The Basic Plan and Ontogeny of the Spinal Cord

Abstract

The CNS consists of the brain and the spinal cord, which are contained entirely within the cranial vault and the vertebral canal. The CNS can be distinguished from the PNS, which is made up of neurons (most often contained in ganglia) and their processes (which bundle together toform nerves) that are distributed throughout the body. Given the delicate consistency of the CNS—something akin to aspic—it is fortunate that Mother Nature has encased this vulnerable organ within a hard bony fortress. Within the fortress, the CNS is further protected by three meningeal coatings. From outside in, the meninges are the thick and leathertough dura mater, the diaphanous arachnoid mater, and the pia mater, which is everywhere adherent to the surface of the CNS. The CNS is further cushioned by a layer of viscous liquid called the cerebrospinal fluid. It fills a space between the arachnoid and the pia mater called the subarachnoid space. The cerebrospinal fluid also fills the hollow core of the brain, the ventricular system, increasing further the shock-absorbing capacity of the brain. Although these raiments of the brain serve their wearer well, their indirect relationship to the workings of neuronal circuits places them beyond the scope of this survey. Here, we’ll focus on the wearer