388 MORPHOLOGY OF NERVES. 



Segmentation. It has been shown in some of the lower vertebrates, especially elasmo- 

 branchs, that at least nine segments are included in the constitution of the head (see 

 Embryology, p. 161), and it is probable that the number is not less in the higher vertebrates, 

 although it may be that the segments have not the same morphological value in all cases, 

 since there is reason for believing that only six of these belonged primitively to the head, 

 while three or more, originally spinal, have also become incorporated in the head in later 

 stages of phylogenetic development. Assuming provisionally, however, that the nerves of nine 

 segments are represented more or less completely in the ten cranial nerves, there is still much 

 uncertainty as to the manner in which some of these nerves are to be allotted to the several 

 segments. It is also to be observed that, as in the case of the spinal nerves, the segmentation 

 which is to be recognized in the central grey matter does not always correspond to the 

 segmentation represented in the nerve-trunks, since fibres which arise from what appears to 

 be a single segmental nucleus may pass out in nerves at different levels, and conversely, a 

 single nerve may contain fibres derived from the nuclei of more than one segment. 



The third nerve belongs to the first head-segment. It includes a large-fibred mesial 

 somatic efferent part, distributed to the orbital muscles developed from the first somite, and a 

 small-fibred splanchnic efferent part passing to the ciliary ganglion. The lateral somatic 

 efferent fibres of this segment are perhaps represented, as suggested by Gaskell. by the ocular 

 facial, which arises from the oculomotor nucleus (p. 251). The ophthalmic division of the 

 fifth appears to be the afferent nerve of this segment. In the elasmobranch Hoffmann finds 

 that the " ramus ophthalmicus profundus," which corresponds to the nasal nerve of man 

 (Ewart), is developed independently of the rest of the fifth nerve, and in close relation to the 

 first somite. Gaskell considers, however, that the afferent fibres of the first segmental nerve. 

 with their stationary ganglion, have undergone degeneration, and are now represented only by 

 the vestigial structure in the roots of the third nerve ; and his view receives support from the 

 observation of Martin that in the early embryo of the cat the third nerve is provided with a 

 dorsal root, which subsequently disappears. 1 



The fourth nerve, supplying the superior oblique muscle formed from the second somite, 

 is the mesial somatic part of the second segmental nerve. The fifth nerve, excluding the 

 ophthalmic division, also belongs to this segment (Hoffmann), of which the small root of the 

 fifth will therefore form the lateral somatic efferent nerve, while the superior and inferior 

 maxillary portions of the large root will be the afferent nerve. The existence of splanchnic 

 efferent fibres is uncertain, since it is not known whether any efferent fibres are furnished by 

 the fifth nerve to the spheno-palatine, submaxillary, and otic ganglia, all of which must be 

 referred to this segment if, as is most probable, they are derived from the Gasserian ganglion, 

 although they receive efferent fibres from other segmental nerves (of. siqira). 



The third segmental nerve includes the sixth, seventh, and eighth cranial nerves. The 

 sixth is the mesial, and the facial proper the lateral somatic efferent portion together with, in 

 the large superficial petrosal branch, a part of the splanchnic efferent. In the portio inter- 

 media are comprised also splanchnic efferent fibres to the submaxillary ganglion and splanchnic 

 afferent fibres which are connected with the geniculate ganglion. The somatic afferent part 

 is the auditory, with the acoustic ganglion, the resemblance of which to the posterior root of 

 a spinal nerve has already been pointed out (p. 256). 



The fourth segmental nerve, according to Hoffmann, is suppressed. A rudiment is present 

 in the early embryo of the elasmobranch, but it afterwards aborts. It would seem possible 

 that the portio intermedia, in view of its central connection and partially independent course, 

 might belong to this segment. 



The nerve of the fifth segment is the glosso-pharyngeal. In this, mesial efferent, and 

 afferent somatic fibres are altogether wanting. Lateral somatic efferent fibres are present in 

 the branch to the stylo-pharyngeus, and probably arise from the accessory vago-glosso- 

 pharyngeal nucleus. Splanchnic efferent fibres run in the small superficial petrosal nerve to 

 the otic ganglion. The chief part of the nerve however is composed of splanchnic afferent 

 fibres, which enter the funiculus solitarius. 



The sixth and seventh segmental nerves are united in the vagus (Hoffmann), which con- 

 tains fibres of all groups except mesial somatic efferent (see p. 386). The latter fibres of the 

 seventh segment may be included in the hypoglossal nerve, which is composed of the mesial 

 somatic efferent parts of the last three or more cephalic (originally first spinal) segmental 

 nerves. Otherwise, these segmental nerves are only represented (doubtfully) by the oral 

 fibres of the facial which are said to arise from the hypoglossal nucleus (lateral somatic 

 efferent), and by the bulbar part of the spinal accessory (lateral somatic efferent and 

 splanchnic efferent), except in those cases in which the hypoglossal nerve is provided with a 

 gangliated dorsal root. 



The arrangement sketched out above is summed up in the annexed table. 



1 What appears to be a gangliated dorsal root to the oculomotor nerve has also been described by 

 Kupffer in Ammocostes, and by Froriep in Torpedo. 



