1378 



HUMAN ANATOMY. 



lary velum. As in the case of the third, so for the trochlear an abortive transient dorsal 

 ganglion and root have been described (Martin). If present these must be regarded as ex- 

 ceptional and not constant features. 



The trigeminal nerve is a mixed nerve and therefore takes its origin differently for its 

 two roots. The motor one is developed from a series of neuroblasts, which lie at some distance 

 from the mid-line within the wall of the neural tube, at a position corresponding to the junction 

 of the dorsal and ventral zones of the mid-brain and metencephalon. The axones of these 

 neuroblasts grow forward and converge to the surface of the later pons at a position close to 

 where the ingrowing sensory fibres join the neural tube. The sensory fibres are the axones of 

 neurones located within the Gasserian ganglion. The latter is derived as a ventrally directed 

 outgrowth from the ectoblast of the roof of the hind-brain, with which it remains attached for a 

 short time, but later becomes entirely separated. The neuroblasts acquire a bipolar form, one 

 set of processes, the axones, growing centrally to establish secondary connections with the 

 hind-brain as the large sensory root, while the others, the dendrites, extend peripherally into 

 the substance of the fronto-nasal and maxillary processes to form the ophthalmic and maxillary 

 nerves and into the mandibular process to form, in conjunction with the smaller motor root. 



Fig. I 142. 



Reconstruction of brain and cranial nerves of pig embryo; cranial nerves indicated by figures; C1-C3, cervical 

 spinal nerves ; in connection with seventh nerve., /.j./>, large superficial petrosal ; ch.ty., chorda tympani : /'a., facial ; 

 7., «., vagus ganglia of root and trunk ; com., commissural extension of ganglion of root ; F, Froriep's hypoglossal 

 ganglion. {F. T. Lewis.) 



the mandibular division of the trigeminus from the ganglion ridge. Provision for the ciliary 

 ganglion is made early by the migration of cells from the major ganglion along the de- 

 veloping ophthalmic division. Similar migrations along the other divisions give rise to the 

 spheno-palatine, the otic and the submaxillary ganglia. The later histological characteristics 

 of these cells, as well as their mode of origin, warrant the view that the ciliary ganglion, as well 

 as the others connected with the trigeminus, belong to the sympathetic system. On entering 

 the wall of the brain-tube, the bulk of the sensory trigeminal fibres assume a longitudinal course 

 and early establish the tract of the spinal cord. 



The abducent nerve developes, in a manner identical with the third and fourth, from a 

 median group of cells occupying the ventral zone of the upper part of the hind-brain. In the 

 human embryo of about four and a half weeks (Fig. 1141), the nerve appears at its super- 

 ficial origin mesial to the Gasserian ganglion. The root-fibres early consolidate into a compact 

 strand. 



The facial nerve being a mixed one also arises from a double source, its motor fibres 

 taking origin from efferent neuroblasts situated in the ventro-lateral wall of the metencephalon. 

 In contrast to the direct ventral course of the axones of the mesial motor nerves, those of the 

 facial pursue a path to the surface of the brain-stem even more indirect than that taken by the 

 lateral motor fibres of the other mixed nerves. Proceeding as the axones of neuroblasts lying 

 within the lateral part of the ventral zone of the wall of the hind-brain, they are directed dor- 

 sally, then grow forward, turn outward and, finally, ventrally to gain emergence from the brain. 

 The sensory portion of the facial is topographically closely connected during its development 

 with the auditory, the nuclei of the two nerves often being designated the facial-acoustic com- 

 plex. The three components of this aggregation — the geniculate, the cochlear and the vestibu- 

 lar ganglia — are primarily derived from an ectoblastic cell-mass in the vicinity of the otic vesicle. 



