The mid-brain or mesencephalon (Fig. 90681) is the short, constricted portion which connects the pons
and cerebellum with the thalamencephalon and cerebral hemispheres.
It is directed upward and forward, and consists of (1) a ventrolateral
portion, composed of a pair of cylindrical bodies, named the cerebral
(2) a dorsal portion, consisting of four rounded eminences,
named the corpora quadrigemina; and (3) an intervening passage
or tunnel, the cerebral aqueduct, which represents the original
cavity of the mid-brain and connects the third with the fourth ventricle
(Fig. 90710).

FIG. 90710– Coronal
section through mid-brain. (Schematic.) (Testut.) 1. Corpora quadrigemina.
2. Cerebral aqueduct. 3. Central gray stratum. 4. Interpeduncular
space. 5. Sulcus lateralis. 6. Substantia nigra. 7. Red nucleus
of tegmentum. 8. Oculomotor nerve, with 8’, its nucleus of
origin. a. Lemniscus (in blue) with the medial
lemniscus and a” the lateral lemniscus. b. Medial
longitudinal fasciculus. c. Raphé. d. Temporopontine
fibers. e. Portion of medial lemniscus, which runs to the
lentiform nucleus and insula. f. Cerebrospinal fibers. g.
Frontopontine fibers.
  The cerebral peduncles (pedunculus cerebri;
crus cerebri
) are two cylindrical masses situated at the base
of the brain, and largely hidden by the temporal lobes of the cerebrum,
which must be drawn aside or removed in order to expose them. They
emerge from the upper surface of the pons, one on either side of the
middle line, and, diverging as they pass upward and forward, disappear
into the substance of the cerebral hemispheres. The depressed area
between the crura is termed the interpeduncular fossa, and
consists of a layer of grayish substance, the posterior perforated
which is pierced by small apertures for the transmission
of bloodvessels; its lower part lies on the ventral aspect of the
medial portions of the tegmenta, and contains a nucleus named the
interpeduncular ganglion (page 802); its upper part assists
in forming the floor of the third ventricle. The ventral surface

of each peduncle is crossed from the medial to the lateral side by
the superior cerebellar and posterior cerebral arteries; its lateral
surface is in relation to the gyrus hippocampi of the cerebral hemisphere
and is crossed from behind forward by the trochlear nerve. Close to
the point of disappearance of the peduncle into the cerebral hemisphere,
the optic tract winds forward around its ventro-lateral surface. The
medial surface of the peduncle forms the lateral boundary of the interpeduncular
fossa, and is marked by a longitudinal furrow, the oculomotor sulcus,
from which the roots of the oculomotor nerve emerge. On the lateral
surface of each peduncle there is a second longitudinal furrow, termed
the lateral sulcus; the fibers of the lateral lemniscus come
to the surface in this sulcus, and pass backward and upward, to disappear
under the inferior colliculus.

FIG. 90711– Transverse
section of mid-brain at level of inferior colliculi.

FIG. 90712– Transverse
section of mid-brain at level of superior colliculi.
Structure of the Cerebral Peduncles (Figs.
90711, 90712).
—On transverse
section, each peduncle is seen to consist of a dorsal and a ventral
part, separated by a deeply pigmented lamina of gray substance, termed
the substantia nigra. The dorsal part

