The middle ear, or cavum tympani, is an ellipsoidal cavity situated in the tympanic bulla. On removing the medial wall of the bulla, a bony plate is seen separating its cavity into two chambers. This plate of bone extends in from the lateral wall of the bulla, forming a complete partition except at the dorsocaudal part, where a notch in the bone forms a free communication between the two chambers. The lateral chamber, or tympanum proper, contains the bones of the ear and is limited externally by the tympanic membrane. At its cranial end is seen the opening of the tuba auditiva, or Eustachian tube.
Fig. 171.—Malleus and Incus, United in the Natural Position.
1, malleus; 2, incus. a, handle of malleus; b, neck; c, head; d, process for attachment of tendon of M. tensor tympani; e, wing-like plate and process of neck; f, g, the two processes of the incus.
The tuba auditiva, or Eustachian tube, is a cartilaginous tube about one and a half to two centimeters long, passing from the nasopharynx to the middle ear and placing these two cavities in communication. The pharyngeal opening is at about the middle of the length of the nasopharynx, near the dorsal border of the lateral wall; it is a narrow slit two or three millimeters long. Thence the tube passes caudolaterad on the ventral surface of the sphenoid bone, and enters the tympanic cavity through the opening just laterad of the styliform process of the tympanic bulla. The tube has a thick, cartilaginous medial wall, while the lateral wall is thin and formed of connective tissue. The lumen of the tube is a curved slit in cross-section.
Bones of the Ear (ossicula auditus).—There are three small bones in the middle ear, the malleus (hammer) ([Fig. 171], 1), incus (anvil) ([Fig. 171], 2), and stapes (stirrup) ([Fig. 172]).
The malleus ([Fig. 171], 1) consists of a slender handle (a) with a small neck (b) attached at an angle to the caudal end of the handle. At the end of the neck is a rounded head (c) which articulates with the incus (2), and together with one part of the incus lies in a small fossa dorsad of the fenestra vestibuli. From the medial aspect of the neck extends a short process of bone (d) to which is attached the tendon of the tensor tympani muscle. One side of the neck is produced into a thin wing-like plate, bearing on its edge a pointed process (e).
The incus ([Fig. 171], 2) bears much resemblance to a two-fanged molar tooth. It lies in the fossa with the head of the malleus. Its head or crown is directed craniad and has a concave surface which articulates with the head of the malleus. One of the fangs (f) extends caudad in the fossa above mentioned and is held in position by a ligament. The other fang (g) extends ventromediad and articulates with the head of the stapes. This fang terminates in a minute rounded tubercle known as the lenticular process; this in early stages of development is a separate bone, the os lenticulare.
Fig. 172. Stapes.