THE MORPHOLOGY OF THE CRANIAL NERVES. 659 
THE MORPHOLOGY OF THE CRANIAL NERVES. 
The head and face, possibly the oldest, and from every point of view the most fundamental 
and important portion of the bodily fabric, present in some respects a more conservative type of 
structure, and in other aspects have been subject to more profound alterations than other parts 
of the body. Segmentation is characteristic of the trunk, pervading bones, muscles, vessels, and 
nerves. An absence of true segmentation is characteristic of the head region—omitting for the 
moment the cranial nerves. The head is characterised by the possession of an unsegmented 
tubular nervous system, enclosed in a bony capsule not properly segmented, with which the 
capsules of the sense-organs become united. The pre-oral and post-oral visceral arches and clefts 
are not truly segmental like the costal arches of the trunk. The branchial clefts are said to be 
inter-segmental : and their muscles (associated with the myoblast surrounding the developing 
heart) are described ,as visceral, and not myotomic, so that the branchial vessels and nerves 
(similarly) are not to be regarded as comparable to the segmental vessels and nerves of the trunk. 
The truly segmental structures present are certain persistent myotomes or muscle plates, which 
give rise to muscles innervated by the third, fourth, sixth, and twelfth cranial nerves. 
Another difficulty in the morphology of the head arises in the absence of body cavity, and 
the consequent difficulty of differentiating the somatic and splanchnic mesoblast, and the somatic 
and splanchnic distribution of a given nerve. 
Under these circumstances there is little help to be derived from head structures other than 
the nerves themselves in seeking a solution of the question of the morphological relations of the 
cranial nerves. ‘The spinal nerves are, generally speaking, all alike. The cranial nerves, on the 
other hand, are all different. Scarcely any two nerves are alike; and no single cranial nerve 
possesses in itself all the characteristic features of a spinal nerve. As seen in relation to their 
development, the cranial nervous system possesses a series of dorsal ganglia, comparable in 
position and development to the spinal ganglia ; and the efferent roots arise in the same way, 
and occupy somewhat the same position as the ventral roots of the spinal nerves. But there is 
no single complete segmental nerve in the head. The very essence of the architecture of the 
head is a want of segmentation ; and this character is shared by the cranial nerves. In addition 
it must be borne in mind that, in relation to the mammalian head, there are organs which have 
no homologues in the trunk, and on whose existence the arrangement of the cranial nerves 
depends—e.g. sense-organs and gill-arches. 
Among the cranial nerves there are several which possess a resemblance to one or other of 
the elements of a typical spinal nerve. In the neck the origin of the fibres of the spinal 
accessory nerve is from the side of the spinal cord, and it is in series with the motor roots of 
the vago-glossopharyngeal, facial, and fifth nerves. His (as shown in the account of the 
development of the nerves) has described the neuroblastic origin of the motor roots of these 
nerves from the lateral part of the basal lamina of the primitive brain. They thus form a series 
apart—lateral motor roots—separable from the series of motor roots originating from the mesial 
part of the basal lamina, comprising those of the third, fourth, sixth, and twelfth nerves ; the 
latter nerve roots being comparable to and in series with the ventral roots of the spinal nerves. 
The lateral motor roots are not represented in the spinal series except in the neck. It is 
questionable if there is any fundamental distinction between the lateral and ventral motor roots 
of the cranial nerves. The spinal accessory fibres, for example, when traced into the spinal cord, 
have an origin from the anterior cornu of the cord, and only differ from the motor or ventral 
root fibres of a spinal nerve in their different course to the surface. The ganglia in association 
with the cranial nerves are comparable to the spinal ganglia. The fifth nerve, with the Gasserian 
ganglion, the ganglion of the facial, the ganglia of the auditory, of the glosso-pharyngeal and the 
vagus, and the transitory (Froriep’s) ganglion of the hypoglossal nerves, arise from the brain in a 
comparable position, and in the same way as the spinal ganglia. But another series of structures 
—the sense organs of the lateral line, and the so-called “ epibranchial” organs, which are highly 
developed in lower vertebrates (e.g. elasmobranchs), and which appear transitorily only, or are 
absent altogether in mammalian development, may possibly have a share in the formation ot 
certain of these ganglia or parts of them (eg. ciliary ganglion, geniculate ganglion, ganglia of the 
auditory nerve, petrosal ganglion of the glosso-pharyngeal, and the ganglion of the trunk of the 
vagus). 
Certain of the cranial nerves are apparently distinctly segmental, supplying muscles derived 
from the persisting myotomes of the head. The first three myotomes are said to give rise to the 
muscles of the eyeball. The jirst produces the superior rectus, inferior rectus, internal rectus, 
and inferior oblique muscles, and its segmental nerve is the oculo-motor. The second myotome 
is said to produce the superior oblique muscle, and its segmental nerve is the trochlear. The 
third myotome is said to produce the external rectus muscle, and its segmental nerve is the 
abducent. It has been asserted that the tongue muscles are derived from the last three or four 
cephalic and first cervical myotomes, and that the hypoglossal nerve is the segmental nerve for 
these myotomes, comprising the motor elements of several (four or five) segmental nerves. The 
intervening myotomes between the first three and this occipital series disappearing, the cor- 
responding elements of segmental nerves are supposed to be absent also (Fig. 473). 
Certain of the cranial nerves are essentially related to the structures derived from and asso- 
ciated with the pre-oral and post-oral visceral clefts and arches (Fig. 474). The trigeminal nerve 
is essentially the nerve of the mandibular arch. By its efferent root it supplies the muscles of 
that arch. By its afferent root and branches it is related to (1) the fronto-nasal process (oph- 
thalmic division and ciliary ganglion); (2) the maxillary arch (superior maxillary nerve) ; and 
