Individual variability in the size and organization of the human arcuate nucleus of the medulla.

The arcuate nucleus (Arc) of the medulla is found in almost all human brains and in a small percentage of chimpanzee brains. It is absent in the brains of other mammalian species including mice, rats, cats, and macaque monkeys. The Arc is classically considered a precerebellar relay nucleus, receiving input from the cerebral cortex and projecting to the cerebellum via the inferior cerebellar peduncle.

The Claustrum in the Squirrel Monkey.

The claustrum (CLA) is a subcortical structure that is reciprocally and topographically connected with the cerebral cortex. The complexity of the cerebral cortex varies dramatically across mammals, raising the question of whether there might also be differences in CLA organization, circuitry, and function. Species variations in the shape of the CLA are well documented.