. Chordate anatomy. Chordata; Anatomy, Comparative. 390 CHORDATE ANATOMY From each of these cells protoplasmic processes extend in two directions— one towards the central nervous system and one towards the periphery. The olfactory nerve is peculiar in its derivation from cells in the olfactory pit which extend their neurites towards the brain and hence are unipolar. The neural crest is formed of cells left between the neural tube and the skin when the neural tube separates from the skin. The crest, which is primarily continuous, becomes secondarily broken up into the series of cranial and spin
. Chordate anatomy. Chordata; Anatomy, Comparative. 390 CHORDATE ANATOMY From each of these cells protoplasmic processes extend in two directions— one towards the central nervous system and one towards the periphery. The olfactory nerve is peculiar in its derivation from cells in the olfactory pit which extend their neurites towards the brain and hence are unipolar. The neural crest is formed of cells left between the neural tube and the skin when the neural tube separates from the skin. The crest, which is primarily continuous, becomes secondarily broken up into the series of cranial and spinal ganglia. Development of S3nnpathetic Ganglia. The sympathetic ganglia of vertebrates are derived, like the neurilemma cells, from the dorsal (sensory) ganglia by the migration of cells ventrally along the nerves toward PLEXUS OF AUERBACH^^ PLtXUS OF UElSSNERIk. jLONGITUDINAL -PERITONEUM Fig. 346.—A stereogram of a portion of tlie small intestine, showing the arrangement of sympathetic neurons in the plexuses of Meissner and Auerbach. Motor cells are shown in black, sensory cells with white nuclei. (Redrawn after Kahn.) the dorsal aorta. They first appear as clusters of cells, each cluster connected with the nerve from which it arose, at the level of the aorta. In the head the ciliary, sphenopalatine, otic, and submaxillary ganglia are formed in this way. In the trunk the superior and inferior cervical ganglia, and the series of vertebral and prevertebral ganglia belonging to the sympathetic are derived from the neural crest by the prolonged migration of nerve cells. In the sympathetic ganglia the nerve cells "spin" the postganglionic fibers to the blood-vessels and viscera. Con- nexions with the nerves from which the sympathetic ganglia arise appear as rami commimicantes. The metameric ganglia become secondarily connected by a sympathetic cord which runs parallel with the dorsal aorta. The prevertebral plexuses, cardiac, celiac, and hypogastric, arise by the mo
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