EAR. 

 t The Middle Ear comprise* the cavity of the Tympanum, with 



iU onotonU ; the oelU in the bony prominence behind the ear, called 

 d Prooessj, with which the tympanum communicates; and 



the Mastoid 



the Eustachian Tube, or passsge leading from the tympanum into the 

 upper and back part of the throat, where it open* in the form of an 

 expanded alit on each tide behind the posterior nares. 



The Tympanum is an irregular cavity scooped in the petrous portion 

 of the temporal bone between the vestibule and the external meatus. 

 The principal entrances to it are the fenestra ovalis and the round or 

 somewhat oral opening at the bottom of the external passage upon 

 which the membrana tympana is stretched. Between these there is 

 extended a chain of three small bones, obliquely articulated to each 

 other with perfect joints, so placed that the chain somewhat resembles 

 in figure the letter Z. 



These bones are called respectively the Stapes (stirrup), the Incus, 

 (anvil), and Malleus (hammer), from some similarity in form to those 

 implements. The base of the stapes is applied to the fenestra ovalis, 

 exactly fitting it, and is attached firmly to its membrane. The 

 extremity of the longer leg of the incus is articulated to the head of 

 the stapes, and there is a minute bone between them of the size of a 

 small shot, which is generally considered to be only a process of the 

 incus. It is however called from its spherical shape the Os Orbiculare, 

 and is sometimes reckoned as a fourth bone. (Fig. 3, o.) The shorter 

 leg of the incus (fig. 2, c,) rests against the bony parietes of the 

 tympanum at the back part, near the mastoid cells. Upon the hol- 

 lowed cavity in the head of the incus (Jig. 2, a) the lateral depression 

 of the head of the malleus (Jig. 2, t) is articulated, and moves easily ; 

 the long handle of the latter is attached by its extremity (Jig. 2, It) 

 to the middle of the membrana tympani, as well as by a portion of 

 the side of the handle, which lies close to and parallel with the 

 membrane. The long slender process of the malleus called the Pro- 

 ceesus Oracilis (jiy. 2, g) lies in a slit passing to the articulation of the 

 jaw called the glenoid fissure. 



Fig. 2. Magnified view of the osslcnla auditus : M, malleus ; I, incus ; 

 8, stapes ; o, shape of the fcnestra ovalis ; a, cavity of the incut, which is 

 articulated to the malleus ; d, longer process of the incus with the on orbiculare 

 attached at A ; r, its shorter process ; r t bead of the malleus ; /, IU shoit pro. 

 esss, or prominent point for the attachment of the tensor tympani; *, the 

 Ji Illusion which articulates with the incus ; g, proceseus graellls of the 

 ; *, Its handle, or manubrium. 



Kg. 3. 



Fif. t. The same bones of their natural size : m, malleus ; i, incus ; 

 ..Upes; o, orblcolare. 



At first sight the use of these bones would appear to be to transmit 

 the vibrations of the membrana tympani to the membrane of the 

 frneotra ovalis, and thence to the internal ear, but when it is found 

 that sounds can be heard with distinctness even when the membrana 

 tympani and the ossicula have been removed by disease, it is evident 

 that this function can be performed independent of them. They have 

 evidently another use which would be incompatible with a single 

 bone passing between those membranes, as in birds and most reptiles; 

 this is to permit the membrana tympani to be drawn into a conical 

 shape so as to tighten it, and adapt it either to resist the impulse of 

 too loud a sound, or favour a more acute or gentle one. The muscle 

 which chiefly effects this object, called the tensor tympani (.'.</. 4, <0, 

 is attached near the head of the malleus to a point projecting from 

 it (Fig. 2,/.) Other muscles, to steady and antagonise iU action, 

 called the laxator major and minor tympani are also attached to the 

 malleus, the former (Jig. 4, 4) to the processus gracilu, the latter 

 (fg. 4, c) to the handle of the bone. A further description of the 

 directions and outer attachment* of these minute muscles wotiM 1- 

 tedious and unintelligible to the general reader. No muscle is 

 attached to the incus, but a small one of great importance is inserted 

 into the neck of the step**, called the stapideus; the effect of this i. 

 to counteract the obliquity of traction or tilting of the stapes, which 

 would otherwise ensue from the movements of the other bones ; by 

 this means the motion of the stapes is directed either immediately to 



EAR. 4M 



or from the fenestra ovalis, the membrane of which is also further 

 preserved from injury by the oblique arrangement of the joints of 

 these minute bones, by means of which, although the membrane of 

 the tympanum oscillates through a considerable space in passing from 

 tension to relaxation, that of the fenestra is moved to a much smaller 

 extent It is to be observed that the same action which draws the 

 membrana tympani into a cone thrusts the base of the stapes farther 

 into the fenestra ovalis. 



