Meninges Of Brain And Spinal Cord Compare Contrast Essay

Meninges of the Brain and Spinal Cord

Organization

The interior of the cranium and vertebral canal are lined with a layer of the meninges called the dura mater – which is actually separated from the vertebral column by the epidural space, with only a “potential” epidural space between the cranium and this layer of the meninges. The surface of the brain and spinal cord are covered with another layer of the meninges called pia mater. Between these two layers of the meninges are two spaces – the subdural space and subarachnoid space, and a third layer of the meninges called the arachnoid mater.

The subdural space – which is a potential site for haemorrhaging blood, in subdural haematoma (see clinical correlation below), intercept the arachnoid mater and dura mater, while the subarachnoid space lies beneath the arachnoid mater (hence the name “sub”-arachnoid) and between it (arachnoid mater) and the pia mater.

Introduction

The meninges (singular: meninx) is a Greek word for membrane. Meninges are connective tissue membranes that line the cranium and vertebral canal, and enclose the central nervous system (CNS-brain and spinal cord). It is made up of three layers, which from outwards to inwards or cranium/vertebrae to brain/spinal cord are:

  • dura mater,
  • arachnoid mater, and
  • pia mater.

Hence the pia mater are the closest to the CNS structures, while the arachnoid and dura mater are farthest from the CNS.

The meningeal coverings provide protection to the brain and spinal cord, accompany blood vessels to and from the CNS tissues, and channel the cerebrospinal fluid (CSF) around the surfaces of the brain and spinal cord.

The meninges also support these blood vessels and contains the CSF, within its subarachnoid space. These are the structures involved in meningitis – an inflammation of the meninges, which if severe, may become encephalitis – an inflammation of the brain.

Additionally, the dura mater is a thick tough fibrous and dense inelastic membrane which is conventionally described as consisting of two layers. The dura mater is considered the “hard meninges or pachymeninx”. The arachnoid mater is connected to the pia mater by many fine filamentous processes or webs (hence its name - “arachnoid”, meaning spiderlike). The pia mater is a thin, highly vascular membrane investing the brain and spinal cord. The arachnoid and pia mater are considered the “soft meninges or leptomeninges (leptomenix)”. The region between the arachnoid and pia mater is the subarachnoid space which is filled with CSF.

Embryology

Development of the Cranial Meninges

Embryogenesis of the meninges varies across vertebrate species. However, the human cranial meningeal development is still a matter of great controversy.

To the extent of present knowledge, the meningeal coverings of the brain develop from both neural crest-derived cells and a mesenchymal layer of the neural tube, during the early embryonic development. These cells of the mesenchymal layer and neural crest form the primordium of the meninges called primary meninx. The primary meninx differentiates, during embryonic development, into two layers of cells: the pachymeninges – a thick layer and future dura mater, and the leptomeninges which develop into the arachnoid and the pia mater.

Development of the Spinal Meninges

Early in the 4th week of embryonic development, the mesenchyme surrounding the neural tube condenses to form a membrane called the primordial meninx or meninges. The external layer of this membrane thickens to form the dura mater. The internal layer, the pia-arachnoid which is composed of the pia mater and arachnoid mater (leptomeninges), is derived from neural crest cells.

Fluid-filled spaces appear within the leptomeninges that soon coalesce to form the subarachnoid space. The development of the pia and arachnoid mater from a single layer is indicated in the adult by arachnoid trabeculae – numerous delicate strands of connective tissue that pass between the pia and arachnoid mater. These arachnoid trabeculae are the webs or fine filamentous processes formed by the arachnoid mater after embryonic development. Cerebrospinal fluid begins to form during the 5th week of development.

Histology

The meninges are essentially composed of a series of fibroblasts and/or arachnoid cells, as well as varying amounts of extracellular materials, fibres, and fluid-filled cisterns (openings or spaces).

Dura mater

Dura mater

The dura mater is a rigid but simple covering of the brain as well as an internal covering of the calvaria. It forms venous sinuses that drain blood and CSF. The border layer of the dura mater contains electron-dense cytoplasm and it is also characterized by:
  1. extracellular cisterns containing fuzzy amorphous materials,
  2. devoid of connective tissue fibres,
  3. contains a good number of cell junctions, and
  4. lacks tight junctions.

Dura mater is composed of greater (periosteal) and lesser (meningeal) laminae, which are formed mainly of collagen fibres that are aligned differently. A small amount of elongated, flattened fibrocytes are intermingled with large number of extracellular collagen fibrils.

