The cisterna magna, an opening in the subarachnoid cavity in the brain, is so named because of its large size. In fact, it is the largest of the openings that can be found in this gap of the central nervous system. The cisterna magna is also known as the cerebellomedullary cistern.
The alternate name of the cisterna magna, cerebellomedullary cistern, is derived from the opening's location. It runs along the cerebellum, the part of the brain that is responsible for one's movement coordination, as well as for contributing to other functions such as alertness, speech and reaction to emotions such as fear and joy. It is also located close to the upper surface of the medulla oblongata. This is the lower section of the posterior part of the brain called the brain stem, enabling functions such as breathing, blood pressure, heart and respiratory rate, and sexual arousal.
The subarachnoid cavity, where the cisterna magna is located, is also known as the subarachnoid space. It creates a space between the arachnoid mater and the pia mater. These are membranes that represent two of the three meninges covering the central nervous system, specifically the brain and spinal cord. The arachnoid is named for its shape, which resembles that of a spider web.
The third membrane that covers the central nervous system, the dura mater, sits atop both the arachnoid and the pia mater as the system's outermost layer. Surrounding the brain and spinal cord, it keeps a clear, colorless fluid called the cerebrospinal fluid, or liquor cerebrospinalis. It is produced in the choroid plexus and pours into the cistern magna from the lateral and median apertures of one of the brain's fluid-filled cavities, referred to as the fourth ventricle. By infiltrating the cistern among several other regions, the cerebrospinal fluid acts as a floating and protective device for the entire brain.
Accompanying the cisterna magna are several other openings, all of which are smaller in size. The pontine cistern and the interpeduncular cistern can also be found in the subarachnoid space. The former contains a blood vessel called the Basilar artery, which provides oxygenated blood to the brain, while the latter is named after its enclosure of stalk-like structures in the cerebrum, called the cerebral peduncles. Other notable cisterns besides the cisterna magna include the superior cistern, which contains a large blood vessel known as the great cerebral vein; and the ambient cistern, which can be considered an extension of the aforementioned opening.