The =medulla oblongata= lies on the basilar part of the occipital bone.
It is quadrilateral in outline, but much wider in front than behind, and
compressed dorso-ventrally. Its length, measured from the root of the
first cervical nerve to the pons, is about two inches (ca. 5 cm.).
Its =ventral surface= is convex in the transverse direction. It is
marked by a =median fissure= (Fissura mediana ventralis) which is
continuous behind with the similar fissure of the spinal cord. The
posterior part of the fissure is faintly marked, but in front it becomes
deeper and ends in a small depression (Foramen cæcum) behind the central
part of a transverse band, the =corpus trapezoideum=. On either side of
the fissure is a rounded tract, the =pyramid= (Pyramis), which is
bounded externally by a faint lateral groove (Sulcus intermedius
ventralis). The pyramids join the pons in front; behind they become
narrower and disappear into the substance of the medulla, in which their
fibers intercross, forming the =decussation of the pyramids= (Decussatio
pyramidum).[190]
The superficial origin of the =sixth cranial nerve= (N. oculomotorius)
is just lateral to the anterior end of the pyramid. The =corpus
trapezoideum= is a transverse band which extends across the surface
immediately behind the pons. It is crossed by the pyramids, which cut
off a small central part. The lateral part extends out to the roots of
the seventh and eighth nerves on either side. Behind the outer part of
the corpus trapezoideum there is a variably developed rounded eminence,
the =tuberculum faciale=. The root-fibers of the twelfth or hypoglossal
nerve form an oblique linear series lateral to the posterior part of the
pyramid.
[Illustration:
FIG. 499.—BASE OF BRAIN OF HORSE, NATURAL SIZE.
]
The =dorsal surface= is largely concealed by the cerebellum and forms
the greater part of the floor of the fourth ventricle. The =dorsal
median fissure= (Fissura mediana dorsalis), the direct continuation of
the corresponding groove of the spinal cord, extends forward to about
the middle of the surface. Here the =restiform bodies=, which constitute
the lips of the fissure, diverge to form the lateral boundaries of a
triangular depression; this is the posterior part of the rhomboid fossa
or floor of the fourth ventricle of the brain. The dorso-lateral fissure
winds outward and forward to the lateral aspect of the medulla, where it
presents the roots of the =ninth=, =tenth=, and =eleventh cranial
nerves=. External to it is a distinct oval eminence on the anterior part
of the lateral column, termed the =tuberculum cinereum=. The central
canal of the cord is continued in the posterior part of the medulla,
inclines dorsally, and opens in the posterior angle of the fourth
ventricle. Hence it is customary to distinguish a closed and an open
part of the medulla. The dorsal aspect of the latter, which is concealed
at present, will be considered later in the description of the fourth
ventricle.
[Illustration:
FIG. 500.—LINE DRAWING OF BASE OF BRAIN OF HORSE. (KEY TO FIG. 499.)
]
The lateral surface is narrow behind, wider and rounded in front. From
it the root-fibers of the ninth, tenth, and eleventh cranial nerves
arise in a linear series, and alongside of it the spinal part of the
eleventh nerve passes forward to join the medullary root. Close
inspection reveals the presence of striæ which curve obliquely downward
and backward from the surface of the restiform body toward the
hypoglossal root-fibers; these are the =external arcuate fibers= (Fibræ
arcuatæ externæ). The recess between the lateral aspect of the medulla
and the cerebellum is occupied by an irregular mass of villous
projections of the pia mater, containing tufts of vessels; this is the
=chorioid plexus of the fourth ventricle=, and is the lateral edge of
the =tela chorioidea= of the ventricle. On raising the chorioid plexus
it is seen that the tela chorioidea is attached to the dorsal aspect of
the medulla, and reinforces here the wall of the fourth ventricle; also
that the restiform body terminates in front by entering the base of the
cerebellum, forming its posterior peduncle.
In the medulla the fiber tracts of the spinal cord either terminate in
the nuclei of the gray matter or undergo changes in their relative
position, and new tracts appear. The gray matter is highly modified
and forms masses which have no homologues in the cord. The central
canal of the closed part of the medulla is surrounded by a thicker
layer of gray matter than is the case in the spinal cord. As the
medulla opens out this gray matter is naturally spread in the floor of
the fourth ventricle. The dorsal horns of gray matter become wide,
spread apart, and are broken up to a great extent in the formatio
reticularis. Two elongated masses of cells appear above the central
gray matter; these are the =nucleus gracilis= and =nucleus cuneatus=,
and in them the fasciculi of like name gradually end. In front of
these are the =terminal nuclei= of the afferent or sensory cranial
nerves and of the sensory portions of the mixed nerves. The ventral
horns are succeeded by the nuclei of =origin= of the efferent or motor
cranial nerves and the motor root-fibers of the mixed nerves. Of the
twelve pairs of cranial nerves, the last eight are connected with
nuclei in the medulla and pons. The =posterior olivary nucleus=
(Nucleus olivaris caudalis) is a conspicuous gray mass which lies
above the pyramid on each side. On cross-sections it appears as a wavy
layer of gray matter which is folded on itself so as to inclose a mass
of white matter. It is about 2 cm. long in the horse, but is smaller
in circumference than in man and hence does not cause any very
distinct external enlargement (olivary eminence), as in the latter. It
is connected with the opposite side of the cerebellum by the
cerebello-olivary fibers. At its inner side are two small accessory
olivary nuclei. The =pyramidal tracts=, which are small in ungulates,
send most of their fibers across to the opposite side in the posterior
part of the medulla, forming the =pyramidal decussation=. Some fibers,
however, continue in the ventral column of the same side of the cord,
and others are connected with the nuclei of the motor nerve-roots.
From the nucleus gracilis and nucleus cuneatus fibers arise which are
traceable forward to the thalamus. These are the =internal arcuate
fibers= (Fibræ arcuatæ internæ), which curve across the median plane
below the central gray matter and form with those of the opposite side
the =decussation of the fillet= (Decussatio lemniscorum). Beyond the
decussation the fibers form an important longitudinal tract which
extends forward in the mid-brain. This is the =fillet= or =lemniscus=,
the chief continuation of the sensory conducting path from the dorsal
roots of the spinal nerves. The =external arcuate fibers=, some of
which were seen on the lateral aspect of the medulla, have a similar
origin. Part of them (Fibræ dorsales) pass directly to the dorsal
aspect of the restiform body of the same side; others (Fibræ
ventrales) cross to the opposite side, descend close to the ventral
fissure, and then curve upward and forward to the restiform body. The
decussation of the arcuate fibers forms the distinct median =raphé=
seen on cross-sections of the medulla anterior to the pyramidal
decussation. The restiform body, situated dorso-laterally, contains,
in addition to the arcuate fibers, the =cerebello-olivary fasciculus=
before mentioned, and the =cerebellospinal fasciculus= or direct
cerebellar tract. The =dorsal longitudinal fasciculus= corresponds to
the ventral ground-bundle of the spinal cord, displaced dorsally by
the decussation of the pyramids and fillet. In the posterior part of
the medulla it is not marked off from the fillet, along the dorsal
edge of which it lies. From the level of the hypoglossal nucleus
forward it is distinct and can be traced as a conspicuous tract in the
ventral margin of the gray matter of the floor of the fourth ventricle
and of the central gray matter of the mid-brain. Below the restiform
body and related externally to the external arcuate fibers, there is a
considerable bundle of longitudinal fibers, the =spinal root= (Tractus
spinalis) =of the fifth nerve=; internal to it is the terminal nucleus
of the sensory root of the nerve.
[Illustration: