. Text-book of embryology. Embryology. II CEANIAL NEEVES 123 In the first place we find in the head region as in the trunk a tendency for the nerve-fibres to come off from the central nervous system in segmentally arranged clumps, and for the motor fibres to be situated more ventrally and the sensory more dorsally. In the head region however the dorsal root becomes reinforced by a large mass of motor fibres which have become shifted dorsalwards and incorporated with it. A neural crest develops resembling that of the trunk and in the Birds it can be seen similarly to have a paired origin, arisi
. Text-book of embryology. Embryology. II CEANIAL NEEVES 123 In the first place we find in the head region as in the trunk a tendency for the nerve-fibres to come off from the central nervous system in segmentally arranged clumps, and for the motor fibres to be situated more ventrally and the sensory more dorsally. In the head region however the dorsal root becomes reinforced by a large mass of motor fibres which have become shifted dorsalwards and incorporated with it. A neural crest develops resembling that of the trunk and in the Birds it can be seen similarly to have a paired origin, arising before the complete closure of the medullary tube. This neural crest of the brain region forms an anterior prolongation of that in the trunk: it is quite continuous with the latter, it develops outgrowths similarly, and the intervening portions here also persist for a time as a longi- tudinal commissure. A number of the most important cranial nerves. int. Fig. 68.—Acanthias, stage 23, 9 mm. long, showing ganglia of cranial and spinal nerves. (After Scarnmon, 1911.) int, intestine ; I, lens ; li, liver; pan, pancreas ; , ganglia of spinal nerves ; 27?,, thyroid ; V, ventricle ; , visceral cleft; , yolk-stalk ; IV, V, etc., ganglia of cranial nerves. are simply prolongations of the outgrowths in question—V, VII, VIII, IX and X. A conspicuous feature in the development of the cranial nerves is that in portions of their length they receive components directly from localized thickenings (placodes) of the ectoderm (Kupffer, Beard) a possible reminiscence of the time when nerve-trunks became evolved out of a plexus in direct relation to the external ectoderm. I. The Olfactory nerve is unrivalled amongst all the sensory nerves of the Vertebrates as a subject for investigation on account of its large size, its short uncomplicated course and its retention of comparatively primitive conditions even in the adult. Besearch should. therefore be specially concentrated upon i
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