Artem has a doctor of veterinary medicine degree.
An orange has three protective layers. The first one is the thick and hard layer that gives the fruit its color. Beneath that is this white layer that resembles a spider's web as you pull the orange peel away. And the layer underneath that, the one that attaches directly to the pulp, is this very thin, semi-transparent, delicate membrane.
And, perhaps not too surprisingly, these layers remind me of the layers of the meninges, three fibrous membranes that enclose the central nervous system, which consists of the brain and spinal cord.
Let's see what they are called and why they remind me of an orange.
Alright so, our orange has an orange peel. This is the thick and hard outermost layer that surrounds the fruity goodness inside.
The central nervous system's thick and tough outermost meningeal layer is called the dura mater. 'Dura' means 'hard' and 'mater' means 'mother' in Latin, and so, the dura mater translates literally as 'hard mother.'
What in the world, does this connective tissue have to do with your mother? Well, the interesting story goes like this. A man named Stephen of Antioch, an Italian translator of the 12th century, was translating the work of a 10th century Persian physician called Haly Abbas into Latin, the common language of the learned European men at that period in time.
At the time of Abbas's writing, Arabic medicine believed the meninges were the source of all of the membranes in the body and used real-world familial relationships like mother, son and so forth, to describe the relationships between different tissues. And as you'll soon learn, all the layers of the meninges end with the Latin for mother, mater.
Anyways, inside of the cranium, the portion of the skull holding the brain, the dura mater is split into two layers itself:
- The outermost layer of the dura mater inside the cranium is equivalent to the internal periosteal lining of the bones of the cranium. It is like an inner lining of the bones that hold the brain (the cranial bones). This layer is also referred to as the endocranium, endosteal layer, or endosteum of the skull as well.
- The other layer, deeper to the endocranium, is known as the inner meningeal layer, and it's the one that covers the brain itself.
These two layers stick to one another in the cranium except when they are separated by structures called the dural venous sinuses (venous channels that allow for blood flow), ones that are similar to the epidural venous plexus of the vertebral (spinal) canal.
In the vertebral (aka spinal) canal, the cavity in the backbone where the spinal cord passes through, there is no periosteal layer to the dura mater, only the meningeal layer encloses the spinal cord. The vertebral canal (aka spinal canal, aka the spinal cavity) should not be confused with the epidural space, the space lying within the vertebral canal but outside the dura mater. An epidural (extradural) space is also possible between the cranial bones and the endocranium although this becomes less likely barring an underlying pathology as an individual gets older because the endocranium becomes superglued to the cranial bones with age.
As you peel the hard external orange layer of the orange itself, you'll notice this whitish layer pulling away from the fruit itself. As it pulls away, you can see strands create this spider web-like layer (see the second orange in the image above). You can even hear the strands ripping apart as you peel the hard external layer away.
This reminds me of the arachnoid mater, the delicate, web-like middle meningeal layer. Knowing what you now know about the arachnoid mater, its terminological origin shouldn't be a surprise since arachnoid refers to something that resembles a cobweb.
Under normal conditions, there shouldn't be any space between the arachnoid mater and dura mater. However, when there is an underlying pathology something known as a subdural space may exist between the two. This can happen, for example, during trauma from something like a car accident.
Finally, when you've peeled the thick outer orange peel and web-like white stuff from the orange, you're left with this final thin and delicate membrane covering the orange's pulp (see the third orange in the image above).
This is like the pia mater, the very thin, delicate, vascular and innermost layer of the meninges. 'Pia' means 'delicate.' Its role is to provide oxygen and nutrients to the brain and spinal cord.
To help remember the order of the meninges, think of the acronym PAD. A pad soaks things up from the bottom up. So the layers of the meninges from the bottom up are pia, arachnoid and then dura.
Anyways, the arachnoid mater is separated from the pia mater by the subarachnoid space, the space between the pia mater and arachnoid mater, filled with cerebrospinal fluid. The subarachnoid space has these strands of connective tissue crossing it, called arachnoid trabeculae, and they serve to connect the arachnoid mater and pia mater.
The cerebrospinal fluid (CSF) found in the subarachnoid space, ventricles of the brain and the central canal of the spinal cord serves to protect and cushion the brain and spinal cord. Another lesson goes into more detail about its function.
CSF is produced by the choroid plexus, a structure found in the ventricles (large spaces within) the brain. The choroid plexus itself is formed by the fusion of protruding invaginations of the pia mater and the ependymal cells lining the ventricles into the ventricles themselves.
The CSF makes its way from the ventricles, then into the subarachnoid space and finally into the venous dural sinuses (namely the superior sagittal sinus) by way of arachnoid granulations. Arachnoid granulations (aka arachnoid villi) are protrusions of the arachnoid mater through the dura mater. The arachnoid granulations allow for CSF to leave the subarachnoid space and be absorbed into the blood stream.
Now you're a pro at peeling and pulling apart oranges and the meninges!
The meninges are three fibrous membranes that enclose the central nervous system, which consists of the brain and spinal cord.
The thick and tough outermost meningeal layer is called the dura mater. In the cranium, it is composed of the endocranium and the inner meningeal layer. The spinal vertebral canal doesn't have an endosteal layer to its dura mater. The vertebral (spinal) canal, aka the spinal cavity, should not be confused with the epidural space, the space lying within the vertebral canal but outside the dura mater.
Lying deeper to the dura mater is the arachnoid mater, the delicate web-like, middle meningeal layer. It is separated from the pia mater, the very thin, delicate, vascular and innermost layer of the meninges, by the subarachnoid space, the space between the pia mater and arachnoid mater, filled with cerebrospinal fluid.
Watch the lesson on the layers of the meninges and then attempt to realize these objectives:
- Name the three meninges of the central nervous system
- Identify the two layers of the dura mater
- Specify the functions of cerebrospinal fluid and the choroid plexus
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