ABSTRACTS

OF

THE PAPERS PRINTED IN

THE PHILOSOPHICAL TRANSACTIONS.

The Croonian Lecture. On the Structure and Uses of the Membrana Tympani of the Ear. By Everard Home, Esq. F.R.S. Read Nov. 7, 1799. [Phil. Trans. 1800, p. 1.]

THE part of the ear which was the subject of the present lecture, has always been considered as a common membrane, which, being stretched or relaxed by means of muscles belonging to the malleus, became fitted in its various degrees of tension to convey the immense variety of external sounds to the internal organ.

Though this description have been generally adopted, yet it will appear upon further inquiry that, owing no doubt in a great measure to the extreme minuteness of the part, the structure and some of the properties of this membrane had not as yet been properly investigated. And the discovery here announced is brought forward with the greater propriety on this occasion, as it affords a new instance of the application of muscular action, which may ultimately account for certain phænomena in the sense of hearing in a more satisfactory manner than has hitherto been proposed.

This discovery we owe in some measure to the opportunity Mr. Home had to dissect the ear of an elephant, where the parts are so much larger even than they ought to be in proportion to the size of the animal, that the structure of this membrane, which is usually denominated the Drum of the Ear, becomes obvious even to the naked eye. On close examination it was found, that instead of being an uniform coat or skin, a great number of muscular fibres, which seem incorporated in it, pass along its texture in a radiated manner from the rim which surrounds it, towards the handle of the malleus, with which the central part of the membrane is firmly connected.

Having thus been put upon the scent, the membrane in the human ear was carefully separated from its contiguous parts, and being viewed in a microscope, magnifying 23 times, exhibited the same

B

texture, the muscular fibres appearing similar throughout the whole surface; without any central tendons as in the diaphragm, and chiefly forming the internal layer, on which side they appeared most conspicuous.

The blood-vessels of this membrane, the number of which is proportionate to its action, resemble in a great measure those of the iris, and are nearly as numerous. They anastomose with one another in a similar manner, and their general dissection is from the circumference of the membrane to the handle of the malleus. From near this handle a small trunk sends off branches in a radiated manner, which likewise anastomose with those that have an opposite course. This correspondence in the number and distribution of blood-vessels, between the membrana tympani and the iris, is given as a proof of that membrane being endowed with muscular action; and indeed the author henceforth speaks without hesitation of a radiated muscle distinct from the membrane. Having examined the structure of this muscle in various animals, Mr. Home found the application of a principle laid down in a former lecture, namely, that while the organization necessary for muscular contraction may in some measure exist in an apparent membrane, yet wherever muscular action is to overcome a resistance, a fasciculated structure becomes absolutely necessary. Thus in birds, where from the smallness of the organ the resistance is very trifling, this membrane is of a simple structure, like the coat of the hydatid; whereas in the elephant the fibres forming fasciculi are very distinct and obvious.

This muscular structure in the membrana tympani serves to explain various phænomena in hearing, which were not hitherto clearly understood. It is observed that the sounds produced by percussion on an extended membrane like the drum and tambourine, cannot but be imperfect; since, the tension being from side to side, the instrument may be considered as consisting of a number of parallel strings varying in their lengths like the parallel chords of a circle, and bearing consequently no musical proportion to each other. Such also must have been the imperfection of the membrana tympani, according to the received opinion of its structure: but after the present discovery it will appear obvious that the radiated muscle proceeding from the circumference of the circle to a common centre, produces a system of strings all equal in length, and uniformly varying in musical effect, according to their tension.

In order to illustrate the manner in which this radiated muscle adapts the membrana tympani to different sounds, the author finds it necessary to enumerate the more important parts of the organ, and to point out the use commonly assigned to each of them.

Without entering into this detail, we can here only observe that, availing himself of the present discovery, he compares the tensor and radiated muscles of the membrana tympani to a monochord, of which the membrane is the string, the tensor muscle the screw giving the necessary tension to make the string perform its proper scale of vibrations, and the radiated muscle acting upon the membrane like the

moveable bridge of the monochord, adjusting it to the vibrations required to be produced. Thus the combined effects of the action of these muscles give the perceptions of grave and acute tones; and in proportion as their original conformation is more or less perfect, so will their action be, and consequently the perceptions of sound which they communicate.

This mode by which the membrana tympani is capable of being adjusted to certain tones, or rather musical keys, will it is thought fully account for the difference between a musical ear, and one which is too imperfect to discriminate different notes with any degree of nicety. This delicacy of the ear, as it is found to depend on muscular action, may therefore be in some measure acquired, and is likewise liable to be impaired by illness or other accidental causes, of which some striking instances are here related.

In endeavouring to explain the uses of the more internal parts of the ear, considerable advantage, it is thought, may be derived from classing them in two divisions, namely, those which are formed for the purpose of receiving impressions conveyed through the medium of liquids or of solid substances; and those adapted to receive impressions made by the impulses of an elastic fluid such as common air. The former are the ears of fish, which are found to have fewer parts than those of birds, quadrupeds and man; but in the latter we find that the organ is susceptible of impressions by both vehicles. Thus men can hear the ticking of a watch by applying it to the forehead, and shutting the ears: the sound in this instance being evidently conducted through the bones of the skull, it appears manifest that only the interior parts of the ear, namely, the vestibulum and semicircular canals, co-operate to produce this sensation; and these in fact are the principal parts of that organ in fish.

In birds the membrana tympani has no tensor muscle to vary its adjustment, and hence their scale of sounds cannot descend so low as in the human ear. The cochlea, which has hitherto been considered as the part of the organ by which sounds are modulated, is also wanting in birds, which, however, are known to have a singular nicety in discriminating inarticulate sounds; and hence, as well as on account of its being filled with water instead of air, which renders it less capable of modifying sounds, it is manifest that this is not the real use for which the cochlea is destined. What is its precise use, as well as of the semicircular canals, remains yet to be investigated.

Lastly, it is observed that in the elephant there is no bony septum separating the cells of the skull belonging to one ear from those which open into the other, but a free communication exists between them: from this, the enlarged proportions of the organ and some other circumstances here mentioned, it is inferred that the sense of hearing must be quicker in this than in any other animal. And in fact some curious instances are mentioned which seem fully to confirm this assertion.