Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/353

 299 Part. VI. A N A T O M Y. which being fixed to the edges of the half-canal, each of thefe laminse maybe fubdivided into feveral others. gina, forms an infire tube therewith. The depreflion in the middle of the membrana tympa- At the extremity of this bony half-canal, where we ni is caufed by the adhefion of the little bone called mal- obferve the cavity fhaped like the mouth of a fpoon, this leus, the handle of which is clofely joined to the infide mufcle ends a tendon, which is bent round the tranfof the membrane from the upper part of the circumfe- verfe bony orin ligamentary ridge in the laft-named cavity, rence all the way cb the center to which the end of the as over a pulley, and is inferted in the neck of the malhandle is fixed. This handle feems to lie in a very fine leus above the fmall apophyfis, advancing likewife as far membranous duplicature, by means of which it is tied as the handle. to the membrana tympani, and which ferves it for a pe- The mufcle of the ftapes is Ihcrt and thick, and lies riofleum. within the fmall bony pyramid at the bottom The periofteum of the tympanum or barrel of the ear concealed of the tympanum. The cavity which it fills, touches produces that of the fmall bones; it is likewife continued very the bony canal of the portio dura of the auover the two feneftrte, and enters the euftachian tube, ditorynearly nerve; and it terminates in a fmall tendon which where it is loft in the inner membrane of that dud. of the cavity through the fmall hole in the apex The cellulse maftoidaei are very irregular cavities in the ofgoestheoutpyramid. As it goes through the hole it turns fubftance of the maftoid apophyfis, which communicate and is inferted in the neck of the ftapes on the with each other, and have a common opening towards forward, of the longeft and moft crooked leg of that bone. the infide, and a little above the pofterior edge of the or- fideThe three parts of the labyrinth, that is, the veftibicular groove. Thefe cells are lined by a fine membrane, bulum, femicircular canals, and cochlea, are lined by a which is partly a continuation of the periofteum of the tympanum, and partly feems to be of. a glandular ftruc- fine periofteum, which is continued over all the fides of ture like a kind of the membrana pituitaria. The ma- their cavities, and ftiuts the two feneftrae of. the tymftoid opening is oppofite to the fmall opening of the Eu- panum.. The periofteum of the two fides of the bony fpiral laftachian tube, but a little higher. The ligaments of the officula come next in order. The mina advances beyond the edge of that lamina, and forms incus is tied by a ftrong fhort ligament, fixed in the point a membranous duplicature, which extending to the oppoof the fhort leg, to the edge of the maftoid opening. fite fide compkats the fpiral feptum. Between the incus and malleus we find a fmall, thin car- This feptum feparates the two half-canals from the batilage. The malleus is conneded through the whole lls to the apex ; but there it leaves a fmall opening, by length of its handle to the infide of the membrana tym- which the fmall extremities of the half-canals communicate with each other. The large extremity of the exterpani. The malleus has three mufcles, one external, one an- nal half-canal ends by an oblique turn in the feneftra' roterior, and one internal; and the ftapes has one mufcle. tunda, which is Ihut by a continuation of the periofteum The external or fuperior mufcle of the malleus, is a thin of that canal. The large extremity of the other halffafciculus of flefhy fibres lying along the upper part of the canal opens into the veftibulum; and thefe two extremibony meatus auditorius, between the periofteum and the ties are intirely feparated by a continuation of the peother integuments. The outer part of it is pretty broad, riofteum. and it contrads by degrees as it advances towards the up- The ear is the organ of which we can moft diftin<ftjy unper part or break of the orbicular groove of the tympa- fold the ftrudture, and demonftrate the greateft number of num, into which it enters by a fmall tendon, above the parts, that is, of fmall machines of which it is made up. membrana tympani, and is inferred in the neck of the We know likewife in general, that it is the organ of hearmalleus, near the fmall eminence or Ihort apophyfis of the ing ; .but. when we endeavour to difcover the ufes of each handle. of thefe parts, that is, how each contributes to the great The anterior mufcle of the malleus, is fleftiy, long, defign of the whole, after having thoroughly examined and thin. It runs along the outfide of the Euftachian them, we muft be obliged to own, that the greateft part lube, to which it adheres very clofely through its whole of what the moft able philofophers have faid upon this length. Its anterior extremity is fixed in that fide of the fubjed, is without any real foundation. tube juft before the fphenoidal fpine; and the pofterior extremity ends in a long thin tendon, which runs in the S e c t. VIII. The M o u t h. articular or glenoid fiffure of the os temporis, through a fmall oblique notch; in which fiflure it enters the tympa- The mouth may be diftinguilhed into external and innum, and is inferted in the long thin> apophyfis of the ternal, and the parts of which it confifts may likewife malleus. It is partly accompanied by a. nerve, .which come under tire fame two general heads. The parts of the neck ftill undefcribed are only the forms what is called the chorda tympani. The internal mufcle of the malleus is very fielhy and larynx, pharynx, glindulse thyroidese, andthe mufculus diftind. It lies along the infide of the Euftachian tube, cutaneus, which really belong to the head; and therepartly on the cartilaginous, and partly on the bony por- fore, inftead of making, a particular feftion for fo fmall a tion, being fixed by one extremity in the apophyfis pc- number of parts, efpecially fince the larynx and pharynx trofa. Afterwards it runs along the cavity of the bony have fo near a relation to the internal parts of the mouth, half-canal of the tympanum, within which cavity it is in- we are under a neceflity of defcribing. them, before provefted by a pprtion of a-membranous or jigamentary ya- ceeding to the mouth in particular. The;:.