HEARING. 



567 



the two stations ought to be conjugate foci of 

 the reflecting surface, i. e. such that if the 

 reflecting surface were polished, rays of light 

 diverging from one would be made after re- 

 flexion to converge to the other. Hence, if a 

 vault be in the form of a hollow ellipsoid of 

 revolution, and a speaker be placed in one 

 focus, his words will be heard by an auditor in 

 the other, as if his ears were close to the other's 

 lips. The same will hold good if the vault be 

 composed of two segments of paraboloids having 

 a common axis, and their concavities turned 

 towards each other; only in this case sounds 

 excited in the focus of one segment will be 

 collected in the focus of the other after two 

 reflexions. 



The most favourable circumstances for the 

 production of a distinct echo from plane sur- 

 faces is when the auditor is placed between two 

 such exactly half-way. In this situation the 

 sounds reverberated from both will reach him 

 at the same instant, and reinforce each other : 

 if nearer to one surface than the other, the one 

 will reach him sooner than the other, and the 

 echo will be double and confused.* 



We propose to enquire the part which each 

 portion of the complex auditory apparatus of 

 man performs in the function of hearing. 



I. Of the internal ear. The fact that a 

 part, answering precisely to the vestibule, is to 

 be met with in every class of animals in whom 

 an auditory apparatus can be detected, affords 

 a strong presumption that this portion of the 

 labyrinth is the essential part of the organ. 

 Here is the seat of the principal expansion of 

 the auditory nerve upon the saccule and 

 common sinus, which floating in the perilymph 

 communicate, through the medium of that fluid, 

 with the membrane of the fenestra ovalis, and 

 consequently with the air contained in the tym- 

 panum. Any vibrations or oscillations then 

 excited in the membrane of the fenestra ovalis, 

 cannot fail to affect the perilymph to a propor- 

 tional extent, and through it the membranous 

 vestibule. In the simple ear of Crustaceans 

 as well as that of Cephalopods and the lowest 

 Cyclostomous fishes, the sonorous impressions 

 are conveyed directly to the vestibular cavity 

 through the solid material in which that cavity 

 is formed, or, as in some Crustaceans, through 

 the vibration of an external membrane. 



In the higher organized fishes, too, the 

 labyrinth constitutes the whole of the auditory 

 apparatus, nor has it any kind of opening to or 

 communication with the external air, being 

 lodged in the walls or cavity of the cranium, 

 the sonorous impressions must be conveyed 

 through the solid cranial parietes; for, in truth, 

 there is no other mode in which they can be 

 conveyed, and we know that solids are even 

 better conductors of sound than either liquids 

 or aerial fluids.f 



As to the function performed by the otolithes, 



* Herschel, Encycl. Metrop. 



t Hunter, Monro, Weber, and Treviranus, how- 

 ever, describe a communication with the exterior in 

 Rays and the Shark by two long canals ; but Scarpa, 

 Bell, and Blainville positively deny that these 

 ducts perform the office of auditory canals. 



or the calcareous dust, otokonics, which are 

 found in the sacculus vestibuli of the ears of 

 Cephalopods and Fishes, no satisfactory theory 

 has as yet been offered by any physiologist. 

 Although it is now admitted that similar cal- 

 careous particles exist in the vestibules of all 

 vertebrated animals, still they are only in a 

 rudimental condition when compared with 

 those of fishes ; indeed it seems not unreason- 

 able to suppose that the calcareous dust or 

 otokonie of cartilaginous fishes (the ray ot 

 shark for example) is rudimental of the hard, 

 porcellaneous, and artfully formed otolithe of 

 the osseous fishes.* A sort of loose notion 

 seems to prevail, that the presence of this hard 

 body in the vestibule favours the communication 

 of sound, by impinging upon the expansion of 

 the auditory nerve. The following obser- 

 vations of Camper no doubt propagated this 

 idea, if they did not originally give rise to it : 

 " Pour etre convaincu," says this distinguished 

 physiologist, " qu'un corps plus ou moins 

 dur, mais flottant dans une substance gela- 

 tineuse recoit la plus legere commotion ou 

 mouvement extdrieur, on n'a qu'a remplir un 

 verre de gelee de corne de cerf, et y plonger 

 quelque corps, on sentira aux doigts le mouve- 

 ment de ce corps des qu'on remuera le verre, 

 ou qu'on lui donnera un petit choc avec un 

 doigt de 1'autre main. Quand on enferme dans 

 une petite vessie quelque corps dur, le moindre 

 mouvement de la vessie fait branler ce corps, 

 qui produit une sensation tres forte sur le doigt 

 qui tieut la vessie. "f 



Sir A. Carlisle thinks that the nature of this 

 substance has reference to the habits of the 

 particular class of fishes in which it exists. 

 " Fishes," he says, " are only provided with 

 more simple organs of hearing, ordained to 

 inform them of collisions among rocks and 

 stones, or the rushing of water or moving 

 bodies in that element : and since the collisions 

 of stones or of water are only variable in their 

 magnitude or intensity, fishes are provided 

 with these dense ossicles to repeat the sem- 

 blable acute tones of similarly dense substances, 

 such as rocks, stones, gravel, &c." Again, 

 " There is an especial sac of calcareous pulp 

 given to skates and some other cartilaginous 

 fishes in the place of the dense ossicles, ap- 

 parently intended to respond to the movements 

 of sand and muddy strata on which they are 

 doomed to reside ; and it is remarkable that the 

 sturgeon has its auditory ossicles consisting 

 partly of hard substance and partly of calca- 

 reous pulp." | 



Weber believes that the otolithes in fishes 

 supply the place of the cochlea which is want- 

 ing in these animals: the auditory nerves being 

 connected with them receive the vibrations 



* So definite does the form of these otolithes 

 appear to be in osseous fishes, that Cuvier says 

 the osseous fishea may be determined by their 

 otolithes as well as by any other character. 



t Mem. de 1'Acad. des Sciences, an. 1779, and 

 quoted in Scarpa De auditu et olfactu, p. 23. 



$ Quoted from a Mss. Essay on sound, in the 

 Huntcrian Catalogue, vol. iii. p. 193. Miiller calls 

 the otolithe " Eine freier solidcr Schwingungen 

 repercutirender Kbrper." 