is named the tegmentum; the ventral, the base or crusta;
the two bases are separated from each other, but the tegmenta are
joined in the median plane by a forward prolongation of the raphé
of the pons. Laterally, the tegmenta are free; dorsally, they blend
with the corpora quadrigemina.
  The base (basis pedunculi; crusta or pes)
is semilunar on transverse section, and consists almost entirely of
longitudinal bundles of efferent fibers, which arise from the cells
of the cerebral cortex and are grouped into three principal sets,
viz., cerebrospinal, frontopontine, and temporopontine (Fig. 90710). The cerebrospinal fibers, derived from the cells
of the motor area of the cerebral cortex, occupy the middle three-fifths
of the base; they are continued partly to the nuclei of the motor
cranial nerves, but mainly into the pyramids of the medulla oblongata.
The frontopontine fibers are situated in the medial fifth of
the base; they arise from the cells of the frontal lobe and end in
the nuclei of the pons. The temporopontine fibers are lateral
to the cerebrospinal fibers; they originate in the temporal lobe and
end in the nuclei pontis. 123
  The substantia nigra (intercalatum) is
a layer of gray substance containing numerous deeply pigmented, multipolar
nerve cells. It is semilunar on transverse section, its concavity
being directed toward the tegmentum; from its convexity, prolongations
extend between the fibers of the base of the peduncle. Thicker medially
than laterally, it reaches from the oculomotor sulcus to the lateral
sulcus, and extends from the upper surface of the pons to the subthalamic
region; its medial part is traversed by the fibers of the oculomotor
nerve as these stream forward to reach the oculomotor sulcus. The
connections of the cells of the substantia nigra have not been definitely
established. It receives collaterals from the medial lemniscus and
the pyramidal bundles. Bechterew is of the opinion that the fibers
from the motor area of the cerebral cortex form synapses with cells
whose axons pass to the motor nucleus of the trigeminal nerve and
serve for the coördination of the muscles of mastication.
  The tegmentum is continuous below with the reticular
formation of the pons, and, like it, consists of longitudinal and
transverse fibers, together with a considerable amount of gray substance.
The principal gray masses of the tegmentum are the red nucleus and
the interpeduncular ganglion; of its fibers the chief longitudinal
tracts are the superior peduncle, the medial longitudinal fasciculus,
and the lemniscus.
red nucleus is situated in the anterior part of the tegmentum,
and is continued upward into the posterior part of the subthalamic
region. In sections at the level of the superior colliculus it appears
as a circular mass which is traversed by the fibers of the oculomotor
nerve. It receives many terminals and collaterals from the superior
cerebellar peduncle also collaterals from the ventral longitudinal
bundle, from Gudden’s bundle and the median lemniscus. The axons
of its larger cells cross the middle line and are continued downward
into the lateral funiculus of the medulla spinalis as the rubrospinal
tract (page 761); those of its smaller cells end mainly in the thalamus.
The rubrospinal tract forms an important part of the pathway from
the cerebellum to the lower motor centers.
  The interpeduncular ganglion is a median collection
of nerve cells situated in the ventral part of the tegmentum. The
fibers of the fasciculus retroflexus of Meynert, which have their
origin in the cells of the ganglion habenulæ (page 812), end
in it.
  Besides the two nuclei mentioned, there are small collections
of cells which form the dorsal and ventral nuclei and the central
nucleus or nucleus of the raphé.
The origin and course of the superior peduncle have already
been described (page 792).
  (2) The medial (posterior) longitudinal
is continuous below with the proper fasciculi of the
anterior and lateral funiculi of the medulla spinalis. In the medulla
oblongata and pons it runs close to the middle line, near the floor
of the fourth ventricle; in the mid-brain it is situated on the ventral
aspect of the cerebral aqueduct, below the nuclei of the oculomotor
and trochlear nerves. Its connections are imperfectly known, but it
consists largely of ascending and descending intersegmental or association
fibers, which connect the nuclei of the hind-brain and mid-brain to
each other. Many of the fibers arise in Deiters’s nucleus
(lateral vestibular nucleus) and divide into ascending and
descending branches which send terminals and collaterals to the motor
nuclei of the cranial and spinal nerves. Its spinal portion is located
in the anterior funiculus and is known as the vestibulospinal fasciculus.
Other fibers pass to the median longitudinal bundle from cells in
the reticular formation of the medulla, pons and mid-brain and also
from certain large cells in the terminal nucleus of the trigeminal
nerve. According to Edinger it extends to the so-called nucleus of
the posterior longitudinal bundle in the hypothalamic region, but
this is uncertain and the fibers above the nucleus of the oculomotor
are smaller in diameter than the rest of the bundle. According to
Held fibers from the posterior commissure can be traced into the posterior
longitudinal bundle, and according to the same author many of the
descending fibers arise in the superior colliculus, and, after decussating
in the middle line, end in the motor nuclei of the pons and medulla
oblongata. These fibers from the superior colliculus probably pass
into the ventral longitudinal bundle. Fibers are said to pass through
the medial longitudinal fasciculus from the nucleus of the abducent
nerve into the oculomotor nerve of the opposite side, and through
this nerve to the Rectus medialis oculi. Fraser, however, denies the
existence of such fibers. Again, fibers are said to be prolonged through
this fasciculus from the nucleus of the oculomotor nerve into the
facial nerve, and are distributed to the Orbicularis oculi, the Corrugator,
and the Frontalis. 124
  The ventral longitudinal bundle consists for
the most part of the tectospinal fasciculus, and arises from
the superior colliculus, the fibers arch ventrally around the central
gray matter and cross the midline in the fountain-decussation of Meynert.
They then descend in the tegmentum, part of them passing through the
red nucleus ventral to the medial longitudinal bundle. In the medulla
oblongata and spinal cord its fibers are more or less intermingled
with the medial longitudinal bundle and the rubrospinal tract. It
descends in the adjoining region of the ventral and lateral funiculi.
Collaterals and terminals are given off to the red nucleus and probably
other nuclei of the brain stem and to the anterior column of the spinal
cord. It is probably concerned in optic reflexes.
  (3) The medial lemniscus or medial fillet
(Fig. 90713).—The fibers of the medial
lemniscus take origin in the gracile and cuneate nuclei of the medulla
oblongata, and as internal arcuate fibers they cross to the opposite
side in the sensory decussation (page 777). They then pass in the
interolivary stratum upward through the medulla oblongata, in which
they are situated behind the cerebrospinal fibers and between the
olives. In the pons and lower part of the mid-brain it occupies the
ventral part of the reticular formation and tegmentum close to the
raphé, while above it gradually shifts to the dorso-lateral part
of the tegmentum in the angle between the red nucleus and the substantia
nigra. In the pons it assumes a flattened ribbon-like appearance,
and is placed dorsal to the trapezium. As the lemniscus ascends, it
receives additional fibers from the terminal sensory nuclei of the