There are four areas of infoldings (reflections) in the dura mater, and at these points, numerous arachnoid villi can be found. These areas of infoldings are:

  • the falx cerebri,
  • the tentorium cerebelli,
  • the falx cerebelli, and
  • the diaphragma sellae.

Arachnoid mater

Arachnoid mater

The arachnoid contains mechanical strong, flexible, and functional structures, having the following three distinct roles:
  • serve as a covering for the pulsatile brain,
  • a pathway of the CSF circulation, and
  • an anatomical structure for CSF absorption.

For these purposes, the arachnoid cells form numerous morphologically distinct cell junctions, intense cell to cell communications, pinocytotic activity, extracellular cisterns, and specialized segments such as arachnoid villi (arachnoid granulations).

The outer part of the arachnoid mater forms the arachnoid barrier layer, which is comprised of closely packed cells like those of an actual covering membrane. While the inner part of the arachnoid forms the arachnoid trabeculae which is steeped in the CSF of the subarachnoid space, to support the arachnoid barrier layer. Arachnoid barrier cells have a large, polygonal, and electron-lucent cytoplasm with an oval nucleus, showing a marked contrast to the long and flattened dural border cells.

The arachnoid barrier layer is composed of layer of cells with numerous cell junctions, and the presence of many tight junctions which serve as a barrier to the CSF.

Pia mater

This is a layer of cells with long, flattened processes forming the innermost part of the meninges. Pial cells show similarities with arachnoid trabecular cells and have close contact with their attenuated processes. They are composed of a smooth-surfaced, thin layer of cells joined by desmosomes and gap junctions, but no tight junctions are observed.

Gross Anatomy

The gross features of the meninges will be discussed under each of its layers as follows:

Dura mater

Dura mater spinalis

This layer is conventionally described as consisting of an outer and an inner layer. The outer layer is often called the endosteal layer (periosteal dura), and the inner, the fibrous or meningeal layer (meningeal dura). However, there is no need for these somewhat confusing alternatives; all are fibrous and all are part of the meninges. This two layer concept is to some extent, false because the outer layer is actually the periosteum which invests the surface of the cranium (just like periosteum invests the surface of any bone). The inner layer of the dura mater is very different; it consists of a dense, strong fibrous membrane, which is really the dura mater proper. The middle meningeal artery and middle meningeal veins lie between the periosteal dura and meningeal dura.

There are folds or reflections of the inner layer (meningeal layer) which project into the cranial cavity. One such fold, the tentorium cerebelli roofs in the posterior cranial fossa; another forms the falx cerebri, lying in the midline between the two cerebral hemispheres (fig.3). The falx cerebelli and diaphragma sellae are smaller derivatives of the inner dura layer; all four folds are described as follows:

Falx cerebri

This is the largest of all the folds. It is sickle-shaped and lies in the midline between the two cerebral hemispheres. Its front end is narrow and is attached to the internal frontal crest and crista galli. Posteriorly it is broad and blends with the upper surface of the tentorium cerebelli. In the upper fixed margin of the falx cerebri, runs the superior sagittal sinus, while in the lower free margin runs the inferior sagittal sinus. The straight sinus runs along its attachment to the tentorium cerebelli.

Falx cerebelli

This is a vertical infolding that lies inferior to the tentorium cerebelli, separating the cerebellar hemispheres. It is a small, sickle-shaped septum (fold). It attaches (anteriorly) to the occipital crest, and its posterior free margin contains the occipital sinus.

Tentorium cerebelli

This is the second largest septum or fold of the meningeal dura. It is crescent-shaped and forms a roof over the posterior cranial fossa. It covers the upper surface of the cerebellum and supports the occipital lobes of the cerebral hemispheres. On the anterior aspect of the tentorium cerebelli, there is a gap – the tentorial notch, for the passage of the midbrain.

Three sinuses related to the tentorium cerebelli are:

Diaphragma sellae (Sellar diaphragm)

This is the smallest folding covering the pituitary gland and sella turcica. It is a circular-shaped fold that forms the roof of the sella turcica, and has a small opening in its centre that allows the passage of the stalk of the pituitary gland.

These four septa sent inwards, by the meningeal dura, divide the cranial cavity into freely communicating spaces lodging the subdivisions of the brain. They primarily function to restrict the rotatory displacement of the brain.

