THE DEVELOPMENT OF THE CEEEBEAL NEEVES. 683 



cup, the narrow tube connecting the vesicle to the brain becoming the optic stalk. This 

 stalk becomes solid, and forms the basis of the optic tract, optic chiasma, and optic 

 nerve. The optic cup, bilaminar in form, and by its edge clasping the lens, is imbedded 

 in mesodermic 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 tissues of the retina proper. They form neuroblasts 

 with peripheral 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 optic nerve, optic chiasma, and optic tract. Spongioblasts in the inner lamina 

 of the optic cup produce the sustentacular tissue of the retina (Mtiller's fibres). The 

 mesodermic tissue surrounding the optic cup and lens gives rise to the rest of the structure 

 of tlie eyeball, the formation of which is described in the section which deals with the 

 organs of sense. 



III. The oculomotor nerve arises, like the ventral root of a spinal nerve, from a 

 group of neuroblasts in the medial part of the basal lamina of the mid -brain. The 

 peripheral fibres extend forwards, to end around the optic cup in the mesodermic tissue, 

 from which the eye muscles are derived. Numerous cells are carried along with the cell 

 processes in their course, and these have been described as being concerned in the 

 formation of the ciliary ganglion. 



IV. The trochlear nerve also arises from a group of neuroblasts occupying the 

 medial 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 

 decussating with the fibres of the opposite nerve, just behind the quadrigeminal lamina. 



V. The trigeminal nerve is developed by means of a large posterior and a small 

 anterior root. Their origin to a large extent resembles the mode of formation of the 

 roots of a spinal nerve. 



The large posterior (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 semi- 

 lunar 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 processes, separating into three groups, proceed along the fronto-nasal and 

 maxillary processes, 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 ciliary on the ophthalmic nerve, the' 

 spheno-palatine on the maxillary nerve, and the otic ganglion on the mandibular nerve. 



The small anterior (efferent) root of the trigeminal nerve, like the motor anterior 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 mandibular division of 

 the nerve. 



VI. The abducens nerve resembles in its mode of development the oculomotor 

 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 medial 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 mesodermic 

 tissue round the optic cup, which is destined to form the eye muscles. 



VII. The facial nerve has developmen tally a double origin. (1) In connexion with 

 the formation of the acoustic 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 genicular ganglion 

 which becomes incorporated with the efferent part of the facial nerve, and is connected to 

 the brain by a slender root, known as the nervus intermedius (O.T. pars intermedia). 

 (2) The efferent root of the nerve arises from a group of neuroblasts in the lateral part 

 of the basal lamina of the hind-brain, in series with efferent fibres of the vago-glosso- 

 pharyngeal nerves ; 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 in their course round the auditory vesicle become imbedded in the 

 auditory capsule (canalis facialis). The chorda tympani nerve appears early as a branch 

 of the facial nerve. It is probable that the nervus intermedius, the genicular ganglion, 

 and the chorda tympani nerve together represent the posterior afferent element in 

 the constitution of this nerve. 



VIII. The acoustic nerve arises as a cellular bud from the alar lamina of the hind- 



