The arachnoid mater (or simply arachnoid) is one of the three

meninges In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...

, the protective

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...

s that cover the

brain A brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It is located in the head, usually close to the sensory organs for senses such as vision. It is the most complex organ in a v ...

and

spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...

. It is so named because of its resemblance to a

spider web A spider web, spiderweb, spider's web, or cobweb (from the archaic word '' coppe'', meaning "spider") is a structure created by a spider out of proteinaceous spider silk extruded from its spinnerets, generally meant to catch its prey. Spid ...

. The arachnoid mater is a derivative of the

neural crest Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...

mesoectoderm in the embryo.

Structure

It is interposed between the two other meninges, the more

superficial Superficial may refer to: * Superficial anatomy, is the study of the external features of the body *Superficiality, the discourses in philosophy regarding social relation * Superficial charm, the tendency to be smooth, engaging, charming, slick an ...

and much thicker

dura mater In neuroanatomy, dura mater is a thick membrane made of dense irregular connective tissue that surrounds the brain and spinal cord. It is the outermost of the three layers of membrane called the meninges that protect the central nervous system. ...

and the deeper

pia mater Pia mater ( or ),Entry "pia mater"
in
subarachnoid space In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...

. The delicate arachnoid layer is not attached to the inside of the dura but against it and surrounds the brain and spinal cord. It does not line the brain down into its

sulci Sulci or Sulki (in Greek , Steph. B., Ptol.; , Strabo; , Paus.), was one of the most considerable cities of ancient Sardinia, situated in the southwest corner of the island, on a small island, now called Isola di Sant'Antioco, which is, howev ...

(folds), as does the pia mater, with the exception of the

longitudinal fissure The longitudinal fissure (or cerebral fissure, great longitudinal fissure, median longitudinal fissure, interhemispheric fissure) is the deep groove that separates the two cerebral hemispheres of the vertebrate brain. Lying within it is a continu ...

, which divides the left and right

cerebral hemisphere The vertebrate cerebrum (brain) is formed by two cerebral hemispheres that are separated by a groove, the longitudinal fissure. The brain can thus be described as being divided into left and right cerebral hemispheres. Each of these hemispheres ...

Cerebrospinal fluid Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates. CSF is produced by specialised ependymal cells in the choroid plexus of the ventricles of the bra ...

(CSF) flows under the arachnoid in the subarachnoid space, within a meshwork of

trabeculae A trabecula (plural trabeculae, from Latin for "small beam") is a small, often microscopic, tissue element in the form of a small beam, strut or rod that supports or anchors a framework of parts within a body or organ. A trabecula generally has ...

which span between the arachnoid and the pia. The arachnoid mater makes

arachnoid villi Arachnoid granulations (also arachnoid villi, and pacchionian granulations or bodies) are small protrusions of the arachnoid mater (the thin second layer covering the brain) into the outer membrane of the dura mater (the thick outer layer). They ...

, small protrusions through the dura mater into the

venous sinuses The dural venous sinuses (also called dural sinuses, cerebral sinuses, or cranial sinuses) are venous channels found between the endosteal and meningeal layers of dura mater in the brain. They receive blood from the cerebral veins, receive cere ...

of the brain, which allow CSF to exit the subarachnoid space and enter the blood stream. Unlike the dura mater, which receives a rich vascular supply from numerous arteries, the arachnoid mater and the deeper pia mater are generally avascular. The arachnoid mater and dura mater are very close together throughout the cranium and spinal canal all the way to S2, where the two layers fuse into one and end in the

filum terminale The filum terminale ("terminal thread") is a delicate strand of fibrous tissue, about 20 cm in length, proceeding downward from the apex of the conus medullaris. It is one of the modifications of pia mater. It gives longitudinal support to th ...

, which attaches to the coccygeal end of the spinal canal. Sandwiched between the dura and arachnoid maters lie some veins that connect the brain's venous system with the venous system in the dura mater. The arachnoid mater covering the brain is referred to as the "arachnoidea encephali," and the portion covering the spinal cord as the "arachnoidea spinalis." The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version,

leptomeninges In anatomy, the meninges (, ''singular:'' meninx ( or ), ) are the three membranes that envelop the brain and spinal cord. In mammals, the meninges are the dura mater, the arachnoid mater, and the pia mater. Cerebrospinal fluid is located in th ...

