Outline of the arcuate nucleus in the human medulla oblongata.

The outline of the arcuate nucleus in the human medulla oblongata was studied in a series of serial sections of brain stems of newborn and young children. The nucleus lies on the ventral aspect of the pyramid. At higher levels the nucleus lies on the ventral and medial aspect of the pyramid. In the upper regions of the medulla the two nuclei fuse together, giving rise to a median dorsal extension, which in the uppermost part of the medulla extends to the floor of the fourth ventricle. The nucleus sends numerous branches to the corticospinal portion of the pyramidal tract, while the dorsal extension of the arcuate nucleus sends numerous transverse rays on either side which consist of nerve fibres and cells. These rays extend in the areas of the medulla occupied by the medial lemniscus, tectospinal portion, and medial longitudinal bundle. The significance of the findings is discussed.