nerves of the opposite side. Many of the fibers which arise from the
terminal sensory nuclei of the cranial nerves pass upward in the formatio
reticularis as a separate bundle, known as the central tract of
the cranial nerves,
to the thalamus.
  Many fibers either terminate in or send off collaterals
to the gray matter of the medulla, the pons, and the mid-brain. Large
numbers of fibers pass to or from the substantia nigra. Many collaterals
enter the red nucleus and other fibers are said to run to the superior
colliculus. The great bulk of the fibers, however, enter the ventro-lateral
portion of the thalamus, give off collaterals to the posterior semilunar
nucleus and then terminate in the principal sensory nucleus of the

FIG. 90713– Scheme
showing the course of the fibers of the lemniscus; medial lemniscus
in blue, lateral in red.
  In the cerebral peduncle, a few of its fibers pass upward
in the lateral part of the base of the peduncle, on the dorsal aspect
of the temporopontine fibers, and reach the lentiform nucleus and
the insula. The greater part of the medial lemniscus, on the other
hand, is prolonged through the tegmentum, and most of its fibers end
in the thalamus; probably some are continued directly through the
occipital part of the internal capsule to the cerebral cortex. From
the cells of the thalamus a relay of fibers is prolonged to the cerebral
  The medial lemniscus may be considered as the upward
continuation of the posterior funiculus of the spinal cord and to
convey conscious impulses of muscle sense and tactile discrimination.
  The central or thalamic tract of the cranial
is closely associated with the medial lemniscus. The fibers
of the spinothalamic fasciculi are continued from the spinal cord
into this tract which passes upward in the reticular formation and
the tegmentum to the thalamus along the dorsal side of the median
lemniscus. It receives fibers from the opposite terminal sensory nuclei
of the vagus, glossopharyngeal, facial, trigeminal and probably the
vestibular nerves. Many of the secondary sensory fibers of the trigeminal
cross the raphé from its terminal nucleus and pass upward to
the thalamus by a more or less separate but closely associated pathway
known as the central tract of the trigeminal nerve which also
lies on the dorsal aspect of the lemniscus. These two tracts give
off collaterals to the posterior semilunar nucleus of the thalamus
and terminate in the anterior semilunar nucleus of the ventro-lateral
region of the thalamus sending collaterals into the zona incerta.

