a, head; b, base; c, process for attachment of the stapedius muscle.
The stapes ([Fig. 172]) has the form of a stirrup, and is the last in the chain of bones. The small end is termed the head (a) and the broader part the base (b). The head of the stapes articulates with one of the processes of the incus. The base of the stapes fits into the fenestra vestibuli of the petrous bone and completely closes it. The stapes is held in position by the stapedius muscle, which is attached to a minute process (c) near the head.
Thus the three bones of the middle ear form a chain which connects the medial surface of the tympanic membrane with the fenestra vestibuli. As the fenestra vestibuli forms a portion of the wall of the vestibule, the vibrations of the tympanic membrane are transmitted through the chain of bones to the fluid within the vestibule.
Muscles of the Middle Ear.
M. tensor tympani.—The tensor tympani muscle is a short conical muscle having origin in a small fossa ([Fig. 25], d, [p. 34]) in the petrous bone dorsocraniad of the fenestra vestibuli. Its very short tendon passes laterad and is inserted into a projection ([Fig. 171], d) on the neck of the malleus.
M. stapedius.—The stapedius muscle is a short muscle taking origin from a fossa ([Fig. 25], f) in the lateral surface of the petrous bone caudad of that for the incus. It is inserted into the head of the stapes.
The Internal Ear ([Fig. 173]).
The internal ear or labyrinth is completely enclosed in the petrous bone. In the dry bone the internal ear communicates with the middle ear by the fenestra cochleæ and the fenestra vestibuli, which have been described in the account of the petrous bone ([page 33]). In the recent state the fenestra vestibuli is closed by the base of the stapes, while the fenestra cochleæ is covered by a delicate membrane,—so that the internal ear forms a closed cavity. It is possible to distinguish a bony labyrinth and a membranous labyrinth ([Fig. 173]), the former being the cavity in the petrous bone, while the latter is the real auditory organ, situated within the cavity. The membranous labyrinth repeats in general the form of the bony labyrinth, so that it will not be necessary to distinguish the two in our account of the main features of the structure. The auditory nerve is distributed to the membranous labyrinth. For an account of the minute structure of this organ reference should be made to a text-book of general histology or physiology.
Fig. 173.—Membranous Labyrinth.