. ("Lepto"- from the Greek root meaning "thin"). Similarly, the dura in this situation is called the pachymeninx. There are two subdivisions of arachnoid mater surrounding the subarachnoid space, the dorsal layer and the ventral layer. The dorsal layer covers internal cerebral veins and fixes them to the surrounding

tela choroidea The tela choroidea (or tela chorioidea) is a region of meningeal pia mater that adheres to the underlying ependyma, and gives rise to the choroid plexus in each of the brain’s four ventricles. ''Tela'' is Latin for ''woven'' and is used to desc ...

. The ventral layer of arachnoid membrane, on the other hand, is a direct anterior extension of this arachnoid envelope that the dorsal layer forms over the pineal region.

Function

CSF circulates in the subarachnoid space (between arachnoid and pia mater). Cerebrospinal fluid is produced by the

choroid plexus The choroid plexus, or plica choroidea, is a plexus of cells that arises from the tela choroidea in each of the ventricles of the brain. Regions of the choroid plexus produce and secrete most of the cerebrospinal fluid (CSF) of the central nerv ...

(inside the ventricles of the brain, which are in direct communication with the subarachnoid space so the CSF can flow freely through the nervous system). Cerebrospinal fluid is a transparent, colourless fluid and it is produced at about 500 ml/day. Its electrolyte levels, glucose levels, and pH are very similar to those in plasma, but the presence of blood in cerebrospinal fluid is always abnormal."The intracranial arachnoid mater". Child's Nervous system; Jan2013, Vol29, Issue 1, p17-33, 17p. Authors: Adeeb, Nimer; Deep, Aman; Griessernauer, Christoph; Mortazavi, Martin; Watanabe, Koichi; Loukas, Marios; Tubbs, R.; Cohen, Aaron.

Etymology

The arachnoid mater is named after the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

word ''

arachne Arachne (; from , cognate with Latin ) is the protagonist of a tale in Greek mythology known primarily from the version told by the Roman poet Ovid (43 BCE–17 CE), which is the earliest extant source for the story. In Book Six of his ...

'' ("spider"), the suffix ''-oid'' ("in the image of"), and the Latin word ''mater'' ("mother"), because of the fine

-like appearance of the delicate fibres of the arachnoid (

arachnoid trabeculae The arachnoid trabeculae are delicate strands of connective tissue that loosely connect the two innermost layers of the meninges – the arachnoid mater and the pia mater. They are found within the subarachnoid space where cerebrospinal flui ...

) which extend down through the subarachnoid space and attach to the pia mater.

Additional images

File:Meninges-en.svg, Meninges File:Gray1196.png, Diagrammatic section of scalp. file:Sobo 1909 590.png, The arachnoid mater lies under the

, and arteries and veins run on top of it. File:Brain with arachnoid and pia mater.jpg, Brain with arachnoid mater, and an area where it is removed, showing cerebral gyri covered by the translucent pia mater. File:Spinal_dura_mater_1.jpg, Spinal dura mater opened, arachnoid mater visible. File:Gray767.png, The

medulla spinalis The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spin ...

and its membranes. File:Slide5sese.GIF, Spinal cord. Spinal membranes and nerve roots. Deep dissection. Posterior view. File:Slide6Neo.JPG, Meninges and superficial cerebral veins. Deep dissection. Superior view. File:Slide7Neo.JPG, Meninges and superficial cerebral veins. Deep dissection. Superior view. The arachnoid cells continue inside the brain covering the so call

Virchow-Robin space A perivascular space, also known as a Virchow–Robin space, is a fluid-filled space surrounding certain blood vessels in several organs, including the brain, potentially having an immunological function, but more broadly a dispersive role for ...

s or

perivascular space A perivascular space, also known as a Virchow–Robin space, is a fluid-filled space surrounding certain blood vessels in several organs, including the brain, potentially having an immunological function, but more broadly a dispersive role for ...

s. For that reason some meningiomas can appear as completely inside the brain.

References

* Orlando Regional Healthcare, Education and Development. 2004
"Overview of Adult Traumatic Brain Injuries."
Retrieved on 2008-01-16.

External links

*
Slide
{{Authority control Meninges