An American text-book of physiology . tem is best understood by means of schemas(Figs. 162 and 163). As the typical arrangement is found in the spinal cord,a cross section through this part will most readily express the facts. 640 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. The dorsal root-fibres among the spinal and cranial nerves, together withtheir homologues in the retina and tiie olfactory region, are the only channels forthe entrance of impulses intothe central system. Once havingarrived there, the impulses causeother cells to discharge, and thesein turn still others, through anindefinite serie
An American text-book of physiology . tem is best understood by means of schemas(Figs. 162 and 163). As the typical arrangement is found in the spinal cord,a cross section through this part will most readily express the facts. 640 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. The dorsal root-fibres among the spinal and cranial nerves, together withtheir homologues in the retina and tiie olfactory region, are the only channels forthe entrance of impulses intothe central system. Once havingarrived there, the impulses causeother cells to discharge, and thesein turn still others, through anindefinite series. The originalimpulse may thus arouse manyother impulses within the system,antl these spread until some ofthem reach cell-bodies which giverise to efferent fibres and whichdischarge awav from the centralsystem. The efferent fibres passout mainly by the ventral roots, butin part by the lateral (when pre-sent) or by the dorsal roots (). Such efferent fibres endeither directly in striated muscletissue, or in the neighborhood DP y-^ Fig. 162.—Schema of the arrangement of thehuman spinal cord ^s seen in cross section; forclearness the afferent fibres arc shown on theleft side only, efferent and central cells on theright side only (von Lenhossek): D. li, dorsalroot; V. i?, ventral root; D. P, direct pyramidalfibres; C. P, crossed pyramidal fibres; (, directcerebellar tract; A. Z,, antero-lateral tract: D. C,dorsal columns. The various classes of cell-bodies are indicated by the manner of draw-ing.
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