Let’s see some more details. These are nerve roots - we’ll look at them in a minute. The cord is attached to the dura by a series of fine, triangular ligaments, the denticulate ligaments. To see how the spinal nerves arise, we’ll go up to the cervical region. The denticulate ligaments have been divided.
Each spinal nerve arises from a small bundle of dorsal filaments, which unite to form the dorsal, sensory root of the nerve, and a similar bundle of ventral filaments, which unite to form the ventral, motor root.
In the cervical region, the nerve roots follow a slightly oblique, downward course. In the thoracic region their course becomes more oblique.
Here right at the lower end of the spinal cord, this continuous line of nerve filaments gives rise to a large number of nerve roots, which run vertically downward, almost hiding the very end of the cord, which is here.
Below this point, the dural sac is occupied not by the cord, but by this leash of vertically running lumbar and sacral nerve roots, the cauda equina. The nerve roots leave the vertebral canal, two at at a time, all the way down to the lower end of the sacrum.
Let’s follow the course of one spinal nerve, as it passes from inside the sub-arachnoid space, to its emergence from the intervertebral foramen.
To see this we’ll look at the cervical spine, in a dissection in which all the surrounding muscles have been removed, and in which the laminae have also been removed, along these lines, as in the previous dissection.
Here are the roots of the nerve, leaving the dural sac. Here’s the nerve emerging from the intervertebral foramen. To see the whole course of the nerve, we need to remove this part of the vertebra.
Here’s the dorsal root of the nerve, here’s the ventral root. The sleeve of dura that surrounds the converging nerve roots merges with the outer layer of the spinal nerve. This thickening at the very beginning of the spinal nerve is the dorsal root ganglion.
The spinal nerve passes forward and laterally, to emerge from the intervertebral foramen. To see that more clearly, we’ll go back to the preceding stage of the dissection. As the spinal nerve emerges, it divides, into this small posterior primary ramus, and this much larger anterior primary ramus.
The posteror primary rami of the spinal nerves pass backward, to supply the muscles and skin on the back of the body. The anterior rami pass forward and laterally, to supply all the rest of the body. This anterior ramus is an unusually large one.
It’s one of a set of five large rami, between C5 and T1, which form the brachial plexus. The major nerves to the upper extremity emerge from the brachial plexus. The anterior rami from L1 to S3 are also large: they form the lumbar and sacral plexuses, which give rise to the nerves for the lower extremity. These plexuses are shown in volumes One and Two of this atlas.
There’s a man-made puzzle that we need to clear up, regarding the numbering of the spinal nerves. In the cervical region, each spinal nerve takes its number from the vertebra below it. But from T1 on down, each nerve takes its number from the vertebra above it. The result is that there’s one nerve, the one that emerges between C7 and T1, for which there’s no corresponding vertebra. It’s called “C8”.