On the spinal cord, the spinal dura mater or theca is a prolongation of the inner layer of the dura mater of the posterior cranial fossa. It extends downwards through the foramen magnum to the level of S2 vertebra, thus extending below the level of spinal cord termination L1, and ensuring sufficient protection of the spinal cord by allowing space below the spinal cord level for CSF collection during a procedure like “Lumbar Puncture”. Superiorly the spinal dura mater is attached firmly to the tectorial membrane and to the posterior longitudinal ligament on the body of the axis vertebra (second cervical vertebra – C2), but it lies freely in the spinal canal. It is separated from the spinal canal by a layer of fat in which lies the internal vertebral venous plexus. It is also pierced by anterior and posterior roots of the spinal nerves.

Arachnoid mater and Subarachnoid space

Arachnoid mater spinalis

This layer of the meninges consists of an impermeable delicate membrane that is reinforced by the inner layer of the dura mater. However, between this impermeable membrane part of the arachnoid mater and the inner layer of the dura mater (meningeal dura), there is a space called the subdural space – containing lymph (a thin film of tissue fluid). Vessels and nerves pierce the dura and arachnoid mater both at the same place, and never run along between the two membranes.

Lying beneath the arachnoid, is the subarachnoid space. Separating the arachnoid and pia mater in this space are web-like strands called arachnoid trabeculae which also connects both mater. In the spinal part of the subarachnoid space, the arachnoid trabeculae are condensed into a thin posterior midline lamina that forms the incomplete posterior median septum.

The subarachnoid space provides a pathway for the circulation and absorption of the CSF after its escape from the 4th ventricle.

Structures connecting the surface of the brain with foramina necessarily pass through the subarachnoid space. Thus all the cranial nerves and roots of the spinal nerves traverse the space, as well as all the arteries and veins of the brain and spinal cord. The space extends down to the termination of the spinal arachnoid and dura at the level of S2 vertebra.

In certain areas, the arachnoid herniates (protrudes) through little holes in the dura mater into the venous sinuses. Such herniae are the arachnoid villi; through their walls, the CSF “oozes” (is absorbed) back into the blood stream. In adult, these arachnoid villi are aggregated into visible clumps called the arachnoid granulations (Pacchionian bodies).

Resulting from the contours of the brain and cranium, spaces form in the subarachnoid. These spaces form the subarachnoid cisterns. The largest of these cisterns is the cerebellomedullary cistern (cisterna magna). Others are the pontine cistern, interpeduncular cistern and the chiasmatic cistern.

The spinal arachnoid mater shows similar arrangement and features to that of the cranial arachnoid mater. However, the arachnoid trabeculae are condensed into a thin posterior midline lamina that forms an incomplete posterior median septum.

The spinal subarachnoid space is relatively large, accommodating about half of the total volume of the CSF (75ml out of 150ml). It communicates through the foramen magnum with the subarachnoid space of the posterior cranial fossa. In the spinal canal, the space ends at the level of the S2 vertebrae.

Pia mater

This meningeal layer invests the brain and spinal cord as periosteum invests bone. It contains blood vessels, and invests the surface of the CNS to the depths of the deepest fissures and sulci, following the curvatures of the brain surfaces – gyri and sulci. It is prolonged out over the cranial nerves and spinal nerve roots to fuse with their epineurium, and it is invaginated into the substance of the brain by the entering cerebral arteries.

On the spinal cord, the pia mater, as in the cranium, invests the surface of the central nervous system. It clothes the spinal cord and enters to line the anterior median sulcus. It is prolonged over the spinal nerve roots and blends with their epineurium. It is projected below the apex of the conus medullaris, whence it extends as the filum terminale to perforate the spinal theca (spinal dura mater) at the level of S2 vertebra.

The pia mater projects laterally, on each side of the midline, to form the denticulate ligament.

Blood Supply

The blood supply to the meninges generally concerns the blood supply to the outer layer of the dura mater (periosteal dura) rather than the inner layer of the inner layer of the dura (meningeal dura), arachnoid mater, or the pia mater. These two meningeal layers and the inner layer of the dura mater, require very little blood to derive nourishment. All the arteries and veins of the meninges lie between the two layers of the dura mater.

In the supratentorial part of the periosteal dura, the arterial supply is by the:

In the anterior cranial fossa, the arterial supply includes:

  • the meningeal branches of the ophthalmic artery
  • the meningeal branches of the anterior ethmoidal artery
  • the meningeal branches of the posterior ethmoidal artery

Over the cavernous sinus, the periosteal dura is supplied by:

In the middle cranial fossa, it is supplied by:

  • the ascending pharyngeal artery
  • the anterior (frontal) and posterior (parietal) branches of the middle meningeal artery

The middle meningeal artery is a branch of the maxillary artery. It is the most clinically significant arterial supply of the meninges because of its location in the extradural space, and the proximity of its frontal (anterior) branch to the pterion (the point of the cranium where the parietal and temporal bones meet the greater wing of the sphenoid bone) of the skull, making it susceptable to damage in head injuries.

