562 



ORGAN OF HEARING. 



Sometimes, however, the auditory passage has 

 been found regular though the auricles were 

 wanting. 



The auditory passage is sometimes found too 

 wide, sometimes too narrow, sometimes too 

 short. Closure of the auditory passage may 

 be either partial or through its whole extent. 

 It is more rarely the effect of disease than of 

 irregular primitive formation. Partial closure 

 may be by an extension of the skin over the 

 mouth of the passage. Authors mention cases 

 of a membraneous septum sometimes deep in 

 the auditory passage and before the membrana 

 tympani, sometimes nearer the entrance of the 

 passage. 



In monstrous foetuses all the accessory parts 

 of the apparatus of hearing together have been 

 found wanting. 



II. PARALLEL BETWEEN THE EAR AND 

 THE EYE. 



A parallel has often been drawn betwixt the 

 ear and the eye. Breschet, in his memoir, 

 already so often cited in the course of this 

 article, compares the perilymph to the aqueous 

 humour, the endolymph to the vitreous hu- 

 mour, and the calcareous concretions to the 

 crystalline body. 



The comparison which I should institute 

 between the component parts of the ear and 

 the eye is the following : 



The osseous labyrinth may be compared to 

 the sclerotica, and the fenestra rotunda, or coch- 

 lear fenestra, to the cornea. 



To find a part in the eye analogous to the 

 vestibular fenestra, we must first consider that 

 the latter is a yielding part of the otherwise 

 solid wall of the labyrinth ; that through the 

 medium of it, the chain of small bones and 

 their muscles in the tympanum exercise on the 

 soft parts contained in the labyrinthic cavity, 

 a certain degree of tension or compression 

 fitted probably to accommodate in some man- 

 ner the ear to the perception of different 

 degrees of sound. In the case of the eye, 

 the sclerotica, which corresponds to the osseous 

 labyrinth, is thinner and more yielding at the 

 middle of its circumference, (remarkably so in 

 the Greenland seal). From this it has been 

 supposed that the action of the muscles of the 

 eye-ball might by their compression produce 

 a change of shape fitted to accommodate the 

 eye to distances. Hence the vestibular fenestra 

 and middle thin part of the sclerotica might 

 be compared to each other in as far as regards 

 the function which each performs in the eco- 

 nomy of its own organ. However this may be, 

 the vestibular fenestra of the ear and the thin 

 part of the sclerotica correspond to each other 

 as far as can be in relative position ; and if we 

 admit the action just mentioned of the muscles 

 upon the eye-ball, we have, as I shall imme- 

 diately show, their counterparts in the muscles 

 of the small bones of the tympanum. 



The tympanic scala of the cochlea may be 

 compared to the anterior chamber of the 

 aqueous humour, and the vestibular scala to 

 the posterior chamber. 



The spiral lamina, considering its vascu- 

 larity and richness in nerves, and its forming 

 a partition between two chambers containing 

 an aqueous humour, may, as I have already 

 said in a former part of this article, be con- 

 sidered the counterpart of the iris, and the 

 helicotrema that of the pupil. 



The membrane lining the labyrinthic cavity 

 bears the same relation to the latter as the 

 arachnoidea oculi* does to the sclerotica. The 

 space filled with perilymph, between the osse- 

 ous and membraneous labyrinth, may be con- 

 sidered analogous to that between the sclerotica 

 and choroid. It however communicates with 

 the scalse of the cochlea, the parts analogous 

 to the chambers of the aqueous humour, be- 

 cause there is nothing in the ear to be com- 

 pared to the ciliary ligament. 



Forming the membraneous labyrinth we find, 



1. a delicate cellular tissue supporting the 

 branches of the bloodvessels, and which is 

 sometimes found containing black pigment ; 



2. a firm transparent membraneous coat, within 

 which, 3. is a nervous expansion ; 4. the endo- 

 lymph; 5. suspended in the latter the mass of 

 calcareous matter. The cellule-vascular layer 

 containing pigment, together with the rest of 

 the walls of the membraneous labyrinth, may 

 be compared to the choroid coat of the eye, 

 the nervous expansion to the retina, the endo- 

 lymph to the vitreous humour, and the calca- 

 reous mass to the lens. 



In the lower animals the cochlea is the first 

 part of the ear-bulb to disappear; in regard to 

 the eye-ball, the aqueous chambers to which 

 I have compared the scalae of the cochlea, are 

 in like manner the first parts which in the de- 

 preciation of the structure of the eye, in the 

 animal series, disappear, e. g. the eye of the 

 Cephalopodous Mollusca. 



Is the cochlear nerve the same in function 

 with the vestibular? The vestibular nerve is 

 the special nerve of hearing ; but does not the 

 cochlear nerve perform some function in the 

 economy of the ear analogous to what the 

 ciliary nerves perform in that of the eye ? 



If an example is required in which the optic 

 nervous filaments enter the eye separately as 

 do the nervous filaments of the ear-bulb, it is 

 to be found in the Cephalopodous Mol- 

 lusca.f 



As in front of the eyeball there is, or rather 

 would be, if it was not that the eyelids are 

 constantly in contact with the eyeball, a space 

 lined by a mucous membrane, the conjunctiva, 

 so at the peripheral surface of the ear-bulb, 

 there is a space, the tympanic cavity, lined by 

 a mucous membrane also. Moreover, as there 

 is a passage into the nose from the space 

 bounded by the conjunctiva, so does the tym- 



* See my figure and description of a horizontal 

 section of the human eye, in Mackenzie's Practical 

 Treatise on Diseases of the Eye. Second Edition. 

 London, 1835. 



t See a paper " On the Retina of the Cuttle- 

 fish," in the London and Edinburgh Philosophical 

 Magazine for January, 1836. 