FIG. 90714– Transverse
section passing through the sensory decussation. Schematic. (Testut.)
1. Anterior median fissure. 2. Posterior median sulcus. 3, 3’.
Head and base of anterior column (in red). 4. Hypoglossal nerve.
5. Bases of posterior column. 6. Gracile nucleus. 7. Cuneate nucleus.
8, 8. Lemniscus. 9. Sensory decussation. 10. Cerebrospinal fasciculus.
  The fibers of the rubrospinal tract (bundle
of Monakow
) arise in the red nucleus, cross the midline in the
decussation of Forel and pass downward in the formatio reticularis
of the brainstem into the lateral funiculus of the spinal cord ventral
to the crossed pyramidal tract.
  The lateral lemniscus (lemniscus lateralis)
comes to the surface of the mid-brain along its lateral sulcus, and
disappears under the inferior colliculus. It consists of fibers from
the terminal nuclei of the cochlear division of the acoustic nerve,
together with others from the superior olivary and trapezoid nuclei.
Most of these fibers are crossed, but some are uncrossed. Many of
them pass to the inferior colliculus of the same or opposite side,
but others are prolonged to the thalamus, and thence through the occipital
part of the internal capsule to the middle and superior temporal gyri.
  The corpora quadrigemina (Fig. 90720) are four rounded eminences which form the dorsal part of
the mid-brain. They are situated above and in front of the anterior
medullary velum and superior peduncle, and below and behind the third
ventricle and posterior commissure. They are covered by the splenium
of the corpus callosum, and are partly overlapped on either side by
the medial angle, or pulvinar, of the posterior end of the
thalamus; on the lateral aspect, under cover of the pulvinar, is an
oval eminence, named the medial geniculate body. The corpora
quadrigemina are arranged in pairs (superior and inferior colliculi),
and are separated from one another by a crucial sulcus. The longitudinal
part of this sulcus expands superiorly to form a slight depression
which supports the pineal body, a cone-like structure which
projects backward from the thalamencephalon and partly obscures the
superior colliculi. From the inferior end of the longitudinal sulcus,
a white band, termed the frenulum veli, is prolonged downward
to the anterior medullary velum; on either side of this band the trochlear
nerve emerges, and passes forward on the lateral aspect of the cerebral
peduncle to reach the base of the brain. The superior colliculi
are larger and darker in color than the inferior, and are oval in
shape. The inferior colliculi are hemispherical, and somewhat
more prominent than the superior. The superior colliculi are associated
with the sense of sight, the inferior with that of hearing.
  From the lateral aspect of each colliculus a white band,
termed the brachium, is prolonged upward and forward. The superior
extends lateralward from the superior colliculus, and,
passing between the pulvinar and medial geniculate