In addition, the frontal branch also lie over the precentral gyrus, and as a result, haemorrhage from it causes pressure on the motor area.

In the posterior cranial fossa, the cranial periosteal dura is supplied by:

  • meningeal branches of the vertebral artery
  • occipital arteries
  • ascending pharyngeal arteries

Like the cranial meninges, the pia mater, arachnoid mater and meningeal dura of the spinal meninges require very little nourishment from blood. Hence the spinal theca is supplied by:

  • meningeal branches of the vertebral artery

Venous Drainage

The venous return of the dura mater is by the middle meningeal veins which accompany the branches of the middle meningeal artery.

Innervation

The nerve supply of the dura mater is as described below.

In the supratentorial part, it is innervated by:

In the anterior cranial fossa, its nerve supply includes:

  • the ophthalmic division of the trigeminal nerve (nasociliary branch)
  • the anterior ethmoidal nerve
  • the posterior ethmoidal nerve

In the middle cranial fossa, the nerve supply includes:

In the posterior cranial fossa, it is supplied by:

  • 2nd and 3rd cervical branches of the C2 and C3 nerves
  • meningeal branches of the vagus (CN X) and hypoglossal (CN XII) nerves, that is the tenth and twelfth cranial nerves; with contributions from C1 and C2 – the first and second cervical nerves

Clinical Application

Lumbar Puncture

Knowledge of anatomy of the meninges is highly useful in this clinical procedure. The needle is usually inserted between the spines of L3 and L4 or L4 and L5 vertebrae when the patient's back is flexed. This procedure is usually performed to extract cerebrospinal fluid for clinical diagnosis. Since the spinal cord ends at the level of L1 vertebra, it is in no danger by the needle, which usually penetrates the dura and arachnoid mater to access the CSF at the level of the vertebral – between L3 and L4 or L4 and L5. Recall that the spinal dura and arachnoid mater extend to terminate at the level of S2 vertebra.

Spinal Anaesthesia

In this procedure, the anaesthetic solution is injected into the subarachnoid space (with the needle in a similar position to that used for lumbar puncture), so mixing with the CSF surrounding the nerve roots and percolating (filtering gradually) into them.

Epidural Anaesthesia

This is commonly used in child birth. The anaesthetic solution is injected into the epidural (extradural) space, without penetrating as far as the dura mater. In this way the solution can infiltrate through the meningeal sheaths containing the lumbar and sacral nerve roots. The approach is also similar to that for lumbar puncture.

Subdural Haematoma (SDH)

SDH, as illustrated in the diagram above, is a collection of blood (clotting blood) that forms in the subdural space. Usually resulting from tears in bridging veins which cross the subdural space, subdural hemorrhages may cause an increase inintracranial pressure (ICP), which can cause compression of and damage to delicate brain tissue. A severe SDH may lead to loss of consciousness (but very rare), and lucidness of a few hours. Treatment includes craniotomy and clot evacuation.

Epidural Haematoma (EDH)

This is also called extradural haematoma, and it is the accumulation of blood in the epidural space due to injury, for example from a road traffic accident or sports injury, involving the middle meningeal artery. Treatment may require decompression of the haematoma, usually by craniotomy to ease-out the pressure cause by the clothed blood on the brain.

Meningitis

This is also referred to as leptomeningitis, and it is an inflammation of the arachnoid and pia mater, and most often also involving the subarachnoid space. It is caused by bacteria, viruses or fungi infections, and may lead to coma and death in less than 24 hours. Long term effects of meningitis include deafness and blindness, which may be caused by the compression of cranial nerves and brain areas responsible for the sense of hearing and sight.

Meningioma

This is tumor arising from the meninges. It is mostly benign (meaning non-cancerous), but can also be malignant (cancerous). However, there are also atypical meningiomas, that is, meningiomas that neither can be categorized as benign nor malignant. Meningiomas occur in males at different age, but older woman are most susceptable.

Henry Gray   Anatomy of the Human Body.  
 
 
The brain and medulla spinalis are enclosed within three membranes. These are named from without inward: the dura mater, the arachnoid, and the pia mater.
 
