Chapter 3 The Different Parts of the Nervous System 75 Figure 3.5 Two segments of the spinal cord (seen from the ventral aspect). In the upper part, the white matter has been removed. The dorsal and ventral roots emerge from the posterior and anterior lateral sulci, respectively, and unite to form spinal nerves. Note the location of the spinal ganglion at the site where the roots unite. lies in the intervertebral foramen just where the dorsal and ventral roots unite (Fig. 3.6). There is an important functional difference between the ventral and dorsal roots: The ventral roots consist of efferent (motor) fibers, and the dorsal roots of afferent (sensory) fibers. In total, 31 spinal nerves are present on each side, forming symmetrical pairs (Fig. 3.3). They all leave the vertebral canal through the intervertebral foramina on each side. As mentioned, the ventral and dorsal roots unite at the level of the intervertebral foramen to form the spinal nerves. The spinal nerves are numbered (as a general rule) in accordance with the number of vertebrae above the nerve. We therefore have 12 pairs of thoracic nerves, five pairs of lumbar nerves, and flve pairs of sacral nerves. In humans, there is only one pair of coccygeal nerves. There are Figure 3.6 Cross section of the vertebral column showing the positions of the spinal cord and the spinal nerves. Note the location of the spinal ganglion in the intervertebral foramen. The spinal cord is surrounded by the cerebrospinal fluid contained within the dura. Outside the dura, there is fat and avenous plexus, also serving as soft padding for the cord and the spinal nerves. => page 76    start page