The inner ear of the common rhea (Rhea americana L.).

The morphology of the inner ear in rheas was examined by light and electron microscopy. The shape is typically bird-like with very long semicircular canals. The anterior and posterior cristae have small septa cruciata. The vestibular sensory epithelia contain two main types of hair cell innervation; bouton-innervated hair cells and calyceal hair cells characterized by a surrounding nerve calyx. The utricular macula has a single zone of calyceal hair cells, while all other previously examined birds, except the mute swan, have 2 zones. The height of the tallest sensory hairs of the cristae is 20-30 microns. In the utricular and lagenar macula, the hairs are 5-7 microns in the striola and 10-20 microns in the main parts of the sense organs. Along the edges of the maculae the longest hairs may reach 20-30 microns. The number of stereovilli on mature vestibular hair cells is 40-60. The sensory hairs of the hearing organ, the basilar papilla, are generally shorter but more numerous than the vestibular sensory hairs. In the proximal end, the tallest of the 175-200 stereovilli are 2.8-3.7 microns; in the distal end of the papilla, the number of stereovilli decrease to 65-100, and their height increases gradually to 7.3-8.7 microns. The neural sensory hairs are generally taller than those of the abneural side.