The Dura Mater
  The dura mater is a thick and dense inelastic membrane. The portion which encloses the brain differs in several essential particulars from that which surrounds the medulla spinalis, and therefore it is necessary to describe them separately; but at the same time it must be distinctly understood that the two form one complete membrane, and are continuous with each other at the foramen magnum.
  The Cranial Dura Mater (dura mater encephali; dura of the brain) lines the interior of the skull, and serves the twofold purpose of an internal periosteum to the bones, and a membrane for the protection of the brain. It is composed of two layers, an inner or meningeal and an outer or endosteal, closely connected together, except in certain situations, where, as already described (page 654), they separate to form sinuses for the passage of venous blood. Its outer surface is rough and fibrillated, and adheres closely to the inner surfaces of the bones, the adhesions being most marked opposite the sutures and at the base of the skull its inner surface is smooth and lined by a layer of endothelium. It sends inward four processes which divide the cavity of the skull into a series of freely communicating compartments, for the lodgement and protection of the different parts of the brain; and it is prolonged to the outer surface of the skull, through the various foramina which exist at the base, and thus becomes continuous with the pericranium; its fibrous layer forms sheaths for the nerves which pass through these apertures. Around the margin of the foramen magnum it is closely adherent to the bone, and is continuous with the spinal dura mater.


 
 
Processes.—The processes of the cranial dura mater, which projects into the cavity of the skull, are formed by reduplications of the inner or meningeal layer of the membrane, and are four in number: the falx cerebri, the tentorium cerebelli, the falx cerebelli, and the diaphragma sellæ.
  The falx cerebri(Fig. 765), so named from its sickle-like form, is a strong, arched process which descends vertically in the longitudinal fissure between the cerebral hemispheres. It is narrow in front, where it is attached to the crista galli of the ethmoid; and broad behind, where it is connected with the upper surface of the tentorium cerebelli. Its upper margin is convex, and attached to the inner surface of the skull in the middle line, as far back as the internal occipital protuberance; it contains the superior sagittal sinus. Its lower margin is free and concave, and contains the inferior sagittal sinus.
  The tentorium cerebelli(Fig. 766) is an arched lamina, elevated in the middle, and inclining downward toward the circumference. It covers the superior surface of the cerebellum, and supports the occipital lobes of the brain. Its anterior border is free and concave, and bounds a large oval opening, the incisura tentorii, for the transmission of the cerebral peduncles. It is attached, behind, by its convex border, to the transverse ridges upon the inner surface of the occipital bone, and there encloses the transverse sinuses; in front, to the superior angle of the petrous part of the temporal bone on either side, enclosing the superior petrosal sinuses. At the apex of the petrous part of the temporal bone the free and attached borders meet, and, crossing one another, are continued forward to be fixed to the anterior and posterior clinoid processes respectively. To the middle line of its upper surface the posterior border of the falx cerebri is attached, the straight sinus being placed at their line of junction.


 
  The falx cerebelli is a small triangular process of dura mater, received into the posterior cerebellar notch. Its base is attached, above, to the under and back part of the tentorium; its posterior margin, to the lower division of the vertical crest on the inner surface of the occipital bone. As it descends, it sometimes divides into two smaller folds, which are lost on the sides of the foramen magnum.
  The diaphragma sellæ is a small circular horizontal fold, which roofs in the sella turcica and almost completely covers the hypophysis; a small central opening transmits the infundibulum.
 
Structure.—The cranial dura mater consists of white fibrous tissue and elastic fibers arranged in flattened laminæ which are imperfectly separated by lacunar spaces and bloodvessels into two layers, endosteal and meningeal. The endosteal layer is the internal periosteum for the cranial bones, and contains the bloodvessels for their supply. At the margin of the foramen magnum it is continuous with the periosteum lining the vertebral canal. The meningeal or supporting layer is lined on its inner surface by a layer of nucleated flattened mesothelium, similar to that found on serous membranes.
  The arteries of the dura mater are very numerous. Those in the anterior fossa are the anterior meningeal branches of the anterior and posterior ethmoidal and internal carotid, and a branch from the middle meningeal. Those in the middle fossa are the middle and accessory meningeal of the internal maxillary; a branch from the ascending pharyngeal, which enters the skull through the foramen lacerum; branches from the internal carotid, and a recurrent branch from the lacrimal. Those in the posterior fossa are meningeal branches from the occipital, one entering the skull through the jugular foramen, and another through the mastoid foramen; the posterior meningeal from the vertebral; occasional meningeal branches from the ascending pharyngeal, entering the skull through the jugular foramen and hypoglossal canal; and a branch from the middle meningeal.
  
