[Illustration:
FIG. 502.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE, PASSING THROUGH
FACIAL NUCLEUS.
_Cr_, Corpus restiforme; _D_, dorsal longitudinal fasciculus; _Fa_,
ascending part of facial nerve; _L_, fillet; _N7_, nucleus of facial
nerve; _N8_, triangular nucleus of vestibular root of eighth nerve;
_N8′_, spinal root of eighth nerve; _Py_, pyramid; _Ra_, raphé;
_R7_, radicular part of facial nerve; _R8_, vestibular root of
eighth nerve; _Sg_, substantia gelatinosa; _Ta_, posterior end of
tuberculum acusticum; _V_, spinal root of trigeminus.
(Ellenberger-Baum, Anat. d. Haustiere.)
]
[Illustration:
FIG. 503.—CROSS-SECTION OF MEDULLA OBLONGATA OF HORSE; SECTION PASSES
THROUGH MIDDLE OF CORPUS TRAPEZOIDEUM.
_Cr_, Corpus restiforme; _D_, dorsal longitudinal bundle; _Fa_,
ascending part of facial nerve; _Fa′_, emergent or descending part
of facial nerve; _R7_, root of facial nerve; _L_, fillet; _M_,
central white matter of cerebellum; _N6_, abducens nucleus; _N8_,
triangular nucleus; _N8′_, nucleus of Deiters; _N8″_, tuberculum
acusticum; _Oo_, anterior olive; _Py_, pyramid; _R6_, root of
abducens nerve; _R7_, root of facial nerve; _Rc_, cochlear nerve;
_Rv_, vestibular nerve; _Sg_, substantia gelatinosa; _T_, corpus
trapezoideum; _V_, spinal root of trigeminus. (Ellenberger-Baum,
Anat. d. Haustiere.)
]
The =pons= (Varolii) is that part of the brain stem which lies between
the medulla and the cerebral peduncles; it is marked off from these
ventrally by anterior and posterior grooves. Viewed from below it is
elongated transversely, convex in both directions, and presents a wide
shallow median groove (Sulcus basilaris), which lodges the basilar
artery. Laterally a large part of its mass curves upward and backward
into the base of the cerebellum, forming the =middle cerebellar
peduncle= (Brachium pontis). The superficial origin of the trigeminal
(fifth) nerve is at the lateral limit of the ventral surface. Transverse
striations indicate the course of its superficial (ventral) fibers which
connect the two sides of the cerebellum. The dorsal surface is blended
on either side with the overlying anterior peduncles of the cerebellum;
the central free portion forms the anterior part of the floor of the
fourth ventricle, and will be considered in the account of that cavity.
On cross-section the pons is seen to be composed of dorsal and ventral
parts. The =dorsal part= (Pars dorsalis pontis) consists superficially
of a layer of gray matter covered by the ependyma of the fourth
ventricle. Beneath this the median raphé of the medulla is continued
into the pons, dividing it into similar halves. In the anterior part
of the pons the fillet divides into an inner and outer part, the
=medial= and =lateral fillets= (Lemniscus medialis, lateralis); the
latter arches outward to reach the outer side of the anterior
cerebellar peduncle. The =dorsal longitudinal fasciculus= becomes
sharply defined into a round bundle which lies close to the raphé
under the gray matter of the floor of the fourth ventricle. In
cross-section the =formatio reticularis= forms a large area below the
superficial gray matter and the longitudinal bundles. Dorso-laterally
is the large rounded section of the =anterior cerebellar peduncle=.
Lower down is a large bundle, the =sensory root of the fifth nerve=.
In front of this is the =motor nucleus= of the same nerve, external to
which is its =motor root=. The =ventral part= of the pons (Pars
basilaris pontis) is composed of transverse and longitudinal fibers,
and a large amount of gray matter which is broken up into small masses
(Nuclei pontis) by the intersection of the fibers. The =transverse
fibers= are gathered laterally into a compact mass which turns upward
and backward and enters the central white matter of the cerebellum,
forming the =middle cerebellar peduncle=. Centrally the fibers are
arranged in bundles which intercross. The transverse fibers are
chiefly of two kinds. Some arise from the Purkinje cells of the
cerebellar cortex and pass either to the opposite side of the
cerebellum or turn at the raphé and run forward and backward in the
brain stem. Others are axones of cells of the nuclei pontis, and pass
to the hemispheres of the cerebellum. The corpus trapezoideum is
mainly the central continuation of the cochlear division of the
auditory nerve. Above it is the small =anterior olivary nucleus=
(Nucleus olivaris nasalis). The =longitudinal fibers= of the ventral
part of the pons consist chiefly of the =cerebrospinal= or =pyramidal
fasciculi=. These come from the ventral part (pes) of the cerebral
peduncles and are situated laterally, interspersed among the deep
transverse fibers in the anterior part of the pons. Toward the
posterior part the bundles incline toward the median plane and become
collected into a compact mass which appears superficially at the
posterior border. Many fibers come from the cerebral cortex and
terminate in the nuclei of the gray matter of the pons; they may be
designated =corticopontile fibers=.