In anatomy, a medullary ray (Ferrein's pyramid) is the middle part of a cortical lobule (or renal lobule). Each consists of a group of nephrons in the renal cortex.^[1] Their name is potentially misleading, as "medullary" refers to their destination, not their location. They travel perpendicular to the capsule, and extend from the cortex to the medulla. They may be visualised during urography.^[1]

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  Medullary rays
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  Medullary rays

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This article incorporates text in the public domain from the 20th edition of Gray's Anatomy (1918)

• Histology image: 16003loa – Histology Learning System at Boston University — "Urinary System: kidney, H&E, cortex and medullary ray"
• Histology image: 15901loa – Histology Learning System at Boston University — "Urinary System: neonatal kidney"
• UIUC Histology Subject 1008

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