Each kidney has a dark brown outer cortex and a light brown inner medulla. The medulla is composed of about a dozen renal pyramids, each having its base oriented toward the cortex and its apex, the renal papilla, projecting medially. The cortex extends into the medulla between adjacent pyramids as the renal columns. Extending from the bases of the renal pyramids into the cortex are striations known as medullary rays.
Definition