THE NERVOUS SYSTEM 1469 



The centripetal or proximal pole, which is the axon of a ganghonic cell. 

 grows into the dorsal part of the wall of the neural tube and forms part of 

 the dorsal or posterior root of a spinal nerve. Within the marginal layer of 

 the neural tube the centripetal pole or nerve-fibre divides into two branches — 

 ascending and descending, which give off collaterals and terminate in arboriza- 

 tions. The centrifugal or distal pole joins the ventral or anterior nerve-root 

 of the same segment of the spinal cord, on the distal side of the ganglion, to 

 form a spinal nerve. 



Though the cells of a spinal gangUon are originally bipolar, they become in 

 the course of growth unipolar, the single pole having a T-shape. This is 

 brought about by an excessive growth of one wall of the bipolar cell, which 

 brings the two original poles into contact, when they fuse, and are now con- 

 nected with the cell by one stalk or pole, which divides into a centripetal and 

 a centrifugal process. 



Whilst the fibres of the dorsal or posterior roots of the spinal nerves grow 

 into the mantle-layer of the neural tube from the cells of the spinal gangUa, 

 the fibres of the ventral or anterior roots arise within the mantle-layer as the 

 axons of its neuroblasts or nerve-cells. The fibres of the anterior roots there- 

 fore grow out from the neural tube. 



Cranial Nerves. — The development of the cranial nerves, with the exception 

 of the olfactory and optic nerves, corresponds for the most part with the 

 development of the spinal nerves. The motor cranial nerve-fibres arise as 

 the axons of nerve-cells of the brain and grow into the brain, whereas the 

 sensory cranial nerve-fibres grow into the brain from the cells of the cephalic 

 gangha. 



Cephalic Ganglia. — The cephalic gangUa, like the spinal ganglia, are 

 developed from the neural crests, and they constitute four pairs of ganghonic 

 groups — namely, trigeminal, acoustico-facial, glosso-pharyngeal, and vagal. 

 They are all comparable to the spinal ganglia 



The trigeminal or Gasserian ganglion is connected with the sensory root of 

 the fifth cranial nerve. The centripetal poles of its nerve-cells pass inwards 

 into the brain, forming the large sensory root of the nerve, and the centrifugal 

 poles of its cells pass peripheradly, forming the ophthalmic, superior maxiUary. 

 and sensory part of the inferior maxillary nerves. The Gasserian gangUon 

 is thus clearly comparable to a spinal ganglion. The small motor root of the 

 fifth nerve is homologous to the motor or anterior root of a spinal nerve, 

 inasmuch as its fibres arise as the axons of nerve cells tvithin the brain. 



The acoustico-facial ganglion resolves itself into facial and acoustic parts. 



The facial ganglion, known as the geniculate ganglion, is coimected with 

 the genu of the facial nerve in the aqueduct of Fallopius. The centripetal 

 poles of the nerve cells of this ganghon form the sensory root of the facial 

 nerve — namely, the pars intermedia of IVrisberg, which passes inwards to the 

 fasciculus solitarius and upper part of the glosso-pharyngeal nucleus. Many 

 of the centrifugal poles of the cells issue from the ganglion as the chorda 

 tympani nerve, which passes to the anterior two- thirds of the tongue as a nerve 

 of special sense. 



The acoustic ganglion is represented by the vestibular and cochlear ganglia 

 in connection with the auditory nerve. 



The vestibular ganglion is connected with the vestibular division of the 

 auditory nerve in the internal auditory meatus. As in the other gangha, 

 the centripetal poles of the cells of this ganglion form the centripetal fibres 

 of the vestibular nerve, which pass inwards to the brain. The centrifugal 

 poles of the cells leave the ganghon and form the peripheral part of the nerve 

 as regards its distribution. 



The cochlear ganglion, known as the ganglion spirale, is connected with 

 the cochlear division of the auditory nerve, and is situated in the spiral canal 

 of the modiolus. Its nerve-celk are related to nerve-fibres as in the case of 

 the vestibular ganglion. 



The glosso-pharyngeal ganglion, which is broken up into a jugular ganglion 

 and a petrous ganglion, is comparable to a spinal ganglion. The centripetal 

 poles of the nerve-cells, which issue from the gangUon, represent the centri- 

 petal sensory fibres of the glosso-pharjTigeal nerve, passing into the brain. 