  
  
  The veins returning the blood from the cranial dura mater anastomose with the diploic veins and end in the various sinuses. Many of the meningeal veins do not open directly into the sinuses, but indirectly through a series of ampullæ, termed venous lacunæ. These are found on either side of the superior sagittal sinus, especially near its middle portion, and are often invaginated by arachnoid granulations; they also exist near the transverse and straight sinuses. They communicate with the underlying cerebral veins, and also with the diploic and emissary veins.
  The nerves of the cranial dura mater are filaments from the semilunar ganglion, from the ophthalmic, maxillary, mandibular, vagus, and hypoglossal nerves, and from the sympathetic.
  The Spinal Dura Mater (dura mater spinalis; spinal dura) (Fig. 767) forms a loose sheath around the medulla spinalis, and represents only the inner or meningeal layer of the cranial dura mater; the outer or endosteal layer ceases at the foramen magnum, its place being taken by the periosteum lining the vertebral canal. The spinal dura mater is separated from the arachnoid by a potential cavity, the subdural cavity; the two membranes are, in fact, in contact with each other, except where they are separated by a minute quantity of fluid, which serves to moisten the apposed surfaces. It is separated from the wall of the vertebral canal by a space, the epidural space, which contains a quantity of loose areolar tissue and a plexus of veins; the situation of these veins between the dura mater and the periosteum of the vertebræ corresponds therefore to that of the cranial sinuses between the meningeal and endosteal layers of the cranial dura mater. The spinal dura mater is attached to the circumference of the foramen magnum, and to the second and third cervical vertebræ; it is also connected to the posterior longitudinal ligament, especially near the lower end of the vertebral canal, by fibrous slips. The subdural cavity ends at the lower border of the second sacral vertebra; below this level the dura mater closely invests the filum terminale and descends to the back of the coccyx, where it blends with the periosteum. The sheath of dura mater is much larger than is necessary for the accommodation of its contents, and its size is greater in the cervical and lumbar regions than in the thoracic. On each side may be seen the double openings which transmit the two roots of the corresponding spinal nerve, the dura mater being continued in the form of tubular prolongations on them as they pass through the intervertebral foramina. These prolongations are short in the upper part of the vertebral column, but gradually become longer below, forming a number of tubes of fibrous membrane, which enclose the lower spinal nerves and are contained in the vertebral canal.


 
 
Structure.—The spinal dura mater resembles in structure the meningeal or supporting layer of the cranial dura mater, consisting of white fibrous and elastic tissue arranged in bands or lamellæ which, for the most part, are parallel with one another and have a longitudinal arrangement. Its internal surface is smooth and covered by a layer of mesothelium. It is sparingly supplied with bloodvessels, and a few nerves have been traced into it.
 
The Arachnoid—The arachnoid is a delicate membrane enveloping the brain and medulla spinalis and lying between the pia mater internally and the dura mater externally; it is separated from the pia mater by the subarachnoid cavity, which is filled with cerebrospinal fluid.
  The Cranial Part (arachnoidea encephali) of the arachnoid invests the brain loosely, and does not dip into the sulci between the gyri, nor into the fissures, with the exception of the longitudinal. On the upper surface of the brain the arachnoid is thin and transparent; at the base it is thicker, and slightly opaque toward the central part, where it extends across between the two temporal lobes in front of the pons, so as to leave a considerable interval between it and the brain.
  The Spinal Part (arachnoidea spinalis) of the arachnoid is a thin, delicate, tubular membrane loosely investing the medulla spinalis. Above, it is continuous with the cranial arachnoid; below, it widens out and invests the cauda equina and the nerves proceeding from it. It is separated from the dura mater by the subdural space, but here and there this space is traversed by isolated connective-tissue trabeculæ, which are most numerous on the posterior surface of the medulla spinalis.
  The arachnoid surrounds the cranial and spinal nerves, and encloses them in loose sheaths as far as their points of exit from the skull and vertebral canal.
 
