476 THE NERVOUS SYSTEM. q 
THE DEEP CONNEXIONS OF THE CRANIAL NERVES ATTACHED TO THE 
MEDULLA AND PONS. 
There are twelve pairs of cranial nerves, of which the lower eight are attached 
to the medulla and pons Varolii. From above downwards these are named the 
fifth or trigeminal, the sixth or abducent, the seventh or facial, the eighth or 
auditory, the ninth or glosso-pharyngeal, the tenth or vagus, the eleventh or spinal 
accessory, and the twelfth or hypoglossal. The hypoglossal, the spinal accessory, 
the greater part of the facial, the abducent, and the motor root of the trigeminal 
are efferent nerves; the auditory, the pars intermedia of the facial, and the sensory 
root of the trigeminal are purely afferent nerves; whilst the vagus and glosso- 
pharyngeal are composed of both efferent and afferent fibres. In all cases, afferent 
fibres arise from ganglionic cells placed outside the brain and penetrate the brain- 
stem, to end in connexion with the cells of certain nuclei of termination. Lfferent 
fibres, on the other hand, take origin within the brain as the axons of cells which 
are grouped together in certain places i in the form of nuclei of origin. 
Nuclei of Origin, or Motor Nuclei.—In the spinal cord there is a continuous 
nucleus of origin or column of cells running within the whole length of the 
anterior horn of gray matter, from which the series of efferent anterior nerve- 
roots take origin. In the medulla and pons the nuclei of origin, or, in other 
words, the motor nuclei, become discontinuous, and are represented by certain 
more or less isolated clumps of compact gray matter, in which are placed the 
clusters of cells from which the fibres of the efferent nerves arise. At the decus- 
sation of the pyramids, the anterior horn of gray matter of the cord is broken 
up by the intercrossing bundles into a detached head and a basal part which 
remains in relation with the ventro-lateral aspect of the central canal. Certain 
of the efferent or motor nuclei of the medulla and pons he in the line of the basal 
portion of the ventral horn of gray matter of the spinal cord, and thus close to 
the mesial plane. These are termed mesial nuclei of origin, and are met with at 
different levels in the brain-stem. This group comprises the hypoglossal nucleus, 
the abducent nucleus (and, in the mesencephalon, the trochlear nucleus and 
the oculo-motor nucleus). Other motor nuclei of origin are present in the form 
of isolated clumps of gray matter, which lie at different levels in the medulla 
and pons in the line of the detached head of the anterior horn of gray matter. 
They are the medullary nucleus of the spinal accessory, the nucleus ambiguus of 
the vagus and glosso-pharyngeal, the facial nucleus, and the nucleus of the motor 
root of the trigeminal nerve. From their position in the tegmental substance of 
the medulla and pons, they constitute a group to which the name of lateral motor 
nuclei is applied. The different nuclei of origin of the efferent fibres which belong 
to the various cranial nerves, both mesial and lateral, are connected with the motor 
area of the cerebral cortex by fibres from the pyramidal tract, which enter the 
nuclei and end in connexion with their cells. 
Nuclei of Termination.—In the brain the nuclei of termination are likewise 
discontinuous, and are represented by more or less isolated clusters of cells. Unlike 
the motor nuclei, however, these nuclei show no regular or definite position within 
the medulla and pons. Some are found in the gray matter which surrounds the 
central canal, and, in its continuation upwards, as the gray matter in the floor of the 
fourth ventricle ; others are placed in the tegmental substance ; whilst two actually 
lie on the surface of the brain-stem, viz. the lateral and ventral nuclei of the 
cochlear or outer division of the auditory nerve. The axons of the cells of the 
nuclei of termination enter the reticular formation of the tegmental substance as 
arcuate fibres, and, crossing the mesial plane, are carried upwards i in the tegmental 
substance of the opposite side to establish direct or indirect connexions with the 
cerebral cortex. 
Hypoglossal Nerve (nervus hypoglossus)—The nucleus of origin of the hypo- 
elossal nerve, the motor nerve of the tongue, hes in the substance of the medulla 
oblongata. It is composed of several groups of large multipolar cells, which closely 
resemble the cells in the ventral horn of oray matter in the spinal cord, and is 
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