Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/352

 Part. VL 29S A N A T O M Y. The bafis of the cochlea is gentl^ hollowed, and to- of the pyramid; and lower down, by another hole, with wards the middle perforated by feveral fmall holes. The the barrel of the ear. fpindle is a kind of Ihort cone, with a very large bafis, The internal parts of the ear are chiefly the membrana which is the middle of the bafis of the cochlea. Thro’ tympani, the periofteum of the barrel, oflicula auditus, its. whole length runs a double fpiral groove, which, labyrinth and all its cavities, the membrana maftoidasa through a microfcope,' Ihews a great number of pores. interna, the mufcles of the oflicula, and the parts which The cochlea makes about two turns and an half from complete the formation of the Euftachian tube. the bafis to the apex; and the two dutts, being ftri&ly The Euftachian tube is a canal or dud which goes from united together through their whole courfe, form an en- the tympanum to the pofterior openings of the nares, or tire common feptum, which muft not be confounded with nafal folfas, and toward the arch of the palate. the half feptum or fpiral lamina, as is often done. The The bony portion of it lies through its whole length firft might be termed the common feptum, the other the immediately above the fiffure of the glenoid or articular cavity of the os temporis, and terminates at the meeting particular feptum or half-feptum. Both of them are clofely joined to the fpindle, being of the fpinal apophyfis of the os fphenoidale with the athicker there than in any other place. Tire common pophyfis petrofa of the os temporis. feptum is complete, and feparates the turns entirely from The other or mixed portion reaches in the fame direceach other; whereas the half-feptum in the llceleton is tion from this place to the internal ala of the apophyfis only a fpiral lami'na, the breadth of which is terminated pterygoides, or to the pofterior and outer edge of the all round by a very thin border lying in the middle cavity nares. It is properly divided into four parts, two ftiperior, of the cochlea. In the natural ftate, there is a membra- and two inferior. nous half-feptum, which completes the partition between The two upper parts or quarters are bony; and of thefe the innermoft is formed by the fide of the apophy-: the two dufts. The two half-canals turn jointly about the fpindle, fis petrofa, the outermoft by the fide of the apophyfis of the os fphenoides, fo that the upper half of one being fituated towards the bafis of the cochlea, the fpinalis this portion of the tube is bony. Of the two inferior other towards the apex: for which reafon they have been parts, is cartilaginous, and the external memtermed the one internal, and the other external; branousthe; fointernal that the lower half of this portion of the tube the divifion of them into the upper and lower flight, not is partly cartilaginous the os fphenoidale, and partbeing agreeable to the natural ftate, but liable to convey ly membranous next thenextapophyfis petrofa. a very falfe idea thereof. Euftachian tube thus formed, is very narrow in The fpiral or volute of the cochlea, begins at the low- theThe er part of the veftibuliim, runs from thence forward to bony part next the ear. The other portion grows the top, then backward down to thp bottom, afterwards gradually wider, efpecially near the pofterior nares, where upwards and forwards, and fo on from the bafis which the inner cartilaginous fide terminates by a prominent is turned inward, to the apex which is turned outward. edge, and the outer fide joins that of the neighbouring The two half-canals communicate fully at the apex of noftril. The cavity of the tube is lined by a membrane the cochlea. Their feparate openings are towards the like that of the internal nares, of which it appears to be. bafis, one. of them being immediately into the lower part a continuation. fituation of the two tubes is oblique, their pofteof the forefide of the veftibulum, the other into the fe- riorThe extremities at the ears Being at a greater diftance than neftra rotunda. the anterior at the nares, and the convex fides* of the The internal auditory hole is in the backfide of the prominent are turned toward each other. The oapophyfis petrofa,, in fame tneafure behind the veftibu- penings of edges the tubes are oval at this place, as is likewife ium and bafis of the cochlea.. It is a kind of blind hole, divided' into two foffulas, one large, the other fmall. their whole cavity, efpecially that of the mixed portion. The large one lies loweft^ and ferves for the portio mol- The membrana tympani is a thin, tranfparent, flattifh lis of the auditory nerve or feventh pair. The fmall one pellicle, the edge of -which is round, and ftrongly fixed is uppermoft, and is the opening of a fmall dud through in the orbicular groove which divides the bony meatus of which the portio dura of the fame nerve palfes. the external ear from the tympanum or barrel. This The inferior folfala is full of little holes, which, in membrane is very much ftretched or very tenfe, and yet she natural ftate, are filled with nervous filaments of the not perfectly flat: for on the fide next the meatus exterportio mollis, which go to the fpindle,. to the femkircu- rms it has a fmall hollownefs, which is pointed in the midlar canals, and to thofe of the cochlea. It is this foffii- dle; and on the fide next the tympanum it is gently conla which forms the (hallow cavity at the bafis of the fpin- vex, and alfo pointed in the middle. This membrane is fituated obliquely, the upper part dle of the cochlea. The paflage for the portio dura- of the auditory nerve of its circumference being turned outward, and the lowruns behind the tympanum, and its orifice is the ftylo- er part inward, fuitably to the direction of the bony maftoid hole. It begins by the fmall foffula, and pierces groove already mentioned. It is made up of feveral very from within, outwards, the upper part of the apophyfis fine laminae, clofely united together. The external lapetrofa,, making there an angleor curvature. From thence mina is in feme meafure a produ&ion of the lidn and cuit is inclined backward, behind the fmall pyramid of the ticula of the external meatus.; the,internal lamina is a tympanum, and runs down to the ftylo-maftoid hole, continuation of the periofteum of the tympanum; and li communicates. likewife,.by a fmall hole, with die finus when the membrane has' been, firft macerated in water, each.