We describe several novel morphological features in the nasal region of the hammerhead shark Sphyrna tudes. Unlike the open, rounded incurrent nostril of non-hammerhead shark species, the incurrent nostril of S. tudes is a thin keyhole-like aperture. We discovered a groove running anterior and parallel to the incurrent nostril. This groove, dubbed the minor nasal groove to distinguish it from the larger, previously described, (major) nasal groove, is common to all eight hammerhead species. Using life-sized plastic models generated at 200 microm resolution from an X-ray scan, we also investigated flow in the nasal region. Even modest oncoming flow speeds stimulate extensive, but not complete, circulation within the model olfactory chamber, with flow passing through the two main olfactory channels. Flow crossed from one channel to another via a gap in the olfactory array, sometimes guided by the interlamellar channels. Major and minor nasal grooves, as well as directing flow into the olfactory chamber, can, in conjunction with the nasal bridge separating incurrent and excurrent nostrils, limit flow passing into the olfactory chamber, possibly to protect the delicate nasal structures. This is the first simulation of internal flow within the olfactory chamber of a shark.
This study demonstrates a novel model generation methodology that addresses several limitations of conventional finite element head models. By operating chiefly in imagespace, new structures can be incorporated or merged, and the mesh either decimated or refined both locally and globally. This methodology is employed in the development of a highly bio-fidelic finite element head model from high resolution scan data. The model is adaptable and presented here in a form optimised for impact and blast (Schneiderman et al. 2008). Considerable research has been devoted to investigating the mechanisms which generate TBI in cases where the head is subjected to impact or blast. Of the various experimental methods available (in vivo human experiments, or tests on cadaveric, animal, physical, or mathematical models) the increasing availability of computing power has seen numerical simulation, and in particular the finite element (FE) method, come to the forefront of this research. The current study details the development of a 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
Finite element modellingFE simulation is an invaluable tool in the exploration of complex trauma mechanisms resulting from dynamic insults such as impacts and blasts. While sophisticated FE models of the head offer the prospect of providing accurate and repeatable experimental analyses of all categories of mechanical head injury, the validity of these models is heavily influenced by the geometric accuracy of the structures within the model. Traditionally computational models are created manually using computer aided design (CAD) tools, but when concerned with the complex anatomy of the head and its internal structures this manual approach has significant disadvantages; there is a large scope for subjectivity and user error, as well as of becoming rapidly computationally intractable as more geometric fidelity is introduced. A novel model generation approach which can reduce the inaccuracies associated with modelling a highly complex structure is known as 'image based meshing'. This refers to the conversion of volume scan data, generally from computed tomography (CT) or magnetic resonance imaging (MRI), directly into an FE mesh by way of fully-and semi-automated processes with minimal user input. This increases not only the accuracy, but also the speed at which computational models of complex geometries can be constructed, thereby allowing a greater number of anatomical features to be distinguished.In 1997, Mehta et al. selected 14 cross-sectional image 'slices' from an MRI scan of the head and highlighted the skull in these images using an image processing tool. These highlighted outlines of the bone could then be read by a C++ computer code and converted into CAD coordinate and spline data, from which a model could be constructed that was, at least partially, based on the ...
From high-resolution (65 lm) 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 ($140 mm 2 ) and a modest expansion in the nasal chamber. Oscillating flow (0.3-0.4 Hz) within the narrow (65-130 lm) sensory channels of the nasal chamber is laminar (Reynolds number $ 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.
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