390 MORPHOLOGY OF NERVES. 



Course and distribution. While it is obvious that in the distribution of the cranial 

 nerves the segmental arrangement, if it ever existed, has been in many cases greatly modified, 

 there are certain striking 1 relations between the course and distribution of some of these 

 nerves and the developmental subdivisions of the head. 



The distribution of the third, fourth, and sixuh nerves to the muscles of the first three 

 somites has already been pointed out. The nasal branch of the ophthalmic division of the 

 fifth is the nerve of the fronto-nasal process. The inferior maxillary is the nerve of the 

 mandibular arch, its motor root supplying- the muscles of that arch (Vol. II, p. 349), and its 

 sensory part extending by its splanchnic lingual branch into the part of the tongue which is 

 formed from the tuberculum impar in the concavity of the arch. The superior maxillary 

 nerve is the offset furnished by the more primitive mandibular nerve to the maxillary process. 

 The three divisions of the fifth supply the skin externally, and the mucous membrane inter- 

 nally, of their respective portions of the face ; and they send dorsal branches supraorbital 

 and supratrochlear, temporo-malar, and auriculo-temporal to ramify over the neural cavity 

 in the same way as the posterior divisions of the spinal nerves, the cutaneous area of which 

 they continue anteriorly. 



The facial is the nerve of the hyoid arch, supplying the muscles of that arch stapedius, 

 posterior belly of digastric, and stylo-hyoid and the system of cutaneous muscles which 

 according to Rabl take their origin in that arch (Vol. II, p. 349). 



The glosso-pharyngeal is the nerve of the third cephalic visceral (first branchial) arch, in 

 which it runs to the root of the tongue, but its lingual distribution extends considerably 

 beyond the part which is formed from that arch. It also supplies the muscle of the third 

 arch the stylo-pharyngeus. 



The vagus is a complex nerve. Its auricular branch, somatic in nature, is the diminished 

 representative of a largely developed somatic portion in fishes, known as the " lateral branch." 

 The superior laryngeal is the nerve of the fourth visceral (second branchial) arch, and the 

 inferior laryngeal of the fifth (third branchial) arch, the latter being drawn down so as to 

 acquire its recurrent character by the shifting of the arteries during development. The 

 remaining portion of the vagus extends far beyond the limits of the head, and supplies nearly 

 the whole of the alimentary canal with its derivatives, and other abdominal organs, the 

 greater part of which, it is to be noted, are developed from the f oregut or cephalic segment of 

 the enteric cavity. The accessory part of the spinal accessory in its distribution cannot be 

 separated from the vagus ; and the spinal portion of the same nerve is not morphologically 

 cranial, but entirely spinal both in origin and distribution. 



Lastly, the hypoglossal nerve, also spinal originally, is distributed to tongue-muscles, 

 which are also foreign to the head, being probably derived from the longitudinal system of 

 the ventral muscles of the trunk (Vol. II, p. 348). It is thus in series with the upper 

 cervical nerves supplying the muscles of the front of the neck, and the connection that 

 is formed between the hypoglossal and these nerves, giving rise to the ansa cervicalis, &c., 

 may be regarded as representing a prolongation of the cervical plexus. 



LITERATUBE OF THE MORPHOLOGY OF THE PERIPHERAL NERVES. 



Adolphi, H., Ueber Variationen der Spinalnerven und der Wirbelsdule anurer Amphibicn, 

 Morphol. Jahrh., xix, 1892. 



Beard, J., The Ciliary or Motoroculi Ganglion and the Ganglion of the ophthalmicus profundus 

 in Sharks, Anatom. Anzeiger, ii, 1887. 



Bolk, "L., Beziehungen zwischen SMet, Musculatur und N erven der- Extremitdten, dargelegt am 

 Betkengurtel, an dessen Musculatur , sowie am Plexus lumbo-sacralis, Morphol. Jahrb., xxi, 1894. 



Davidoff, M., Ueber die Varietdlen des Plexus lumbo-sacralis von Salamandr a maculosa, Morphol. 

 Jahrb., ix, 1884. 



Eisler, P., Der Plexus lumbo-sacralis des Menschen, Abhandlg. d. Naturforsch. Gesellsch. zu Halle, 

 1892. 



Ewart, J. C., On the Cranial Nerves of Elasmobranch Fishes, Proc. Roy. Soc , xlv, 1889 ; On the 

 Development of the Ciliary or Motor Oculi Ganglion, and The Cranial Nerves of the Torpedo, Proc. 

 Roy. Soc., xlvii, 1890. 



Froriep, A,, Ueber ein Ganglion des ffypoglossus und Wirbelanlagen in der Occipitalregion, 

 Arch. f. Anat., 1882 ; Ueber Anlagen von Sinnesorganen am Facialis, Glosso-pharynyeus und Vagus, 

 iiber die genetische Stettung des Vagus zum Hypoglossus, und iiber die Herlcunft der Zungenmusculatur, 

 Arch. f. Anat., 1885 ; Ueber das JJomologon der Chorda tympani bei niederen Wirbeltieren, Anatom. 

 Anzeiger, ii, 1887 ; Zur Frage der sogenannten Ncuromerie, Verhandlg. d. anatom. Gesellsch., 1892. 



Furbringer, M., Zur Lchre von den Umbildungen der Nervenplexus, Morphol. Jahrb., v, 1879. 



G-askell, W. H., On the Structure, Distribution, and Function of the Nerves which innervate the 

 Visceral and Vascular Systems, Journ. Physiol., vii, 1886 ; On the Relation between the Structure, 

 Function and Distribution of the Cranial Nerves, Preliminary Communication, Proc. Roy. Soc., xliii, 

 1888 ; On the Relation between the Structure, Function, Distribution and Origin of the Cranial 

 Nerves; together with a Theory of the Origin of the Nervous System of Vertebrata, Journ. Physiol., x, 

 1889. 



