THE SENSE OF HEARING. 165 



The description in its several parts of the auditory nerve, which distributes 

 filaments in the cavity of the labyrinth for the reception of the advancin"" 

 waves of sound, is rather a precarious task, which we may facilitate however, 

 without any real loss to the reader, by forbearing to enter into those subth^ de- 

 tails which, if a novice, he would scarcely understand. Nor ^vill this bf> wholly 

 a sin of omission, for, truth to say, the wonderful apparatus here displayed to 

 ns by the microscope is, in its physiological signification, not much less than 

 altogether unknown to the physiologists themselves. It is only to-day, so to 

 speak, that the corypha?us of our science has succeeded in detecting the true ana- 

 tomical constitution of the extremities of the nerves of hearing in the cochlea 

 and vestibule, so difficult is it to submit uninjured to observation these infinit-dy 

 delicate structures, hidden deeply in the skull and shut up within osseous Avails, 

 and to grope one's .way in the astonishing complexity of the elementary tissues 

 which the microscope reveals in them. The auditory nerve, running back from 

 its origin at the base of the brain, issues, after a short course through several 

 openings in the petrous bone, into the labyrinth, and here divides into two prin- 

 cipal branches destined for different parts. Each penetrates by a separate aper- 

 ture, the one into the vestibule, the other into the cochlea. The former again 

 divides into numerous filaments which direct their course partly to the two mem- 

 branous sacs of the vestibule, pai'tly to the membranous ampulla situated at the 

 beginning of the semicircular canals, where their extremities are distributed 

 eomething after the manner of a delicate brush. These minute fibres, each of 

 which forms an isolated conductor of the nervous current, terminate finally in 

 delicate bulbs or nerve cells. It is a remarkable peculiarity that at the terminal 

 expansion of the nerves of hearing a finely grained calcareous substance is in- 

 terspersed through the delicate fibrous tissue of the vestibular sacs, consisting, 

 as is clearly apparent under the microscope, of minute but distinctly forni'd 

 crystals. Chemical investigation has shown that this otolith or " sand of the 

 ear," consists of a compound widely dispersed through animate and inanimate 

 nature, the same v/hich, under the name of chalk or marble, forms mountains and 

 rocks, and which, as shell, protects from outward injury the embryo of the 

 feathered tribe ; in a word, of carbonate of lime. It may be mentioned by the 

 way that this calcareous matter is not peculiar to man, but occurs also in the 

 organ of most animals, partly as a finely crystallized mass, partly in larger or 

 emaller crystals ; its significancy as respects the hearing has as yet resisted all 

 efforts of conjecture and experiment. The second principal branch of the audi- 

 tory nerve, the cochlear, passes to the base of the cochlea and ascends within 

 its axis to the apex, becoming more and more attenuated as it continues to give 

 out filaments which proceed from it at right angles between the plates of the 

 bony lamina, to their outer border. Corresponding to the spiral winding of this 

 septum or partition wall, the filamentary offshoots from the trunk form also a 

 spiral line. At the edge of the bony portion of the lamina these filaments issue 

 upon the membranous portion and pursue their course at first along its under 

 surfiice and consequently within the scala tympani, but presently pass through 

 different small foramina into the scala vestibuli and now run transversely along 

 the upper surfiice of the membranous lamina towards the outer Avail of the cochlea, 

 in the neighborhood of Avliich they terminate. That part of these filaments 

 which traverses the upper surface of the membranous lamina in the scala vesti- 

 buli, is, however, no longer the "simple nerve fibre Avhich Ave observed in the trunk 

 and in its course betAveen the plates of the bony portion of the lamina ; it has 

 become a singularly complex structure Avith cells, partly inserted m its progress, 

 partly as appendages of the stem. It would require no little detail to give an ex- 

 act account of the microscopic constitution of these nerve extremities, nor would 

 our inquiry even then be much advanced, since physiology is still greatly in the 

 dark as regards this delicate mechanism. The reader Avill perhaps oblaiu the 

 best idea thereof if he imagines himself ascending, suppose in th" ^cala vesti- 



