682 THE NERVOUS SYSTEM. 



of a hiatus between two ganglia. It is, on the other hand, an attractive view, as it 

 ascribes to one germinal layer (ectoderm) the formation of all the elements of the nervous 

 system, and it brings the sympathetic ganglia into serial homology with the isolated 

 ganglia ciliary, spheno-palatine, and otic associated with the trunks of the trigeminal 

 cerebral nerve. 



THE DEVELOPMENT OF THE CEREBRAL NERVES. 



The cerebral nerves are divisible morphologically into three series : (1) those 

 associated with sense organs the first or olfactory, second or optic, and eighth or 

 acoustic; (2) those connected with the embryonic branchial arches the fifth 

 or trigeminal, seventh or facial, ninth, tenth, and eleventh, glossopharyngeal, 

 vagus, and accessory ; and (3) motor nerves distributed to muscles derived from 

 cephalic myotomes the third or oculomotor, fourth or trochlear, sixth or abducent, 

 and twelfth or hypoglossal. 



Omitting the olfactory and optic nerves, which are special vesicular outgrowths 

 of the brain itself, it is possible to trace a distinct homology in the process of 

 development of the other cerebral and the spinal nerves. In the primitive brain the 

 gray matter is arranged into Alar and Basal Laminas (His), comparable to the 

 postero-lateral and anterior areas of gray matter (columns) of the spinal medulla. 

 Further, the basal lamina may be split up into lateral and medial areas. 



The origin of the third, fourth, sixth, and twelfth cerebral nerves all motor 

 efferent nerves is from the medial part of the basal lamina of the primitive brain, 

 in serial homology with the anterior efferent roots of the spinal nerves. 



The efferent motor roots of the fifth, seventh, ninth, tenth, and eleventh nerves 

 arise from the lateral part of the basal lamina, and so may be differentiated from 

 the preceding series. 



The afferent sensory roots of the fifth, seventh (nervus intermedius), eighth, 

 ninth, and tenth nerves are homologous with the posterior roots of the spinal nerves. 

 They are all gangliated, and are connected with the alar lamina of the brain. 



I. The olfactory nerves are associated in their development with the formation of 

 the olfactory pit and the olfactory bulb. 



The olfactory pits appear on each side of the front of the head at a little later period 

 than the formation of the lens and the auditory vesicle. They become converted into the 

 nasal cavities by the formation of the pre-oral visceral clefts and arches, fronto-nasal 

 and ethmo-vomerine in the median plane, and lateral ethmoid and maxillary processes at 

 the sides (p. 49). 



The Rhinencephalon or olfactory bulb is a hollow outgrowth from each telencephalon 

 or cerebral hemisphere, and appears in the first month. It grows forwards into relation 

 with the deep surface of the nasal pit. In many animals (as in the horse) the olfactory 

 bulb remains hollow ; but in the human subject it loses its lumen and becomes a solid 

 bulb (olfactory bulb) connected to the brain by a narrow stalk, the olfactory tract. 



The epithelium of the olfactory pit is responsible for the formation of the olfactory nerves. 

 There are two views as to the mode of their development from the epithelial cells. Both 

 views admit the proliferation of the epithelium of the nasal pit so as to produce neuroblasts. 

 According to the one view these neuroblasts detach themselves from the epithelial surface, 

 and constitute an olfactory ganglion which becomes applied to and incorporated with the 

 olfactory bulb. The cells of the ganglion become bi-polar, and the peripheral axons 

 constitute the olfactory nerves, while the central axons (in the second month) proceed back- 

 wards to the brain along the olfactory tract. According to the other view (based on Disse's 

 investigations), the proliferating cells of the nasal epithelium remain in the wall of the 

 nasal pit, and become the olfactory cells of the nasal cavity, with peripheral processes 

 projecting to the surface of the epithelium. Their central axons become the olfactory 

 nerve fibres which end in the olfactory bulb, forming dendrites associated with the dendritic 

 processes of the nerve-cells of the bulb. The central axons of these latter cells develop 

 into the fibres of the olfactory tract (see p. 622). 



II. The optic nerve is developed wholly from the brain. Its formation begins with 

 the outgrowth of the optic vesicle, a paired hollow outgrowth from the ventral surface of 

 the diencephalon. The ectodermic invagination of the lens, growing inwards from the 

 surface of the head, causes the collapse of the vesicle and its conversion into the optic 



