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THE MORPHOLOGY OF THE CRANIAL NERVES. 661 
arches and clefts makes it possible to understand the innervation by the vagus of the heart and 
lungs, no satisfactory explanation is forthcoming of the passage of the nerve into the abdomen, 
and its distribution to the stomach and other organs below the diaphragm. 
The spinal accessory nerve consists of two parts. The accessory portion of the nerve 
consists of efferent fibres for the branchial region, in series with the lateral motor roots of the 
glosso-pharyngeal and vagus nerves. The spinal portion of the nerve is also composed of efferent 
fibres, and represents the only lateral motor elements in relation to the spinal cord. 
Olfactory Nerve.— There is absolute uncertainty regarding the morphology of this nerve. 
It consists of three existing elements—(1) the olfactory bulb, derived from the cerebral hemi- 
sphere, solid in man, but a hollow cerebral diverticulum in certain animals, and forming the 
rhinencephalon ; (2) the olfactory ganglion, with its central and peripheral processes, derived 
from the ectoderm; (3) the nasal pit. Attention has been specially fixed on the olfactory 
ganglion, which has been compared to (1) a dorsal spinal ganglion, derived from the anterior end 
of the medullary groove ; and to (2) a lateral line sense-organ. 
The optic nerve also presents an insoluble problem in regard to its morphological position 
in the series of cranial nerves. The optic stalk and optic cup have been regarded as a highly- 
modified dorsal ganglion; but there is 
insuperable difficulty in accepting this 
view. The peripheral processes do not 
become connected with either ectodermal 
or mesoblastic structures, but become the 
tissue of the retina; while the central 
processes, growing backwards, envelop the 
optic stalk, and obtain connexions with 
the brain. The retina must be regarded 
as a highly-modified layer, morphologic- 
ally in series with the wall of the fore- 
brain; and the ectodermal structure of 
superficial origin comparable to the 
olfactory ganglion or the auditory vesicle 
is the lens (which may possibly be homo- 
logous with a lateral line sense-organ). 
The optic nerve, optic commissure, and 
optic tract are then to be looked upon as 
cerebral commissures, and not nerves in 
the ordinary sense. 
The simplest and most primitive con- 
dition of the head, in relation to the 
morphology of the cranial nerves, is found 
before the formation of the gill-clefts, 
when the salient features are a tubular 
and simple brain, and a series of super- 
ficial invaginations which pass from the 
surface inwards to become connected with 
outgrowths corresponding to them from 
the primitive brain. On either side of 
the head three hollow invaginations 
occur. (1) The nasal pit bearing the Fic. 474. SCHEME TO ILLUSTRATE THE EMBRYOLOGICAL 
olfactory epithelium becomes connected ARRANGEMENT OF THE CRANIAL NERVES. 
by the olfactory ganglion with the rhin- I. to XII. Cranial nerves ; Fr, Froriep’s ganglion ; C.I, Roots 
encephalon, an outgrowth from the fore- and trunk of the first cervical nerve. 
brain, and so forms the basis of an 
olfactory organ and nerve; (2) a similar invagination produces the lens, connected with a 
protrusion of the optic vesicle from the fore-brain, by which the basis of the eye and the optic 
nerve is formed ; (3) behind the buccal cavity a third invagination forms the auditory vesicle, 
which is connected with the solid extension from the hind-brain of the acoustic ganglia, to form 
the essentials of the organ of hearing and the auditory nerve. 
The trigeminal nerve is essentially the nerve of the buccal cavity, and the subordinate 
cavities, nasal and oral, derived from it. The branchial arches and clefts are secondary struc- 
tures, and their nerves are (1) the trigeminal for the first (mandibular) arch and the cleft in 
front of it; (2) the facial for the second (hyoid) arch and cleft ; (3) the glosso-pharyngeal for the 
third (thyro-hyoid) arch and cleft ; and (4) the pneumogastric for the succeeding arches and clefts. 
The bulbar part of the spinal accessory nerve is inseparable from the motor portion of the vago- 
glosso-pharyngeal nerves ; the spinal part is beyond the series of the cranial nerves. 
Lastly, there are certain truly segmental nerve elements, motor fibres which, remaining 
associated with certain persistent cephalic myotomes, give rise to the oculo-motor, trochlear, 
abducent, and hypoglossal nerves. 
