. Cunningham's Text-book of anatomy. Anatomy. 798 THE NEKVOUS SYSTEM. in the series of cerebral nerves. The optic stalk and optic cup have been regarded as a highly modified spinal ganglion; but there is insuperable difficulty in accepting this view. The peripheral processes do not become connected with either ectodermal or mesodermal structures, but become the tissue of the retina; while the central processes, growing backwards, envelop the optic stalk, and obtain connexions with the brain. The retina must be regarded as a highly modified nerve-layer, morphologically in series with the wall o
. Cunningham's Text-book of anatomy. Anatomy. 798 THE NEKVOUS SYSTEM. in the series of cerebral nerves. The optic stalk and optic cup have been regarded as a highly modified spinal ganglion; but there is insuperable difficulty in accepting this view. The peripheral processes do not become connected with either ectodermal or mesodermal structures, but become the tissue of the retina; while the central processes, growing backwards, envelop the optic stalk, and obtain connexions with the brain. The retina must be regarded as a highly modified nerve-layer, morphologically in series with the wall of the fore-brain ; and the ectodermal structure of superficial origin comparable to the olfactory ganglion or the auditory vesicle is the lens (which may possibly be homologous with a lateral line sense-organ). The optic nerve, optic chiasma, and optic tract are then to be looked upon as cerebral commissures, and not as nerves in the ordinary sense. The simplest and most primitive condition of the head, in relation to the morphology of the cerebral nerves, is found before the formation of the gill-clefts, when the salient features are Abducent nerve Hind-brain Trigeminus Optic cup and lens Trochlearis Acoustic nerve Otic capsule Second post-oral cleft Glossopharyngeal nerve Third post-oral cleft Auricular branch of vagus Froriep's ganglion Telencephalon .1 Oculo-motor nerve Ophthalmic nerve * Diencephalon. Root and trunk of the first cervical spinal nerve _/ - Accessorius [ner -Trunk of 2nd cervical spin Hypoglossal nerve Roots and trunks of "cervical spinal nerves 3-7 Olfactory bulb Fronto-nasal process Nasal pit Ocular fissure Lateral nasal process Maxillary nerve Maxillary process Mouth cleft Mandibular trunk Mandibular arch Hyoid arch First post-oral cleft Fourth post-oral cleft | Vagus nerve Second branchial arch First branchial arch | Thyreo-hyoid arch Facial nerve Fig. 667.—The Embryological Arrangement of the Cerebral Nerves. (Modified from Mall.) a tubular and s
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