. Anatomie des chordés. Cordés ; anatomie, comparatif. Le système nerveux 341 tigroid neurofibrillae contiennent des substances et caractéristique des cellules nerveuses ou neurones du animaux supérieurs. Physiologiquement, cependant, ils sont plus simples que les cellules neurosensoriels, puisqu'ils transmettent des impulsions nerveuses dans une direction, tandis que les cellules neurosensoriels sont certainement polarisé et transmettre des impulsions dans un seul sens uniquement. Elles sont, par conséquent, inter- preted neurosensoriels comme des cellules qui ont perdu leur connexion principale avec la peau et leur polarité fonctionnelle. (Fig. 303) CELLULE NEUROSENSORIELLE NEURO bipolaire

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. Chordate anatomy. Chordata; Anatomy, Comparative. THE NERVOUS SYSTEM 341 contain tigroid substance and neurofibrillae characteristic of the nerve cells or neurons of the higher animals. Physiologically, however, they are simpler than neurosensory cells, since they transmit nervous impulses in any direction, while neurosensory cells are definitely polarized and transmit impulses in a single direction only. They are, therefore, inter- preted as neurosensory cells which have lost both their primary connexion with the skin and their functional polarity. (Fig. 303) NEUROSENSORY CELL BIPOLAR NEURONE ' ' UNIPOLAR NEURONE. Fig. 302.—A diagram illustrating hypothetical stages in the phylogenesis of the characteristic sensory (afferent) neuron of vertebrates. Earlier stages at the left, final stage at the right. Arrows suggest that the direction of growth of the neurite is away from the source of stimulus. The diagram assumes that the primitive neurosensory cell becomes the definitive sensory neuron and that the definitive sense receptor is secondary. It is quite possible, however, that the neuron is secondary and that the neurosensory cell becomes the definitive receptor. (Redrawn after Ariens-Kappers, modified.) In some coelenterates, the primitive ganglion cells form a loose subcu- taneous network or plexus in which, as experiments show, nervous impulses may be carried in any direction. Morphologists incUne to the opinion that the nerve-net of coelenterates becomes the central nervous system of higher animals. A final step in the evolution of the neuron is taken by the flatworms in which the neurons, like the neurosensory cells of coelenterates, trans-. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.. Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philade

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