The mastoid process of the temporal bone is composed internally of numennts cells, communicating with each other, and finally opening into the back part of the tympanum. These de nut exist in the fcetus.
The cavity of the tympanum contains a chain of small bones called ossicula audi tus, connected by one end to the membra na tympani, and by the other to the fe nestra ovalis. Of these the first, which is compared to a hammer, is called the mal leus; the second is namedthe incus, the third the orbiculare, and the fourth the stapes.
The malleus possesses amanubrium or handle, a long and short process, and a head which forms an articular surface.
The incus resembles a grinding tooth, with its two fangs diverging. We remark in it a body, the surface of which is hol lowed out to receive the head of the mal leus : a long and a short leg.
The orbiculare is of the size of a small grain of sand. It is attached to the ex tremity of the long ems of the incus and the stapes.
The stapes has an exact resemblance to the iron part of a stirrup ; it has a head, two crura, and a basis.
The handle of the malleus is firmly con nected to the membrana tympani ; and hence arises the external concavity and internal convexity of the membrane. The head of that bone is joined to the body of the inclis, whose long leg is articulated to the head of the stapes The basis of the stapes fills up the fenestra ovalis. The ends of the bones forming these articula tions are covered with cartilage, and fur nished with capsules like other joints. The hones of the tympanum have some small muscles connected to them by which they are moved outwards, or towards the membrana tympani, and inwards, or to wards the fenestra avails. The first of these motions relaxes, the latter stretches the membrane. The names of these mus cles are, tensor tympani, laxator tympani, and stapedeus.
The nerve called chordatympani passes across the tympanum between the handle of the malleus and the long leg of the incus.
The use of the ossicula auditus seems to be that of transmittingthe vibrations of the air from the membrana tympani to the labyrinth. The final use of the muscles
which moves these bones is unknown.
The labyrinth of the ear consists of three parts :-1. A spiral bony canal, twisted like a snail-shell, and thence call ed the cochlea. 2. Three semicircular bony canals : and 3. A small cavity, call ed the vestibulum, into which the cochlea and the semicircular canals open. These parts are formed of the hardest bone in the hotly, almost equal in solidity to ivory, and the petrous portion of the temporal bone, which incloses them, is of a similar structure. In the fcctus the labyrinth is surrounded by a softer and looser kind of bone, so that it can be most easily dissect ed at that age.
The'vestibulum is about equal in size to a large pea, and the fenestra ovalis opens into the middle of the cavity. It has also five openings from the semicir cular canals ; the superior and exterior joining by one of their extremities, and opening by a common hole.
The cochlea has two turns and a half. Its canal turns round a bony centre, called the modiolus, to which is attached a thin plate of bone, projecting into the cavity of the cochlea, and named lamina spiralis. This projecting plate divides the canal of the cochlea into two parts: one opening into the vestibulum, the other at the fe nestra rotunda. The latter is called the scala tympani, the former scala vestibuli.
The vestibulurn, cochlea, and semicir cular canals, are lined by a delicate vascu larmembrane, on which the portio mollis of the seventh pair of nerves is distribu ted. This membrane contains a clear water.
The filaments of the audito7 nerve pass from the meatus auditorius internus through a number of very small apertures which lead to the labyrinth, and they ter minate on the vascular membrane of the labyrinth, so that the nervous pulp is ex posed almost bare to the contained fluid. The distribution of the nerve on the coch lea is particularly beautiful. The aque ducts of the ear are two very fine tubes, passing from the vestibulum and cochlea to open on the surface of the dura mater.