The nervous system is a hollow nerve cord with cerebrospinal fluid flowing through the center.  In vertebrate brains, this hollow center expands to form a number of ventricles.  In humans, cerebrospinal fluid synthesized in choroid plexus flows through lateral ventricles (in the cerebrum) to a third ventricle (in the diencephalon) through a cerebral aqueduct (through the midbrain) and into the fourth ventricle (at the base of the cerebellum).  From there the fluid may flow into the central canal of the medulla and spinal cord or be reabsorbed in the meninges.  This set of chambers is shared with other vertebrates.  In lower vertebrates, there is also a ventricle in the midbrain.

SHARK

LUNGFISH

ALLIGATOR

OPOSSUM

SHEEP

PIG

HUMAN