MIDDLE AND INNER EAR

The middle and inner ears show few special features of importance. The auditory tubes are narrow and open on the dorsolateral wall of the nasopharynx, level with the landmark provided by the hamulus of the pterygoid bone, which is palpable through the mouth caudome- dial to the last cheek tooth in the dog.

In the cat an incomplete bony septum bullae subdi­vides the middle ear into a small dorsolateral and a large ventromedial compartment. The two compart­ments communicate with each other through an opening at the caudodorsal margin of the septum near the cochlear window.

In both species middle ear infections (otitis media) may be drained into the nasopharynx through the bulla, which can be palpated through the oropharynx and soft palate, caudal to the hamulus. The inflated tympanic bulla of the cat is also easily found on palpation through the skin, between the wing of the atlas and the zygo­matic arch.

The bulla can be approached surgically from the ventral side, with the use of the medial border of the rostral digastric muscle, the mylohyoid muscle, and the stylohyoid and the tympanohyoid cartilages of the hyoid apparatus as landmarks; care should be taken to avoid damage to the nerves of the pharyngeal plexus (Figure 11-44/22) and the vascular supply of the man­dibular lymph node.

Several nerves pass through the middle ear, but only two are of clinical significance. The facial nerve travels in the facial canal of the petrous temporal bone; in its course it detaches a branch, the chorda tympani, that enters the cavity of the middle ear.

Postganglionic fibers of the cranial cervical ganglion, located just behind the tympanic bulla, participate in a plexus within the middle ear. The resulting dysfunction is Horner syndrome, a complication of otitis media. The signs are miosis and retraction of the globe, which causes protrusion of the third eyelid, and narrowing of the palpebral fissure. The syndrome usually disappears spontaneously in about 3 months.