Spinal Cord

[DEFINITION]

• An ovoid column of nervous tissue about 44 cm. long, flattened anteroposterioly, extending from the medulla to the 3nd lumbar vertebra in the spinal canal. All nerves to the trunk and limbs are issued from the spinal cord, and it is the center of reflex action containing the conducting paths to and from the brain. In cross section, it does not fill the vertebral space, being surrounded by the pia meter, the cerebrospinal fluid, the arachnoid, and the dura mater, which fuses with the periosteum of the inner surfaces of the vertebrae.

  The gray substance approximates the shape of an "H", there being a posterior and anterior horn in either half. The anterior horn is composed of motor cells from which the fibers making up the motor portions of the peripheral nerves arise. Sensory neurous enter posteriorly. The "H" also divides the surrounding white matter into posterior, lateral, and anterior bundles. These serve to connect brain and cord in both directions as well as various portions of the cord itself.