Three-dimensional structure of the nasal passageway of a hagfish and its implications for olfaction

From high-resolution (65 mu m) data acquired by magnetic resonance imaging, we have reconstructed the nasal passageway of a single adult hagfish specimen (probably Eptatretus stoutii). We have used this reconstruction to investigate how the anatomy and morphometry of the nasal passageway influence the olfactory ability of the hagfish. We found that the long, broad section of the passageway preceding the nasal chamber will delay the response to an odor by 1-2 s. Diffusion of odorant to the olfactory epithelium, on which the olfactory sensitivity of an animal depends, will be favored by the relatively large surface area of the olfactory epithelium (similar to 140 mm(2)) and a modest expansion in the nasal chamber. Oscillating flow (0.3-0.4 Hz) within the narrow (65-130 mu m) sensory channels of the nasal chamber is laminar (Reynolds number similar to 5) and quasi-steady (Womersley number generally less than one). Distribution of flow over the olfactory epithelium may be aided by: (a) a narrowing before the nasal chamber; (b) partial blockage of the nasal passageway by a protrusion on the central olfactory lamella; and (c) the inward inclination of the olfactory lamellae.