Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 298 SYSTEMATIC ZOOLOGY. FIG. 108.—Diagrammatic section of spinal cord. d, dorsal nerve-root; g, gray matter; v, ventral nerve- root; w, white matter. The central system consists of an anterior brain, passing behind into the spinal cord. The brain is contained in the cranium; the spinal cord passes through the tube formed by the neural arches of the vertebrae. The spinal cord (fig. 108) is somewhat cylindrical, ta- pering behind, and contains in its centre a small canal. Nerves arise f
Elements of comparative zoology (1904) Elements of comparative zoology elementsofcompar1904king Year: 1904 298 SYSTEMATIC ZOOLOGY. FIG. 108.—Diagrammatic section of spinal cord. d, dorsal nerve-root; g, gray matter; v, ventral nerve- root; w, white matter. The central system consists of an anterior brain, passing behind into the spinal cord. The brain is contained in the cranium; the spinal cord passes through the tube formed by the neural arches of the vertebrae. The spinal cord (fig. 108) is somewhat cylindrical, ta- pering behind, and contains in its centre a small canal. Nerves arise from the cord in pairs in regular sequence, and pass out between the vertebrae to all parts of the body and to the limbs. Each of these spinal nerves has two places of origin (roots) from the cord—one near the dorsal, the other near the ventral surface, but after a short course these roots unite into a common trunk. These roots differ greatly in structure and func- tion. The dorsal root bears a nervous enlargement or ganglion; the ventral has no such structure. Experiment shows that the dorsal root is concerned in bringing sensa- tions to the central nervous system, and, if it be cut, the parts to which it goes will be without feeling. The ven- tral root, on the other hand, is motor; , it controls the action of muscles, glands, etc. If this root be cut, the parts which it supplies are paralyzed. Hence we may speak of the dorsal roots as afferent, since they bring sensa- tions to the central nervous system; while the ventral roots are efferent, because they carry nervous impulses in the opposite direction. The brain must be recognized as an enlarged and special- ized portion of the central nervous system. The canal of the spinal cord continues into the brain, enlarging there into four or more cavities or ventricles, connected by
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