The external ear in Crocodilia
- 1 Crocodilians possess a slit-like external auditory meatus, bounded above by a large superior ear-flap, and below and in front by a small inferior ear-flap. The anterior part of the meatus in living animals is usually open when the top of the head is out of water and closed when it is submerged. The posterior part of the meatus is nearly always closed, even when the head is above the surface.
- 2 The superior ear-flap is operated by two muscles of the hyoidean group, the more superficial one being the M. levator and the deeper one being the M. depressor. Both muscles are attached to the skull and to the auricular plate, a mass of dense connective tissue which reinforces the upper ear-flap. These muscles act in an indirect fashion by rotating the auricular plate around a longitudinal axis, and both are related to the deep aspect of the plate.
- 3 The anterior part of the meatus is opened as a result of downward movement of the inferior ear-flap. This is effected by means of a Y-shaped structure composed of dense connective tissue and here termed the ypsilon. The lateral arm of the ypsilon is attached to the lower ear-flap, the medial arm to the postorbital bone, and the stem, which projects forwards into the orbit, to the depressor auriculae inferior muscle. This muscle runs transversely across the floor of the orbit and is probably a derivative of the levator bulbi muscle.
- 4 The functional significance of the ear-flap in protecting the tympanic membrane from mechanical injury is discussed, and some points of contrast between the external ear in crocodiles and that in mammals are noted.