HRP was injected into the peripheral branches of the trigeminal nerve, of rats and ducks, and the subdivisional organization of the trigeminal ganglion was observed in relation to food seeking behavior. In both the rat and the duck, there were definite regions of cellular localization corresponding to the peripheral local regions. In the rat, particularly the regions demarcating the respective regions were shown to be laid three-dimensionally and zigzagged unevenly, but the adjoining regions in the rat or the duck were less vague than in previous studies using axonal degeneration (chromatolysis). Among the regions of the rat trigeminal ganglion, the largest was the NV
2 region, next the NV
3, and lastly the NV
1. In particular, the space held by the cells innervating the sinus hair occupied almost all the NV
2 region. In the duck, moreover, the Grandry corpuscles of the bill were especially numerous in the upper bill, and were equally innervated from the respective regions of NV
1 and NV
2in the trigeminal ganglion. The region of NV
3 was quantitatively smaller than that of NV
1 or NV
2 in the duck (a reduction of about 20%). The above morphological findings indicate that the animal's food seeking behavior is closely related to the constitution of the trigeminal ganglion, so that it might be said that the behavioral characteristics reflect the morphological structure.
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