Difference Between Spinal Cord And Meninges
The spinal cord is a long, somewhat cylindrical mass of nerve tissue lying in the spinal canal. Its average length is about 17.7 inches (45 centimeters) in men, and about 16.5 inches (42 centimeters) in women; however, the spinal cord makes up only about 2 percent of the weight of the central nervous system. It is surrounded and protected by the meninges and by the bony walls of the spinal canal. The cord has two pronounced enlargements—the cervical, in the upper part of the cord, and the lumbar, in the lower part.
Like the brain, the spinal column has both gray matter and white matter. In the spinal cord, however, the gray matter is on the inside, the white matter on the outside. The gray matter serves as a reflex center, receiving impulses carried along a vast number of sensory fibers. In many cases, reflex action is produced without the assistance of the cells in the brain. The gray matter is also a central switching station for nerve impulses coming into and leaving the spinal cord.
The white matter of the spinal cord consists of thick bundles of nerve fibers coated with myelin. The white matter is divided into a left half and a right half by two fissures running down the front and back of the cord. Each half is made up, in turn, of three distinct bundles of nerve fibers that form columns, or funiculi (singular: funiculus). Those in the front are called anterior; those at the side, lateral; and those at the back, posterior.
Some of the fibers in the posterior columns of the funiculi carry impulses up the cord to the brain. The sensory impulse that begins in the finger is carried by a nerve fiber to cells in the gray matter of the cord. From there it is relayed by the fibers of the white matter to the gray matter of the brain. Other bundles of fibers carry impulses from the brain to the motor cells of the spinal cord. These fibers are found in the anterior columns and, to a certain extent, in the lateral columns. Impulses from the brain—the motor impulses—begin in the gray matter of the brain, descend through the white matter of the brain and cord, and end in the motor cells in the gray matter of the cord.
The spinal cord and brain are enveloped in three membranes called the meninges. The meninges provide a framework that supports and protects the spinal cord and brain.
The pia mater is a thin, delicate structure. It adheres closely to the surface of the spinal cord and brain. The pia mater serves as a supporting cover for the brain and cord. It also carries the blood vessels that supply the outer surfaces of these two areas. The arachnoid is thin as well, and is separated from the pia mater by a space containing a watery fluid—the cerebrospinal fluid. The cerebrospinal fluid acts as a cushion, protecting the spinal cord and brain from injury. It also provides some nutrition to the nerve cells.
The dura mater adheres closely to the bones of the skull. In the spinal column, because the dura mater isn’t attached to the surrounding bones, it hangs loosely. In the skull the dura mater is a thick, double membrane. Its inner layer forms partitions that dip between the larger subdivisions of the brain. Between these partitions, and between the two layers of the dura mater, are enclosed large sinuses, or hollows. Venous blood from the brain collects in these sinuses before leaving the brain.