The precious central nervous system is surrounded with wrappers of various sorts that make possible vertebral movement and turning of the head without injuring the cord or brain. The inner envelopes immediately around the cord are termed the meninges. In higher vertebrates there are three meninges which, from the inside out, are named the pia mater, the arachnoidea, and the dura mater.
The pia mater bears blood vessels and consequently has a nutritive mission primarily. It also carries nerves which supply the blood vessels. It fits with tailor-like snugness close around the cord and the brain, descending into the fissures and every unevenness of the surface. It has been likened to the thin, close-fitting, bitter skin around the kernel of a wrinkled walnut.
Outside the pia is the arachnoid layer, forming a spiderweb-like meshwork of great delicacy, hence its name. This is succeeded by a dense connective tissue of protective nature, corresponding to the shell of the walnut, the dura mater, which fits within the skeletal envelope, that is, the neural arches of the vertebrae.
In the cranium the dura unites with the inner periosteum (endosteum) of the skull bones to form a single sheet of tissue (Fig. 625). In the cord, however, it is not in direct contact with the endorachis, the endosteal layer of connective tissue which lines the neural canal and lies against the inner surfaces of the neural arches and centra. Instead, dura and endorachis are separated from one another by a perimeningeal space filled with a fluid. In addition, the space between the dura and the arachnoid (subdural space), as well as that between the arachnoid and the pia (subarachnoid space), which is relatively of considerable size, is filled with a lymph-like cerebrospinal fluid, thus forming jackets of lubricating material that serve as shock absorbers, making possible the movement of the vertebrae upon each other without causing mechanical injury to the cord.
In lower fishes there is only a single meningeal layer, the meninx primitiva, separated from the endorachis by perimeningeal tissue (Fig. 626a).
In amphibians, reptiles, and birds, the meninx primitiva becomes split into dural and pial layers, with a subdural space between (Fig. 626b), while in mammals the inner pial layer is further differentiated into the arachnoid and the pia (Fig. 626c). The complete list of envelopes of the mammalian cord from the inside out is as follows: pia mater, subarachnoid fluid, arachnoid, subdural fluid, dura mater, perimeningeal fluid, endorachis, vertebral arch, tendons and muscles, and skin. Within the cranium the endorachis and the dura are contiguous, not being separated by a fluid-filled space as in the cord region.