130 NEKVOUS SYSTEM 



neuromere in front of the cerebellum with which the fourth nerve is connected, and an indist 

 ninth at the end of the series. 



The lateral motor roots have been interpreted as representing in the cerebral nerves 

 splanchnic efferent fibres which in the cord leave the axis by the ventral root, and are not 

 therefore separated from the somatic efferent fibres. They may be, on the other hand, represented 

 by fibres which Lenhossek and Ramon y Cajal have shown to arise in the ventral horn of the 

 embryonic cord (chick) and to run out in the dorsal roots. The cell column from which the 

 lateral roots spring may represent the lateral horn column of the cord. 



The accessory is (see below) ontogenetically a part of the vagus, wholly therefore a cerebral 

 nerve. From his studies of the occipital nerves Streeter concludes that ' in all higher vertebrates, 

 accompanying the conversion of certain gill-muscles into the trapezius and sternomastoid, the 

 cranial elements (i.e. vagus complex) make a caudal invasion of the spinal cord.' The result of 

 this invasion, added to the inclusion in the skull of the occipito-spinal segments, is a blurring of 

 the line of demarcation between the nerves of spinal and those of cranial type, which is distinct 

 in spite of the inclusion of the occipito-spinal segments in the skull in lower vertebrates. 1 



Numerous attempts have been made to bring the nerves of the head into a segmental scheme, 

 and to homologise the cerebral with the spinal nerves. None of these are quite convincing, and 

 there is still great uncertainty as to what interpretation should be put on the serial characters 

 of the branchial region. 



DEVELOPMENT OF INDIVIDUAL CEKEBEAL NEEVES. 



The hypoglossal 2 appears in the third week as a number of rootlets 

 arranged in three or four segmental groups in series with the cervical motor roots, 

 and connected with the occipital myotomes. During the fourth week they fuse 

 into one trunk as they pass towards the floor of the primitive mouth. Owing to 

 the bend in the neural tube, the hypoglossal and upper cervical nerves are 

 brought close together ' like the spokes of a wheel ' (Streeter) and grow side by 

 side into a mass of tissue out of which the tongue and hyoid muscles are developed. 

 They are bound more or less together by the developing sheaths, and connexions 

 are established between them. When the muscles take form and draw apart the 

 nerves are separated out into a plexus in which is foreshadowed the adult 

 arrangement of the branches supplying this group of muscles. 



A ganglion (Froriep's ganglion) is occasionally found in connexion with one of 

 the roots (figs. 169 and 173). 



The vag-us complex includes the vag-us and spinal accessory, which 

 develop practically as a single structure. The ganglion- crest from which the vagus 

 ganglion is developed is a continuation forwards of the spinal ganglion-crest as 

 seen in embryos of the third week. It extends from the first and second cervical 

 to the auditory vesicle (fig. 175), where it is interrupted, the gap representing the 

 separation of the vagus from the glossopharyngeal. The motor fibres appear first ; 

 they form a strand which runs mesial to the crest as low as the third or fourth 

 cervical segment, and are connected with the lateral-horn region of the neural tube. 

 This strand is the primary accessory trunk ; in front of its emerging roots and in line 

 with them are a few scattered bundles at the head of the crest. The ganglion 

 of the vagus early shows a duplicity ganglion of the trunk and ganglion of the root. 

 These separated masses have usually been described as due to a subdivision of 

 the neural-crest ganglion ; but Streeter supplies evidence which seems to indicate 

 that the ganglion of the trunk (g. nodosum) may be developed separately. It 

 related to an ectodermal patch (epibranchial placode) above the gill-arch 

 The two ganglia are at first separated by a cellular tract, which is afterwards 

 converted into a fibrous trunk. The ganglion-crest now (by the third week) becomes 

 broken up into separate clumps by the laying down of fibre-paths. The most 

 anterior of these form the definitive ganglion of the root ; the remainder (three or 



1 Loc. cit., p. 111. 



2 The summary of the development of the occipital nerves is mainly founded on Streeter's paper 

 (loc. cit.}. 



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