Structure.—The arachnoid consists of bundles of white fibrous and elastic tissue intimately blended together. Its outer surface is covered with a layer of low cuboidal mesothelium. The inner surface and the trabeculæ are likewise covered by a somewhat low type of cuboidal mesothelium which in places are flattened to a pavement type. Vessels of considerable size, but few in number, and, according to Bochdalek, a rich plexus of nerves derived from the motor root of the trigeminal, the facial, and the accessory nerves, are found in the arachnoid.
  The Subarachnoid Cavity (cavum subarachnoideale; subarachnoid space) is the interval between the arachnoid and pia mater. It is occupied by a spongy tissue consisting of trabeculæ of delicate connective tissue, and intercommunicating channels in which the subarachnoid fluid is contained. This cavity is small on the surface of the hemispheres of the brain; on the summit of each gyrus the pia mater and the arachnoid are in close contact; but in the sulci between the gyri, triangular spaces are left, in which the subarachnoid trabecular tissue is found, for the pia mater dips into the sulci, whereas the arachnoid bridges across them from gyrus to gyrus. At certain parts of the base of the brain, the arachnoid is separated from the pia mater by wide intervals, which communicate freely with each other and are named subarachnoid cisternæ; in these the subarachnoid tissue is less abundant.
 
Subarachnoid Cisternæ (cisternæ subarachnoidales) (Fig. 768).—The cisterna cerebellomedullaris (cisterna magna) is triangular on sagittal section, and results from the arachnoid bridging over the interval between the medulla oblongata and the under surfaces of the hemispheres of the cerebellum; it is continuous with the subarachnoid cavity of the medulla spinalis at the level of the foramen magnum. The cisterna pontis is a considerable space on the ventral aspect of the pons. It contains the basilar artery, and is continuous behind with the subarachnoid cavity of the medulla spinalis, and with the cisterna cerebellomedullaris; and in front of the pons with the cisterna interpeduncularis. The cisterna interpeduncularis (cisterna basalis) is a wide cavity where the arachnoid extends across between the two temporal lobes. It encloses the cerebral peduncles and the structures contained in the interpeduncular fossa, and contains the arterial circle of Willis. In front, the cisterna interpeduncularis extends forward across the optic chiasma, forming the cisterna chiasmatis, and on to the upper surface of the corpus callosum, for the arachnoid stretches across from one cerebral hemisphere to the other immediately beneath the free border of the falx cerebri, and thus leaves a space in which the anterior cerebral arteries are contained. The cisterna fossæ cerebri lateralis is formed in front of either temporal lobe by the arachnoid bridging across the lateral fissure. This cavity contains the middle cerebral artery. The cisterna venæ magnæ cerebri occupies the interval between the splenium of the corpus callosum and the superior surface of the cerebellum; it extends between the layers of the tela chorioidea of the third ventricle and contains the great cerebral vein.


 
  The subarachnoid cavity communicates with the general ventricular cavity of the brain by three openings; one, the foramen of Majendie, is in the middle line at the inferior part of the roof of the fourth ventricle; the other two are at the extremities of the lateral recesses of that ventricle, behind the upper roots of the glossopharyngeal nerves and are known as the foramina of Luschka. It is still somewhat uncertain whether these foramina are actual openings or merely modified areas of the inferior velum which permit the passage of the cerebrospinal fluid from the ventricle into the subarachnoid spaces as through a permeable membrane.
  The spinal part of the subarachnoid cavity is a very wide interval, and is the largest at the lower part of the vertebral canal, where the arachnoid encloses the nerves which form the cauda equina. Above, it is continuous with the cranial subarachnoid cavity; below, it ends at the level of the lower border of the second sacral vertebra. It is partially divided by a longitudinal septum, the subarachnoid septum, which connects the arachnoid with the pia mater opposite the posterior median sulcus of the medulla spinalis, and forms a partition, incomplete and cribriform above, but more perfect in the thoracic region. The spinal subarachnoid cavity is further subdivided by the ligamentum denticulatum, which will be described with the pia mater.
  The cerebrospinal fluid is a clear limpid fluid, having a saltish taste, and a slightly alkaline reaction. According to Lassaigne, it consists of 98.5 parts of water, the remaining 1.5 per cent. being solid matters, animal and saline. It varies in quantity, being most abundant in old persons, and is quickly secreted.
  The Arachnoid Villi (granulationes arachnoideales; glandulæ Pacchioni; Pacchionian bodies) (Fig. 769) are small, fleshy-looking elevations, usually collected into clusters of variable size, which are present upon the outer surface of the dura mater, in the vicinity of the superior sagittal sinus, and in some other situations. Upon laying open the sagittal sinus and the venous lacunæ on either side of it villi will be found protruding into its interior. They are not seen in infancy, and very rarely until the third year. They are usually found after the seventh year; and from this period they increase in number and size as age advances. They are not glandular in structure, but are enlarged normal villi of the arachnoid. As they grow they push the thinned dura mater before them, and cause absorption of the bone from pressure, and so produce the pits or depressions on the inner wall of the calvarium.


