Now we’ll move on, to look at the vitally important contents of the vertebral canal - the spinal cord, the spinal nerves, and the protective layers of tissue that surround them.
We’ll look first at a cross-sectional view of the vertebral canal. This is a cut through the 6th thoracic vertebra. Here’s the spinal cord. It only part-way fills the vertebral canal.
On each side there are two lines of nerve filaments, one arising from the ventral aspect, and one from the dorsal aspect of the cord. These filaments form the spinal nerves, as we’ll see in a minute.
The spinal cord lies within this strong protective layer, the dura. The dura is lined on the inside by a loosely attached membrane, the arachnoid.
The cord is covered on the outside by a firmly attached membrane, the pia. The space between the arachnoid and the pia is called the sub-arachnoid space. In life it’s filed with cerebrospinal fluid.
The space between the dura and the wall of the vertebral canal is called the epidural space. It’s filled with fat, loose connective tissue, and blood vessels.
To see the contents of the vertebral canal from end to end, we’ll take a look from behind, at a dissection in which all the laminae have been divided along these lines, and removed.
Here’s the sacrum. Here’s the base of the skull. The tissues that occupy the epidural space have been removed, to give us a clear look at the dura. This is the dura. The sleeve of dura is called the dural sac. It’s open at the top end, and closed at the bottom.
Here at the base of the skull, the dural sac passes through the foramen magnum, becoming continuous with the layer of dura that surrounds the brain.
At the bottom end, within the vertebral canal of the sacrum, the dural sac tapers down to a point, at the level of the second sacral segment.
To look at the spinal cord, we’ll divide the dura in the mid-line and lay it aside. Here’s the spinal cord.
In the early embryo, the spinal cord extends the whole length of the vertebral column, but as development progresses the vertebral column grows much more rapidly than the cord. The cord ends up filling only the upper two thirds of the vertebral canal. The lower end of the cord in the adult is at the level of the first lumbar vertebra.