Ventromedial hypothalamic nucleus. Photomicrographs of orexin A fibers in the ventromedial hypothalamic nucleus (VMH) of the Long-Evans rat (A), grass rat (B), Syrian hamster (C), and degu (D). Note markedly higher density of orexin fibers in the degu VMH in comparison with the other three species. 3V: third ventricle; Arc: arcuate nucleus. Scale bar = 300 μm.