76 MAIN FEATURES OF STRUCTURE AND FUNCTION seven cervical vertebrae but eight pairs of cervical nerves because the first cervical nerve leaves the vertebral canal above the first cervical vertebra (Fig. 3.3). Therefore, the numbering of the cervical nerves does not follow the general rule given above. The spinal cord extends caudally only to the level of between the first and the second lumbar vertebrae. Whereas the upper spinal nerves pass approximately horizontally from the cord to the intervertebral foramen, the lower ones have to run obliquely downward in the vertebral canal to reach the corresponding intervertebral foramen, the distance between the site of exit from the cord and the site of exit from the canal increasing steadily (Fig. 3.3). Below the conus, the vertebral canal contains only spinal nerve roots running longitudinally. This collection of dorsal and ventral roots is called the cauda equina (the horsetail). The Spinal Cord Is Divided into Segments The part of the spinal cord giving origin to a pair of spinal nerves is called a spinal segment. There are, therefore, as many segments as there are spinal nerves. They are numbered accordingly, the first cervical segment giving origin to the first cervical nerves, and so on. There are no surface markings of the cord to indicate borders between the segments, hut the rootlets nevertheless outline them rather precisely (Fig. 3.5). The cervical enlargement (intumescence) corresponds to the fourth cervical (C4) segment through the second thoracic (T2 segment, the lumbar enlargement to the first lumbar (L1) segment through the second sacral (S2) segment. The difference in rostrocaudallevel between the spinal segments and the exit from the vertebral canal of the spinal nerves is of practical importance. Thus, identical symptoms may be provoked by a process close to the cord at one level and by one close to the intervertebral foramen at a considerably lower level (as should be clear from the above description; however, this does not concern the nerves in the cervical region). The Spinal Cord Consists of Gray and White Matter When cut transversely, the fresh spinal cord can be seen to consist of an outer zone of white matter and a central, H-shaped region of gray matter (Fig. 3.4). The arms of the H, extending dorsally and ventrally, are called the dorsal horn (cornu posterius) and ventral horn (cornu anterius), respectively. The gray matter extends as a column through the length of the spinal cord (Fig. 3.5). The central canal is seen as a narrow opening in the center of the cord. The central canal ends blindly in the caudal end of the cord, whereas it continues rostrally into the ventricular system of the brain (Fig. 3.1). The white matter of the cord contains axons running longitudinally. Some of these axons convey signals from the cord to higher levels of the central nervous system; others, from higher levels to the cord. => page 77    start page