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5a. The Olfactory Nerves
(NN. Olfactorii; First Nerve)
The olfactory nerves (Fig. 771) or nerves of smell are distributed to the mucous membrane of the olfactory region of the nasal cavity: this region comprises the superior nasal concha, and the corresponding part of the nasal septum. The nerves originate from the central or deep processes of the olfactory cells of the nasal mucous membrane. They form a plexiform net-work in the mucous membrane, and are then collected into about twenty branches, which pierce the cribriform plate of the ethmoid bone in two groups, a lateral and a medial group, and end in the glomeruli of the olfactory bulb (Fig. 772). Each branch receives tubular sheaths from the dura mater and pia mater, the former being lost in the periosteum of the nose, the latter in the neurolemma of the nerve.The olfactory nerves are non-medullated, and consist of axis-cylinders surrounded by nucleated sheaths, in which, however, there are fewer nuclei than are found in the sheaths of ordinary non-medullated nerve fibers. The olfactory center in the cortex is generally associated with the rhinencephalon (page 826). The olfactary nerves are developed from the cells of the ectoderm which lines the olfactory pits; these cells undergo proliferation and give rise to what are termed the olfactory cells of the nose. The axons of the olfactory cells grow into the overlying olfactory bulb and form the olfactory nerves.
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