 
 
Structure.—An arachnoidal villus represents an invasion of the dura by the arachnoid membrane, the latter penetrates the dura in such a manner that the arachnoid mesothelial cells come to lie directly beneath the vascular endothelium of the great dural sinuses. It consists of the following parts: (1) In the interior is a core of subarachnoid tissue, continuous with the meshwork of the general subarachnoid tissue through a narrow pedicle, by which the villus is attached to the arachnoid. (2) Around this tissue is a layer of arachnoid membrane, limiting and enclosing the subarachnoid tissue. (3) Outside this is the thinned wall of the lacuna, which is separated from the arachnoid by a potential space which corresponds to and is continuous with the subdural cavity. (4) And finally, if the villus projects into the sagittal sinus, it will be covered by the greatly thinned wall of the sinus which may consist merely of endothelium. It will be seen, therefore, that fluid injected into the subarachnoid cavity will find its way into these villi, and it has been found experimentally that it passes from the villi into the venous sinuses into which they project.
 
The Pia Mater—The pia mater is a vascular membrane, consisting of a minute plexus of bloodvessels, held together by an extremely fine areolar tissue and covered by a reflexion of the mesothelial cells from the arachnoid trabeculæ. It is an incomplete membrane, absent probably at the foramen of Majendie and the two foramina of Luschka and perforated in a peculiar manner by all the bloodvessels as they enter or leave the nervous system. In the perivascular spaces, the pia apparently enters as a mesothelial lining of the outer surface of the space; a variable distance from the exterior these cells become unrecognizable and are apparently lacking, replaced by neuroglia elements. The inner walls of these perivascular spaces seem likewise covered for a certain distance by the mesothelial cells, reflected with the vessels from the arachnoid covering of these vascular channels as they traverse the subarachnoid spaces.
  The Cranial Pia Mater (pia mater encephali; pia of the brain) invests the entire surface of the brain, dips between the cerebral gyri and cerebellar laminæ, and is invaginated to form the tela chorioidea of the third ventricle, and the choroid plexuses of the lateral and third ventricles (pages 840 and 841); as it passes over the roof of the fourth ventricle, it forms the tela chorioidea and the choroid plexuses of this ventricle. On the cerebellum the membrane is more delicate; the vessels from its deep surface are shorter, and its relations to the cortex are not so intimate.


 
  The Spinal Pia Mater (pia mater spinalis; pia of the cord) (Figs. 767,770) is thicker, firmer, and less vascular than the cranial pia mater: this is due to the fact that it consists of two layers, the outer or additional one being composed of bundles of connective-tissue fibers, arranged for the most part longitudinally. Between the layers are cleft-like spaces which communicate with the subarachnoid cavity, and a number of bloodvessels which are enclosed in perivascular lymphatic sheaths. The spinal pia mater covers the entire surface of the medulla spinalis, and is very intimately adherent to it; in front it sends a process backward into the anterior fissure. A longitudinal fibrous band, called the linea splendens, extends along the middle line of the anterior surface; and a somewhat similar band, the ligamentum denticulatum, is situated on either side. Below the conus medullaris, the pia mater is continued as a long, slender filament (filum terminale), which descends through the center of the mass of nerves forming the cauda equina. It blends with the dura mater at the level of the lower border of the second sacral vertebra, and extends downward as far as the base of the coccyx, where it fuses with the periosteum. It assists in maintaining the medulla spinalis in its position during the movements of the trunk, and is, from this circumstance, called the central ligament of the medulla spinalis.
  The pia mater forms sheaths for the cranial and spinal nerves; these sheaths are closely connected with the nerves, and blend with their common membranous investments.
  The ligamentum denticulatum (dentate ligament) (Fig. 767) is a narrow fibrous band situated on either side of the medulla spinalis throughout its entire length, and separating the anterior from the posterior nerve roots. Its medial border is continuous with the pia mater at the side of the medulla spinalis. Its lateral border presents a series of triangular tooth-like processes, the points of which are fixed at intervals to the dura mater. These processes are twenty-one in number, on either side, the first being attached to the dura mater, opposite the margin of the foramen magnum, between the vertebral artery and the hypoglossal nerve; and the last near the lower end of the medulla spinalis.



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