798 



THE NERVOUS SYSTEM. 



in the series of cerebral nerves. The optic stalk and optic cup have been regarded as a high 

 modified spinal ganglion ; but there is insuperable difficulty in accepting this view. T] 

 peripheral processes do not become connected with either ectodermal or mesodermal structure 

 but become the tissue of the retina ; while the central processes, growing backwards, envelc 

 the optic stalk, and obtain connexions with the brain. The retina must be regarded as a high 

 modified nerve-layer, morphologically in series with the wall of the fore-brain ; and the ectoderm 

 structure of superficial origin comparable to the olfactory ganglion or the auditory vesicle is tl 

 lens (which may possibly be homologous with a lateral line sense-organ). The optic nerve, opt 

 chiasma, and optic tract are then to be looked upon as cerebral commissures, and not as nerv 

 in the ordinary sense. 



The simplest and most primitive condition of the head, in relation to the morpholo^ 

 of the cerebral nerves, is found before the formation of the gill-clefts, when the salient features a 



Abducent nerve 

 Trigeminus 



Optic cup and lens I 



Trochlearis 



Telencephalon 



Oculo-motor nerve 



Ophthalmic nerve 

 Diencephal 



Hind-brain 



Acoustic nerve 

 I Otic capsule 



Second post-oral cleft 



Glossopharyngeal nerve 

 Third post-oral cleft 



Auricular branch of vagus 



Froriep's ganglion 



Root and trunk of 

 the first cervical 

 spinal nerve 



Accessor ins 



Trunk of 2nd cervicj 



Hypoglossal nerve 



Roots and trunks 

 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 

 First post-oral cleft 



| Fourth post-oral cleft 

 Vagus nerve 

 Second branchial arch 



First branchial arch 

 Thyreo-hyoid arch 

 Facial nerve 

 Hyoid arch 



FIG. 667. THE EMBRYOLOGICAL ARRANGEMENT OF THE CEREBRAL NERVES. (Modified from Mall.) 



a tubular and simple brain, and a series of superficial invaginations which pass from the surfa 

 inwards to become connected with outgrowths corresponding to them from the primitr 

 brain. On each side of the head three hollow invaginations occur : (1) The nasal pit bearii 

 the olfactory epithelium becomes connected by the olfactory ganglion with the rhinencephalon, 

 outgrowth from the fore-brain, and so forms the basis of an olfactory organ and nerve ; (2) a simiL 

 invagination produces the lens, connected with a protrusion of the optic vesicle from the for 

 brain, by which the basis of the eye and the optic nerve is formed ; (3) behind the oral cavil 

 a third invagination forms the auditory vesicle, which is connected with the solid extensic 

 from the hind-brain of the acoustic ganglia, to form the essentials of the organ of hearir 

 and acoustic nerve. 



The trigeminal nerve is essentially the nerve of the buccal cavity and the subordinai 

 cavities, nasal and oral, derived from it. The branchial arches and clefts are secondary structure 

 and their nerves are (1) the trigeminal, for the first (mandibular) arch and the cleft in froi 

 of it ; (2) the facial, for the second (hyoid) arch and cleft ; (3) the glossopharyngeal, for the thii 

 (thyreo-hyoid) arch and cleft; and (4) the vagus, for the succeeding arches and clefts. Th 

 cerebral part of the accessory nerve is inseparable from the motor portion of the vago-gloss< 

 pharyngeal nerves ; the spinal part is beyond the series of the cerebral nerves. 



Lastly, there are certain truly segmental nerve elements, motor fibres which, remainin 

 associated with certain persistent cephalic myotomes, give rise to the oculo-motor, trochlea: 

 abducent, and hypoglossal nerves. 



NOTE. Since 1913 an additional pair of cerebral nerves, the nervi terminates, has bee 

 known in man. The nerves were discovered in 1894 in protopterus, and since then they hav 

 been demonstrated in all groups of vertebrates. In man each nervus terminalis is a ver 

 small ganglionated nerve which is attached to the inferior surface of the frontal portion of th 

 brain in the region of the olfactory trigone. In the intracranial part of its course it lies medis 

 to the olfactory tract and bulb and its peripheral filaments accompany the filaments of th 

 olfactory nerve. The functions of the nervi terminales, the course of their fibres, and thei 

 associations in the substance of the brain are not known. 



