1380 HUMAN ANATOMY. 



being later closely related, both as to position and fibres. An outgrowth of distally directed 

 fibres establishes the main trunk of the nerve, while a forwardly growing strand represents the 

 later tympanic branch. 



The vagus and spinal accessory nerves are so inseparably related in their development 

 that their origin must be regarded as proceeding from a common vagus complex. The latter 

 comprises three elements: (a) a series of motor roots, which arise from the ventral zone of 

 the hind-brain and extend from near the glosso-pharyngeal anlage in front as far as the third or 

 fourth spinal segment below ; (b) a partially subdivided, but at first continuous, ganglionic 

 mass, which arises from the ganglion-crest of the hind-brain and represents the root-ganglia ; 

 (c) a secondary' ventral cell-mass, the primitive ganglion of the trunk, which, as in the case of 

 the glosso-pharyngeal nerve, is developed in close relation with the ectoblast of the posterior 

 branchial furrows. Whilst the motor rootlets persist and become the efferent root-fibres of the 

 later vagus and accessory nerves, the dorsal or crest-ganglia soon exhibit differences in their 

 growth, the one situated farthest forward outstripping the others and becoming the vagal gang- 

 lion of the root, and the remaining ones becoming the accessory root -ganglia. These latter 

 constitute a chain which below meets with the spinal dorsal ganglia. Primarily, therefore, the 

 entire length of the vagus complex is occupied by a series of mixed nerve strands possessing 

 both motor and sensory elements. The head-end of the series later becomes predominatingly 

 sensory, while in the tail-end of the same the motor character prevails. The ventral vagus 

 nucleus is attached secondarily to the dorsal nucleus by centrally growing fibres, while from its 

 distal end extend the dendritic processes which constitute the trunk of the vagus and its 

 branches. In consequence of the intergrowth of these afferent and efferent fibres, the definite 

 tenth nerve in the usual sense, with its two ganglia, becomes established. Although for a short 

 period the accessory part of the complex is provided with both motor and sensory parts, the 

 latter are subsequently overpowered by the efferent fibres, so that the presence of the rudimen- 

 tary ganglionic elements within the accessorius can be demonstrated only by microscopic exam- 

 ination (Streeter) . From the preceding facts it is evident that the estimate of the eleventh nerve 

 as an integral part of the vagus is well founded. 



The hypoglossal nerve appears in the human embryo, towards the close of the third week, 

 as several strands which grow from the ventral zone of the wall of the hind-brain and are in 

 series with the ventral root-fibres of the upper cervical spinal nerves. Soon the separate root- 

 lets converge and consolidate into a common trunk, from which, by the end of the fifth week, 

 the chief branches of distribution arise. The production of the wide-meshed net-work which 

 distinguishes the communications between the upper cervical and hypoglossal nerves results 

 from the separation of fibres which are at first closely adjacent, the subsequent migration of the 

 growing tongue-muscles drawing the hypoglossal fibres away from the spinal nerves, except at 

 such points where they have become enclosed in a common sheath. There is good reason for 

 regarding the hypoglossal nerve as representing the ventral roots of trunk-nerves, which have 

 been cephalicly displaced and drawn within the cranium. Moreover, the observations of 

 Froriep and others upon adult mammals and of His upon the human embryo have shown the 

 presence of a rudimentary dorsal ganglion and abortive dorsal root-fibres. The occasional 

 presence of a rudimentary ganglionic mass, known as Froriep's ganglion, attached to the 

 fibres of the adult hypoglossal nerve in man is to be interpreted as the persistent dorsal 

 element which ordinarily disappears. 



From the preceding sketch it is evident that in no instance, as observed in the usual adult 

 condition in man, is there complete correspondence between the members of the cephalic 

 series and those of the trunk. The group of purely sensory nerves the olfactory, optic and 

 auditory includes one, the optic, which is so exceptional in its fundamental relations as to lie 

 without the pale of peripheral nerves in their strict sense. The remaining two sensory nerves 

 are held to be primarily the equivalents of constituents of a peculiar system of sensory 

 organs, best developed in fishes, known as the organs of the lateral line. The third, fourth, 

 sixth and twelfth, the ventral motor nerves, are undoubtedly associated with head-somites, 

 although the exact number and nerve relations of such mesoblastic segments are uncertain ; 

 in fundamental significance, therefore, these nerves agree with those of the trunk-series, 

 although modified by the suppression of their dorsal or sensory constituents. The mixed 

 nerves the fifth, seventh, ninth and tenth (the eleventh being reckoned as part of the vagus) 

 are unrepresented in the spinal series and belong to the brunchiomeres represented by the 

 visceral arches. Of these nerves, the trigeminus most nearly accords in constitution with a typical 

 spinal n<-rve, since, with the exception of ventral motor constituents which are wanting, it pos- 

 as does the typical spinal nerve, both somatic (general cutaneous) sensory and visceral 

 sensory fibres. A further resemblance is found in the character of the gray matter constituting 

 the reception-nucleus for the sensory fibres of the trigeminus, since this column is composed of 

 sulistautia iM-latinosa continuous with the Rolandic substance capping the posterior cornu of the 

 cord. A similar, alth< >ugh less intimate, arrangement is seen in the column of gray matter accom- 

 panying the descending root (funiculus solitarius)of the facial, glosso-pharyngeal and vagus nerves. 



