THE SPINAL CORD =Structure of the Cord.=—The spinal cord is called the myelon or medulla spinalis. It is a comparatively large, white, irregularly cylindrical cord, flattened from above downward. It extends from the foramen magnum at the base of the medulla to the caudal portion of the spinal canal where it terminates in a fine filament. In order to allow considerable motion of the spinal column without danger of injury to the spinal cord, the cord is loosely suspended in the canal. The meninges, or coverings, are continued from the brain. In addition to these there are to be found arteries, veins, and nerves entering and others making their exit from the canal and from the cord. At the cervico-dorsal juncture, where the brachial plexus is given off, the cord is enlarged. There is another enlargement at the point where the lumbo-sacral plexus is given off. At this point, superiorly, there is a longitudinal cavity called the _sinus rhomboidalis_. This sinus contains a gelatinous substance. There is a pair of nerves given off at each intervertebral foramen. Thus there are as many pairs of spinal nerves as there are vertebral segments. The inferior root is the motor root and carries the impulses from the cord to the periphery. The superior root is the sensory branch and carries the impulses from the periphery to the cord, and thence the impulse is carried to the centers in the brain. The sensory and the motor roots are of about equal size. The inferior have more numerous filaments. The ganglion on the superior or sensory root is relatively large. In the sacral region the sensory and the motor branches pass through their own bony canal. White _rami communicantes_ are given off at each intervertebral foramen to the sympathetic system and gray rami are received from the sympathetic ganglia. Thus there is established a direct communication between the sympathetic and the spinal system. There is a ganglion on the superior root just outside the spinal cord. The ganglionic portion, or _gray matter_, is arranged in the center of the cord in two comma-shaped parts forming an X. The white matter, or the material forming the fibers, is arranged around the central ganglionic portion. The spinal cord may be divided into two lateral symmetrical halves. There are two longitudinal fissures, one on the upper and the other on the lower half. The upper called the _superior median fissure_ is narrow but deep. The one on the inferior side, called the _inferior median_, is usually more pronounced. The superior parts of the gray matter are called the superior cornua, or horns; the inferior parts the inferior cornua, or horns. The center of the cord is pierced by a _central canal_ which communicates with the fourth ventricle at the calamus scriptorius. STRUCTURE OF THE NERVE TRUNKS AND GANGLIA