body, is partly continued into an eminence called the lateral geniculate
and partly into the optic tract. The inferior brachium
passes forward and upward from the inferior colliculus and disappears
under cover of the medial geniculate body.
  In close relationship with the corpora quadrigemina
are the superior peduncles, which emerge from the upper and medial
parts of the cerebellar hemispheres. They run upward and forward,
and, passing under the inferior colliculi, enter the tegmenta as already
described (page 792).
Structure of the Corpora Quadrigemina.—The inferior
(colliculus inferior; inferior quadrigeminal body;
) consists of a compact nucleus of gray substance containing
large and small multipolar nerve cells, and more or less completely
surrounded by white fibers derived from the lateral lemniscus. Most
of these fibers end in the gray nucleus of the same side, but some
cross the middle line and end in that of the opposite side. From the
cells of the gray nucleus, fibers are prolonged through the inferior
brachium into the tegmentum of the cerebral peduncle, and are carried
to the thalamus and the cortex of the temporal lobe; other fibers
cross the middle line and end in the opposite colliculus.
  The superior colliculus (colliculus superior;
superior quadrigeminal body; pregemina
) is covered by a thin stratum
(stratum zonale) of white fibers, the majority of which are
derived from the optic tract. Beneath this is the stratum cinereum,
a cap-like layer of gray substance, thicker in the center than at
the circumference, and consisting of numerous small multipolar nerve
cells, imbedded in a fine network of nerve fibers. Still deeper is
the stratum opticum, containing large multipolar nerve cells,
separated by numerous fine nerve fibers. Finally, there is the stratum
consisting of fibers derived partly from the lemniscus
and partly from the cells of the stratum opticum; interspersed among
these fibers are many large multipolar nerve cells. The two last-named
strata are sometimes termed the gray-white layers, from the
fact that they consist of both gray and white substance. Of the afferent
fibers which reach the superior colliculus, some are derived from
the lemniscus, but the majority have their origins in the retina and
are conveyed to it through the superior brachium; all of them end
by arborizing around the cells of the gray substance. Of the efferent
fibers, some cross the middle line to the opposite colliculus; many
ascend through the superior brachium, and finally reach the cortex
of the occipital lobe of the cerebrum; while others, after undergoing
decussation (fountain decussation of Meynert) form the tectospinal
fasciculus which descends through the formatio reticularis of the
midbrain, pons, and medulla oblongata into the medulla spinalis, where
it is found partly in the anterior funiculus and partly intermingled
with the fibers of the rubrospinal tract.
  The corpora quadrigemina are larger in the lower animals
than in man. In fishes, reptiles, and birds they are hollow, and only
two in number (corpora bigemina); they represent the superior colliculi
of mammals, and are frequently termed the optic lobes, because of
their intimate connection with the optic tracts.
  The cerebral aqueduct (aqueductus cerebri;
aqueduct of Sylvius
) is a narrow canal, about 15 mm. long, situated
between the corpora quadrigemina and tegmenta, and connecting the
third with the fourth ventricle. Its shape, as seen in transverse
section, varies at different levels, being T-shaped, triangular above,
and oval in the middle; the central part is slightly dilated, and
was named by Retzius the ventricle of the mid-brain. It is
lined by ciliated columnar epithelium, and is surrounded by a layer
of gray substance named the central gray stratum: this is continuous
below with the gray substance in the rhomboid fossa, and above with
that of the third ventricle. Dorsally, it is partly separated from
the gray substance of the quadrigeminal bodies by the fibers of the
lemniscus; ventral to it are the medial longitudinal fasciculus, and
the formatio reticularis of the tegmentum. Scattered throughout the
central gray stratum are numerous nerve

cells of various sizes, interlaced, by a net-work of fine fibers.
Besides these scattered cells it contains three groups which constitute
the nuclei of the oculomotor and trochlear nerves, and the nucleus
of the mesencephalic root of the trigeminal nerve. The nucleus
of the trigeminal nerve
extends along the entire length of the
aqueduct, and occupies the lateral part of the gray stratum, while
the nuclei of the oculomotor and trochlear nerves are situated in
its ventral part. The nucleus of the oculomotor nerve is about
10 cm. long, and lies under the superior colliculus, beyond which,
however, it extends for a short distance into the gray substance of
the third ventricle. The nucleus of the trochlear nerve is
small and nearly circular, and is on a level with a plane carried
transversely through the upper part of the inferior colliculus.
Note 123.  A band of fibers, the tractus peduncularis transversus, is sometimes seen emerging from in front of the superior colliculus; it passes around the ventral aspect of the peduncle about midway between the pons and the optic tract, and dips into the oculomotor sulcus. This band is a constant structure in many mammals, but is only present in about 30 per cent. of human brains. Since it undergoes atrophy after enucleation of the eyeballs, it may be considered as forming a path for visual sensations. [back]
Note 124.  A. Bruce and J. H. Harvey Pirrie, “On the Origin of the Facial Nerve,” Review of Neurology and Psychiatry, December, 1908, No. 12, vol. vi, produce weighty evidence against the view that the facial nerve derives fibers from the nucleus of the oculomotor nerve. [back]


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