796 THE NEKVOUS SYSTEM. 



as the anterior roots of the spinal nerves. But there is no single complete segmental nerve i 

 the head. The very essence of the architecture of the head is a want of segmentation ; and th 

 character is shared by the cerebral nerves. In addition it must be borne in mind that, in relatic 

 to the mammalian head, there are organs which have no homologues in the trunk, and c 

 whose existence the essential arrangement of the cerebral nerves depends e.g. sense-organs an 

 gill-arches. 



Among the cerebral nerves there are several which possess a resemblance to one or other < 

 the elements of a typical spinal nerve. In the neck the origin of the fibres of the accessor 

 nerve is from the side of the spinal medulla, and it is in series with the motor roots of the vagi 

 glossopharyngeal, facial, and fifth nerves. His (as shown in the account of the develoj 

 ment of the nerves) has described the neuroblastic origin of the motor roots of these nervi 

 from the lateral part of the basal lamina of the primitive brain. They thus form a seri 

 apart lateral motor roots separable from the series of motor roots originating from the medi, 

 part of the basal lamina, comprising those of the third, fourth, sixth, and twelfth nerves ; tl 

 latter nerve roots being comparable to and in series with the anterior roots of the spinal nerve 

 The lateral motor roots are not represented in the spinal series except in the neck. It 

 questionable if there is any fundamental distinction between the lateral and anterior motor roo 

 of the cerebral nerves. The accessory fibres, for example, when traced into the spinal medull 

 have an origin from the anterior column of the spinal medulla, and only differ from the motor < 

 anterior root fibres of a spinal nerve in their different course to the surface. The ganglia in associi 

 tion with the cerebral nerves are comparable to the spinal ganglia. The trigeminal nerve, wit 

 the semilunar ganglion, the ganglion of the facial, the ganglia of the acoustic, of the glossi 

 pharyngeal and the vagus, and the transitory (Froriep's) ganglion of the hypoglossal nerves, ari: 

 from the brain in a comparable position, and in the same way as the spinal ganglia. But anothi 

 series of structures the sense organs of the lateral line, and the so-called " epibranchial " orgai 

 which are highly developed in lower vertebrates (e.g. elasmobranchs), and which appear transitori] 

 only, or are absent altogether in mammalian development, may possibly have a share in tl 

 formation of certain of these ganglia or parts of them (e.g. ciliary ganglion, genicular ganglioi 

 ganglia of the acoustic nerve, petrous ganglion of the glossopharyngeal, and the ganglio 

 nodosum of the vagus). 



Certain of the cerebral nerves are apparently distinctly segmental, supplying muscles derive 

 from the persisting myotomes of the head. The first three myotomes are said to give rise to tl 

 muscles of the eyeball. The first produces the superior rectus, inferior rectus, medial rectu 

 and inferior oblique muscles, and its segmental nerve is the oculo-motor. The second myoton 

 is said to produce the superior oblique muscle, and its segmental nerve is the trochlear. Tl 

 third myotome is said to produce the lateral rectus muscle, and its segmental nerve is tl 

 abducent. It has been asserted that the tongue muscles are derived from the last three or foi 

 cephalic and first cervical myotomes, and that the hypoglossal nerve is the segmental nerve f( 

 these myotomes, comprising the motor elements of several (four or five) segmental nerves. Tl 

 intervening myotomes between the first three and this occipital series disappearing, the corr< 

 spending elements of segmental nerves are supposed to be absent also (Fig. 666). 



Certain of the cerebral nerves are essentially related to the structures derived from and ass( 

 ciated with the pre-oral and post-oral visceral clefts and arches (Fig. 667). The trigeminal nen 

 is essentially the nerve of the mandibular arch. By its efferent root it supplies the muscles < 

 that arch. By its afferent root and branches it is related to (1) the fronto-nasal process (opl 

 thalmic division and ciliary ganglion) ; (2) the maxillary arch (maxillary nerve) ; and (3) tl 

 mandibular arch (mandibular nerve). The mandibular is at first the main nerve ; and th 

 maxillary division is sometimes regarded as a subordinate branch (prse-branchial, prse-tremati< 

 for the supply of the anterior margin of the cleft (mouth), with which the nerve is in relatioi 

 The ophthalmic nerve is sometimes regarded as a morphologically separate nerve. The nervt 

 to these arches have been compared to the anterior rami of spirial nerves, the branches whic 

 they supply to the forehead and temple (frontal, zygomatic, and auriculo-temporal) representin 

 the posterior rami. The ganglia on each division of the nerve are formed as extensions froi 

 the semilunar ganglion. 



The facial nerve is essentially the nerve of the second (hyoid) arch and the cleft in front c 

 that arch (spiracular cleft, auditory tube). Its motor root supplies the muscles of that arc 

 (stapedius, stylo-hyoid, and digastric), and the epicranial and facial muscles and platysm? 

 which are developments from 'the hyoid arch (Rabl). The chorda tympani nerve is regarde 

 as the subordinate (prae -branchial, prae-trematic) branch to supply the anterior margin of th 

 first post-oral cleft. It is possible that the genicular ganglion, with the nervus intermediu 

 and the chorda tympani, may, in part at least, represent the ganglionic and afferent element c 

 the nerve. Or the genicular ganglion, and the nerves in relation to it, may be associated wit' 

 an " epibranchial " sense-organ. 



The acoustic nerve, on the other hand, may be either the sensory element of the branchia 

 nerve, associated with the hyoid arch and first post-oral cleft, or it may represent the nerve or nerve 

 belonging to ancestral sense-organs of the lateral line. 



The glossopharyngeal is the branchial nerve of the third post-oral (thyreo-hyoid) arch an< 

 the cleft in front. Its efferent fibres supply the muscle of this arch, the stylopharyngeui 

 The superior constrictor of the pharynx is also assigned to this arch ; the middle and inferio 

 muscles to the fourth (first branchial) arch. The afferent portion of the nerve is possibl; 

 composed of two separate parts ; the petrous ganglion being associated with an epibranchia 

 or lateral line sense-organ, and the rest of the nerve forming the afferent fibres for the gill-cleJ 



