There are several other openings into the tympanic cavity.
(a) On the external surface is a large oval opening, the external auditory meatus bounded by a thickened rim.
(b) Into the outer and anterior part of the cavity the outer end of the Eustachian tube opens; while the inner end passes through a foramen (fig. 75, 22) just external to the foramen lacerum medium, on its way to open into the pharynx.
(c) The internal carotid artery also enters the tympanic cavity by a canal which commences in the foramen lacerum posterius, and passes forwards to open on the inner side of the bulla. The artery then passes forwards, and barely appearing on the ventral surface of the cranium, enters the brain cavity through the foramen lacerum medium (fig. 75, 9).
Immediately behind the tympanic, between it and the mastoid process of the periotic and the paroccipital process of the exoccipital is the stylomastoid foramen (fig. 75, VII).
Within the tympanic cavity are four small bones, the auditory ossicles (cp. fig. 74), called respectively the malleus, incus, lenticular and stapes; these together form a chain extending from the fenestra ovalis to the tympanic membrane.
The malleus has a somewhat rounded head (fig. 100, B, 1) which articulates with the incus, while the other end of the bone is drawn out into a long process, the manubrium, which lies in relation to the tympanic membrane. The head is also more or less connected by a thin plate of bone, the lamella, to another outgrowth, the processus longus. The incus (fig. 100, B, 3) is somewhat anvil-shaped, and is drawn out into a process which is connected with the lenticular, a nodule of bone interposed between the incus and the stapes, with which it early becomes united. The stapes (fig. 100, B, 2) is stirrup-shaped, consisting of a basal portion from which arise two crura, which meet and enclose a space, the canal.
The squamosal (fig. 73, 7) is a large bone occupying much of the side wall of the cranial cavity, and articulating above with the parietal, and behind with the supra-occipital, while in front it overlaps the frontal and alisphenoid. But though it occupies so large a space on the outer wall, it forms very little of the internal wall of the skull, but is really like a bony plate attached to the outer surface of the cranial wall. The squamosal is drawn out into a strong forwardly-directed zygomatic process which meets the jugal or malar. The ventral side of the zygomatic process is hollowed out, forming the glenoid fossa (fig. 75, 8), a smooth laterally elongated surface with which the lower jaw articulates, while the hinder edge of the glenoid fossa is drawn out into a rounded postglenoid process (fig. 75, 23). The articulation is such as to allow but little lateral play of the lower jaw.
(2) Bones in relation to the Optic capsules.
The only bone developed in relation to the optic capsule on each side is the lachrymal. This is a small membrane bone lying between the frontal and palatine behind, and the maxillae and jugal in front. It is perforated by a prominent lachrymal foramen (fig. 73, 13) which opens within the orbit.