132 NERVOUS SYSTEM 



four) are accessory root-ganglia, which gradually diminish in size until they are 

 found as mere rudiments in the root of the accessory nerve (fig. 173). 



The result of the differentiation of the vagus complex is, that the fore- 

 part or vagus division, becomes predominantly sensory, and the back-part or 

 accessory division predominantly motor (Streeter). The series of motor roots is 

 continuous, and both divisions are at first mixed motor and sensory. 



The glossopharyng-eal develops in a fashion identical with the vagus, and 

 in the human embryo it arises independently of it. The petrous ganglion, like the 

 vagal trunk-ganglion, is associated with an ectodermic thickening (epibranchial 

 placode) above the third branchial arch. Though in the case of both vagus and 

 glossopharyngeal the part of the ganglion connected with the placode has possibly 

 an independent origin, Streeter does not describe any appearances which positively 

 prove an origin from the placode, as has been described in lower vertebrates. The 

 main bundle of fibres from the petrous ganglion runs in the third arch forming the 

 lingual branch, while a second bundle runs into the second arch and becomes the 

 tympanic branch (fig. 175). 



The acoustic nerve arises from a ganglionic mass (acoustico-facial complex) 

 which lies just in front of the auditory vesicle and is early separated into acoustic 

 and facial portions. The acoustic portion will be described with the organ of 

 hearing. 



The facial has two roots, a motor and a sensory. The sensory root is derived 

 from a ganglion distinguished by its larger cells, which is separated off from the 

 common acoustico-facial ganglion and is named geniculate. It is directly continuous 

 with an epidermic thickening over the hyo-mandibular cleft (embryo of twenty- 

 third day: Futamura 1 ). 



The acoustico-facial is generally considered a complex nerve connected with the hyoid arch 

 Giglio-Tos, founding on observations in a seventeenth-day embryo, argues for the independence 

 of the two nerves in the earliest phases. According to his description, each of the two nerves 

 has a median proganglion derived from the neural crest and two peripheral proganglia, a lateral 

 and an epibranchial, with corresponding placodes. The lateral placode of the acoustic nerve is 

 the auditory epithelium, that of the facial is a separate and distinct thickening ; the epibranchial 

 proganglia are continuous. The lateral and mesial ganglia are connected by cellular strands 

 (pronerves), and out of this complex the acoustico-facial ganglion is formed. 



The central root-fibres of the geniculate ganglion pass into the neural tube and 

 end in series with those of the glossopharyngeal (pars intermedia}. The distal 

 branches appear about the fourth week as strands (A. Francis Dixon) which 

 become the great superficial petrosal and chorda tympani nerves. They unite 

 secondarily with the branches of the fifth nerve with which they are connected 

 in the adult, and are well developed before the peripheral branches of the trunk of 

 the facial can be recognised. The motor root springs from a group of neuroblasts 

 in the antero-lateral part of the basal lamina, and at an early stage the fibre-paths 

 connected therewith show a general mesial direction (His) towards the nucleus of 

 the sixth nerve, foreshadowing the devious course of the facial root through the 

 floor of the fourth ventricle. The motor fibres are distributed to the hyoid arch, the 

 muscles of which they supply ; the sensory branches pass over into the mandibular 

 arch. 



The abducens arises from the mesial column of neuroblasts in line with the 

 hypoglossal roots. No ganglion has been described in connexion with it. It passes 

 forward mesial to the trigeminal ganglion to the rudiment of its muscle, the 

 external rectus of the eye. 



The trigeminal nerve has a motor and sensory rudiment. The chief motor 

 nucleus appears as a group of neuroblasts which forms a keel-like projection of 



1 Anatomische Hefte, xxx. 1906. 





