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The foramen of Monro, or Monro's foramen, is a term used for the interventricular foramen, which is named after its location in the brain. It connects the right and left sides of the lateral ventricles and is responsible for allowing cerebrospinal fluid (CSF) to get into a third ventricle. The word "foramen" refers to an opening, passage or canal in the body instrumental in connecting the body's vessels, nerves or muscles. Since there are more than one of these openings, a more accurate and inclusive term to use would be the plural of "foramen," which is "foramina." This results in the "foramina of Monro," or the interventricular foramina.
This particular pair of crescent-shaped openings is named after Scottish physician Alexander Monro. Although French anatomist Raymond Vieussens had identified the interventricular foramina of the brain about a century before him, Monro is credited as being the first person to describe them. He published his findings in his 1783 publication, Observations on the Structure and Functions of the Nervous System.
The foramen of Monro can be found at the right and left sides of the lateral ventricles. The openings connect these chambers with the third ventricle at the brain's mid-line. Thus the lateral ventricles flank the third ventricle, and these structures form the major parts of the brain's ventricular system. Each foramen is bordered by a fibrous bundle called the fornix and a grey-matter mass named the thalamus.
By connecting these parts of the brain, each foramen of Monro permits the CSF that the lateral ventricles produce to get into the third ventricle. From there, this clear, colorless fluid spreads to the rest of the ventricular system. The CSF is necessary for protecting the brain. An excess production of the fluid, however, can lead to hydrocephalus, or "water on the brain." Also contained in the foramina is the choroid plexus, which produces the CSF.
The phrase "foramen of Monro" is particularly crucial in setting it apart from another type of interventricular foramen. The field of embryology also has an interventricular foramen, which is an opening that occurs for a certain period of time between the heart's still-developing ventricles, or lower chambers. It is created when the interventricular septum leaves an opening during its separation of the heart into the two ventricles. This area of the organ, however, is not called the foramen of Monro, and there is only one of these openings instead of the pair that characterizes the interventricular foramina of the brain.