THE DEVELOPMENT OF THE CRANIAL NERVES. 657 
becomes solid, and forms the basis of the optic tract, optic commissure, and optic nerve. 
The optic cup, bilaminar in form, and clasping the lens by its margin, is imbedded in 
mesoblastic tissue, which gives rise to the envelopes of the eyeball, etc. The outer layer of 
the optic cup produces the layer of hexagonal pigment cells of the retina. The cells of 
the inner layer produce the tissue of the retina proper. They form neuroblasts with peri- 
pheral and central processes. The peripheral processes are converted into retinal nerve 
tissues ; the central processes extend backwards along the optic stalk, and give rise to the 
fibrous structure of the optic nerve, optic commissure, and optic tract. Spongioblasts in 
the inner lamina of the optic cup produce the sustentacular tissue of the retina (Miiller’s 
fibres). 
The oculo-motor nerve arises, like the ventral root of a spinal nerve, from a 
group of neuroblasts in the mesial part of the basal lamina of the mid-brain. The peri- 
pheral fibres extend forwards, to end around the optic cup in the mesoblastic tissue, from 
which the eye muscles are derived. Numerous cells are carried along with the cell pro- 
cesses in their course, and these have been described as being concerned in the formation 
of the ciliary ganglion. 
The trochlear nerve also arises from a group of neuroblasts occupying the 
mesial part of the basal lamina of the mid-brain, close to its junction with the hind-brain. 
The peripheral processes do not emerge directly from the brain, but extend dorsally from 
their origin along the side of the brain to its dorsal aspect, where they appear, after decus- 
sating w ith the fibres of the opposite nerve, just behind the quadrigeminal lamina. 
The trigeminal nerve is developed by means of a large dorsal and a small ventral 
root. Their origin is in the main similar to the origin of the roots of a spinal nerve. 
The large dorsal (afferent) root is formed by means of a cellular bud from the 
alar lamina of the hind-brain. This bud separates from the brain, and forms the Gasserian 
ganglion. Its cells becoming bipolar, like the cells of a spinal ganglion, are secondarily 
connected with the brain by means of their central processes ; while the peripheral pro- 
cesses, Separating into three groups, proceed along the fronto-nasal and maxillary pro- 
cesses, and along the mandibular arch, to form the three main divisions of the nerve. 
Numerous cells accompany each main division in its course from the ganglion, and form 
eventually the subordinate ganglia—the ecz/zary on the ophthalmic nerve, the spheno-pala- 
tine on the superior maxillary nerve, and the otic ganglion on the inferior maxillary nerve. 
The small ventral (efferent) root of the trigeminal nerve, like the motor ventral root 
of a spinal nerve, is later in its appearance than the sensory root. It arises as the peri- 
pheral fibres of a group of neuroblasts occupying the lateral part of the basal lamina of the 
hind-brain, which proceed directly to the surface to join the inferior maxillary division of 
the nerve. 
The resemblance between the trigeminal nerve and a typical spinal nerve, though 
striking, is incomplete. The efferent root does not quite correspond in place of origin 
with the motor root of a spinal nerve, being lateral and not ventral. The afferent root, 
while ganglionic and dorsal, differs from the typical segmental spinal nerve in separating 
into trunks for the supply of more than one segment. 
The abducent nerve resembles in its mode of development the oculo-motor and 
trochlear nerves, with which in its origin it is in series. It is formed by the peripheral 
processes of a group of neuroblasts in the mesial part of the basal lamina in the upper 
part of the hind-brain. These processes pierce the part of the brain in which, at a later 
stage, the fibres of the pyramid are developed. They then proceed to the mesoblastic 
tissue round the optic cup, which is destined to form the eye muscles. 
The facial nerve has developmentally a double origin. (1) In connexion with 
the formation of the auditory nerve a group of cells becomes separated from the alar 
lamina of the hind-brain opposite the auditory vesicle. This group becomes separated 
into three parts, of which the middle portion is the rudiment of the geniculate ganglion 
(or efferent root). (2) From a group of neuroblasts in the lateral part of the basal lamina 
of the hind-brain the efferent root of the nerve arises in series with efferent fibres of the 
vago-glossopharyngeal fibres ; after a tortuous course within the brain, its fibres emerge 
beneath the above-mentioned cellular mass, opposite the auditory vesicle. They are 
joined by the ganglionic root, and winding round the auditory vesicle, become 
imbedded in the auditory capsule (aqueduct of Fallopius). The chorda tympani nerve 
appears early as a branch of the facial nerve. It is probable that the pars intermedia, 
the geniculate ganglion, and the chorda tympani nerve represent the dorsal afferent 
element in the constitution of this nerve. 
The auditory nerve arises as a cellular bud from the alar lamina of the hind-brain, 
dorsal to the efferent portion of the facial nerve and opposite to the auditory vesicle. 
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