DEVELOPMENT OF PERIPHERAL NERVES. 



1379 



The neuroblasts of the facial constituent, the geniculate ganglion, send their centrally directed 

 processes to the brain-stem as the pars intermedia, whilst their peripherally growing dendrites 

 contribute the sensory fibres, passing by way of the chorda tympani and the greater and lesser 

 superficial petrosal nerves. The geniculate ganglion and the pars intermedia correspond, 

 therefore, to a dorsal root. 



The auditory nerve, although for a time closely related in position (Fig. 1103) with the 

 facial (geniculate) ganglion, developes entirely independently and at no time has more than an 

 incidental relation. The primary auditory nucleus is defined in human embryos by the begin- 

 ning of the fourth week as an elongated ellipsoidal mass in contact with the anterior wall of the 

 otic vesicle. According to Streeter \ the nucleus very shortly exhibits a differentiation into 

 a superior and an inferior part, from the latter of which soon appears a third portion. This 

 third portion, the later ganglion spirale, early manifests a tendency to coil in consequence of 

 its close relations with the 



Fig. 



Vagus root gang. 



II43- 



Accessory root gang. 



IX. root gang. , 



N. tymp. 

 Gang, petros. 



IX. 



Gang, nodos. 

 N. iaryg. sup. 



XII. with r. descend 



Kroriep 



ductus cochlearis. The 

 major part of the primary 

 acousvic comple.x, including 

 the superior and most of 

 the inferior part, becomes 

 the vestibular ganglion, 

 from the neuroblasts of which 

 centrally directed a x o n e s 

 pass to the young brain- 

 stem as the vestibular nerve, 

 wMile the dendrites become 

 connected at certain places 

 with the semicircular canals, 

 the utricle and the saccule. 

 The grouping of the vestibular 

 rami seen in the adult is early 

 foreshadowed in the develop- 

 ing nerve, since from the 

 upper part of the vestibular 

 ganglion grows out the su- 

 perior division of the vestib- 

 ular nerve which, supplies 

 the utricle and the ampulla? 

 of the superior and external 

 semicircular canals (Fig. 

 1070) . The lower part of the 

 ganglion, in addition to fur- 

 nishing the anlage for the 

 cochlear nerve, gives off the 

 inferior division of the vestib- 

 ular nerve, by which the 

 saccule and the posterior 

 canal are supplied. During 

 the subsequent growth of the 

 structures, the neurones of 

 the spiral ganglion send ax- 

 ones towards the brain which become the cochlear nerve, whilst their dendrites-grow peripherally 

 into the ductus cochlearis and are represented by the minute filaments extending from the 

 cells of the spiral ganglion to the auditory cells of Corti's organ. 



The glosso-pharyngeal nerve is a mixed nerve and has, therefore, a double origin. Its 

 motor fibres arise from neuroblasts situated in the dorsal part of the ventral zone of the wall of 

 the hind-brain just posterior to the otic vesicle. The sensory part of the nerve, along with 

 that of the vagus, oflers greater complexity, since it is developed, as shown by Streeter 2, from 

 two sources. The ganglion of the root (g. superius or jugular ganglion) arises very early as 

 a small mass of cells derived from the ganglion-crest of the hind-brain. It varies in size and 

 soon ceases to grow, which behavior, in connection with the preponderating ingrowth of the 

 motor fibres, accounts for the well-known inconstancy of the structure. The ganglion of 

 the trunk (g. petrosum) arises, according to Streeter, not from the neural crest, but in 

 relation with the ectoblast of the second visceral furrow. At first ununited with the smaller 

 ganglion superius, the ganglion of the root subsequently becomes joined to it, the two nodes 



^ Amer. Jour, of Anatomy, vol. vi., 1907. 

 ^Amer. Jour, of Anatomy, vol. iv., 1904. 



Sympathetic 



\'agus 



Reconstruction of peripheral nerves of human embryo of five weeks 

 (14 mm.) /; 13. (Streeter.) 



