Does the cranial dura consist of the periosteum? and if not, then is there a mistake in my textbook?

Question

My textbook (Neuroanatomy an illustrated colour text) states that:

The spinal dura and much of the cranial dura are separate from the periosteum, which forms the inner lining of the surrounding bones.

However, as far as I am aware the cranial dura has two layers, one of which is the outer periosteal layer or the periosteum, so I'm not sure why this is mentioned separate to the cranial dura. The textbook later states that:

At certain locations, however, such as on the floor of the cranial cavity, the dura and periosteum are fused and the cranial dura is tightly adherent to the interior surface of the skull.

This would make sense if they were referring to the pericranium, but the first quote suggests that the textbook is referring to the outer periosteal layer (or the inner periosteum).

Please could you shed some light on this. Thank you.

Answer 1

First about the terms:

• The outer side of the cranial bones is covered by a single layer of periosteum that is specifically called pericranium.
• The inner side of the cranial bones is covered by dura mater that has 2 layers:
  • the periosteal (or sometimes called endosteal) layer that is attached to the inner side of the bones
  • the meningeal layer that covers the periosteal layer and, at places, folds into the skull to make extensions

Image 1: Dura mater with its periosteal and meningeal layer (source: Idris Siddiqui, SlideShare)

Image 2: Extensions of the meningeal dura mater (falx cerebri, falx cerebelli, tentorium cerebelli) (source: Wikipedia, CC license)

The cranial dura mater has the periosteal and meningeal layer, but the spinal dura mater has only a meningeal layer, which encloses the spinal cord. Vertebra have no additional periosteal layer. The periosteal layer of the dura mater that covers the cranial bones does not extend to the spinal canal:

The periosteal or endosteal layer of dura mater is simply a layer of periosteum that covers the inner surface of the skull. The layer does not extend beyond the foramen magnum to become contiguous with the dura mater of the spinal cord. The spinal cord dura mater has no periosteal layer.

The meningeal layer of dura mater is a durable, dense fibrous membrane which passes through the foramen magnum and is continuous with the dura mater of the spinal cord. (V. Kekere, Dura mater, StatPearls)

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Both quotes from the book mentioned in the question suggest that the periosteum and dura are two separate things: periosteum being a part of the bone (as elsewhere in the skeleton) and dura consisting of a single layer. The other explanation, which I presented here, is that the periosteum on the inner side of the skull is a part of the dura (its periosteal layer) that has another layer (the meningeal layer). So, they are two ways how you can look at the structure of dura mater, and I believe, the second one is more widely accepted.