana reticularis, and through 

 minute holes in this net extend the numerous bristle-like filaments of 

 long cylindrical epithelial cells beneath. These cells recline more or 

 less on the outer sides of the pillars of Corti, and are called respectively 

 the inner and outer auditory cells or hair-cells. Each has several unique 

 structures within it whose significance is quite unknown. There are 

 about 18,000 of these cells; one row of the inner cells and three or four 

 rows of the outer. Between the hair-cells are scattered certain support- 

 ing-cells, named for Deiters. The fibrils of the auditory nerve pass 

 from the ganglion spirale within the lamina spiralis to the bases of the 

 hair-cells, and the filaments on the summits of these cells are apparently, 

 the nerve being afferent, the beginnings of the cochlear branch of the 

 auditory nerve. Above the filaments extends outward from the lamina 

 spiralis at the base of Reissner's membrane (separating the scala media 

 from the scala vestibuli above it) the soft membrana tectoria, striated 

 and cuticular, and apparently free in the endolymph of the scala media. 



The membrana basilaris, on which as it curves about in the spiral 

 of the cochlea Corti's organ is placed, is made up of transverse fibers, 

 about 24,000 in number. Were this membrane uncoiled and spread out 

 it would suggest in its structure and shape a xylophone. Contrary to 

 what one might expect, the longest fibers are at the apex of the cochlear 

 spiral and the shortest at its base. The extreme lengths of these fibers are 

 0.05 mm. long and 0.45 mm. It is hard not to suppose on the principle 

 of analogy that the respective lengths of these fibers of the membrana 

 basilaris do not correspond in some way and degree to the differences in 

 pitch of sounds, but this is by no means sure. 



How DOES THE ORGAN OF CORTI ACT? To this question many have 

 replied, but all differently. A notion of its mode of working perhaps 

 more common than others is that which follows, little altered in all the 

 busy years since the master of sonology, Helmholtz, suggested it : When, 

 started by a vibrating body, an air-wave strikes against the membrana 

 tympani, through the agency of the elaborate bony levers of the middle 