Fif. 4. 



Fif. 4. Muscles attached to the ouicula uuditu* : a, tensor tympani ; 

 t, laxator major ; e, lexator minor ; </, stapidcus. 



These small muscles are not under the dominion of the will, being 

 supplied with nerves in a way peculiarly interesting to a physiologist, 

 and acting automatically in correspondence with the i in press i. 

 the auditory nerve. Yet the instinctive consciousness we have of the 

 degree of their contractions in adjusting the tension of the membrana 

 tympani to circumstances, is probably one of our chief means of 

 estimating the intensity of sounds. 



The fenestra ovalis is situated nearly opposite the membrana 

 tympani, on the upper edge of a prominence called the Promontory ; 

 it faces outwards and a little downwards ; and beneath it, concealed 

 by the promontory, is the foramen rotundum, closed by a membrane, 

 and leading into the cochlea by the scala tympani. The object of this 

 last opening is disputed : some think it conveys in part the vilu 

 of the air of the tympanum to the internal ear ; but it seems more 

 reasonable to suppose, with Sir C. Bell, that the end it chiefly serves 

 is to give vent and freedom to those of the fluids pent up in the 

 unyielding bony canals of the labyrinth. Besides these openings 

 from the tympanum, there are others which 1. ad into the mastoid 

 cells behind it ; these are also filled with nir, and ni-e supposed t 

 tribute to the distinctness of the tympanic vibrations. There is also 

 an opening from the tympanum forwards into the Euatachian tube. 

 This canal is nearly two inches long ; the first part of its course from 

 the tympanum is bony : it then becomes cartilaginous, and widens as 

 it approaches the throat, the mucous membrane of which lines it, and 

 thence passing into the tympanum, spreads over the surface of the 

 whole cavity, investing the ossicula and its other contents, as well as 

 the mastoid cells. From this circumstance arises the tendency of 

 the inflammation of cold or sore throat to extend into the tympanum, 

 producing temporary deafness, ear-ache, and sometimes mischief of a 

 more permanent kind. From the deafness which accompanies the 

 closure of the Eustachian tube by that or other causes, the importance 

 of its functions in renewing and giving vent to the air within the 

 tympanum may be appreciated. Besides the foramina already men- 

 tioned, there are others through which nerves and vessels enter the 

 tympanum. We have not space to describe them : we shall only 

 mention that one of the nerves, called the chorda tympani, originally 

 connected with the portio dura of the seventh nerve, after traversing 

 the petrous bone m a circuitous course, enters the cavity of the 

 tympanum, and passing quite across it, is transmitted through the 

 glenoid fissure to a salivary gland under the lower jaw. The object 

 of this singular but uniform course of the chorda tympani is not well 

 understood. 



Deafness arising from the closure of the Euatachian tube has been 

 sometimes cured by dilating that canal by instruments pawed for 

 that purpose into its outer expanded extremity through the nostrils, 

 or from the back of the throat; or by injecting fluids into it by 

 means of a syringe with a small curved pipe. This latter plan has 

 also been successful in curing deafness arising from chronic intlannna 

 tion or morbid secret:*. n within the tympanum. Suppuration within 

 that cavity or in the mastoid cells sometimes results from hi^h inflam- 

 mation, and has been attended with fatal consequences by spreading 

 to the bones of the cranium, or along the nerves to the brain or it* 

 membranes. Cases of this kind generally originate, as we have already 

 stated, in cold with sore throat, and are found to occur chiefly in scro- 

 fulous habits. 



ieni.il i:.n- . .n-i-its nf the Meatun Auditorius Extermu 

 (fit- 5, m), and Concha. The former, commencing from the membrana 

 tympani, is an osseous canal in the first part of its course in the adult, 

 and then becomes nothing more than a tubular continuation 

 expanded cartilage of the concha, or outer appendage of the ear. It 

 is lined throughout with a delicate skin, covered by a thin cuticle, 

 whirh also coven the outer surface of the membrane. Beneath the 

 skin, and opening through it on the surface, are numerous glandular 

 follicle* which secrete the ear wax ..r ei-nuncn. In the fcctus and 

 new-born infant there is hardly any appearance of thin tube ; the 

 membrane of the tympanum being close to the surface of the head, 

 stretched upon the inner margin of a bony ring (aunulus auditorius) 



