1. _Myelencephalon._--The myelencephalon or medulla oblongata is
transitional between the spinal cord and the brain. It has in general
the structural peculiarities of the cord, but these undergo in it a
gradual transformation into the arrangements characteristic of the
brain.
It has the form of a flattened and truncated cone, which widens craniad
and is limited at the cranial end by the pons (Fig. 138, _i_) ventrally
and laterally; by the cerebellum (Fig. 137, _III_) dorsally. The cranial
portion of its dorsal surface is overhung by the cerebellum. The points
of origin of the roots of the first pair of cervical nerves (Fig. 138,
_s_) may be taken as indicating the boundary between the spinal cord and
the medulla oblongata; there is no other external marking to show the
limits of the two.
The central canal of that part of the medulla which is overhung by the
cerebellum is greatly widened (Fig. 141, _h_). The cavity thus formed is
flattened and triangular, with the apex of the triangle caudad, and is
the caudal part of the =fourth ventricle= (Fig. 141, _h_). The roof is
very thin (Fig. 143, _n_) and is intimately connected with the pia
mater, so that in removing the latter the thin roof of the fourth
ventricle is removed with it and the cavity of the fourth ventricle is
left exposed. The thin roof of the fourth ventricle is known as the
=velum medullare posterius= (Fig. 143, _n_).
The ventral (anterior) fissure (Fig. 138, _p_) of the cord passes onto
the medulla oblongata and ends at the pons (_i_), its end being slightly
deeper and forming what is known as the =foramen cæcum= (_q_). The
dorsal (posterior) sulcus (Fig. 141, _a_) is well marked; it ends at the
caudal boundary of the fourth ventricle (_h_).
The anterior lateral sulcus (Fig. 138, _r_) may be traced craniad from
the origin of the ventral roots of the first cervical nerves (_s_),
along the lateral border of the area elliptica (_n_), then curving
mediad to the lateral border of the pyramis (_o_), and finally reaching
the pons (_i_). Its position is marked by the origin of the roots of the
hypoglossal nerve (_XII_).
The posterior lateral sulcus (Fig. 141, _b_), marked on the spinal cord
by the origin of the dorsal nerve-roots, curves laterad at the sides of
the fourth ventricle (_h_) owing to the increasing width of the latter,
and ends at an elevated area of oblique fibres, the =area ovalis= (_f_).
The columns or funiculi bounded by the longitudinal fissures present the
following peculiarities:
The anterior white funiculus of the cord is replaced in the medulla by
the =pyramidal tracts= (=pyramides=) (Fig. 138, _o_). The pyramidal
tracts are formed by fibres which emerge from beneath the pons and pass
caudad to disappear just craniad of the level of the first cervical
nerve (_s_). The pyramidal tracts are bounded medially by the anterior
median fissure (_p_), but laterally each is separated from the anterior
lateral sulcus (_r_) over its caudal part by an elongated =area
elliptica= (_n_), the human homologue of which is uncertain. It perhaps
represents the =oliva=.
Laterad of the cranial portion of the pyramids is an irregular area
known as the =trapezium= (_l_) which abuts caudad on the area elliptica
(_n_) and the area ovalis (_m_). The =area ovalis= (Fig. 138, _m_; Fig.
141, _f_) (or zonula Arnoldi) is abroad band of oblique fibres which
passes from the lateral side of the area elliptica craniodorsad to
disappear under the cerebellum.
The posterior white funiculus was divided in the cervical region into
two, the =fasciculus gracilis= (column of Goll) and the =fasciculus
cuneatus= (column of Burdach). The fasciculus gracilis (Fig. 141, _c_)
extends to the caudal end of the fourth ventricle (_h_) and ends there
in an expansion, the =clava= (_d_), which forms the posterior boundary
of the fourth ventricle. The fasciculus cuneatus (_e_) passes laterad on
account of the width of the fourth ventricle and appears to end at the
area ovalis (_f_), but it may be seen passing beneath the area ovalis,
emerging at its cranial border and turning dorsad to enter the
cerebellum. It forms the side walls of a part of the shallow fourth
ventricle.
The lateral funiculi are divided by longitudinal furrows into three
divisions. The dorsal one of these is the =fasciculus cuneatus
lateralis= or column of Rolando (Fig. 141, _g_). It accompanies the
fasciculus cuneatus into the cerebellum.
The entire mass formed by the fasciculus gracilis, the fasciculus
cuneatus medialis, and fasciculus cuneatus lateralis is known as the
=corpus restiforme=, and since its fibres pass into the cerebellum it is
sometimes known as the =pedunculus cerebelli= (or crus cerebelli ad
medullam).
The following cranial nerves arise from the medulla oblongata (Fig.
138).
The twelfth nerve (_XII_) (=N. hypoglossus=) arises by ten or fifteen
rootlets from the ventral surface of the medulla oblongata in the
anterior lateral fissure (_r_) opposite the caudal portion of the area
elliptica (_n_).
The eleventh cranial nerve (_XI_) (=N. accessorius=) arises by numerous
rootlets from the lateral surface of the medulla oblongata and of the
spinal cord as far caudad as the sixth or seventh cervical nerve. These
rootlets join to form a nerve which enters the cranium through the
foramen magnum and is closely associated at its point of exit with the
glossopharyngeal (_IX_) and vagus (_X_) nerves. The line of origin on
the medulla passes between the dorsal and ventral roots of the cervical
nerves and is along the middle of the lateral white funiculi.
The tenth cranial nerve (_X_) (=N. vagus=) arises by about eighteen very
delicate rootlets from the surface of the area ovalis. The rootlets are
divided into a dorsal and a ventral series. The dorsal series (about
twelve) arise in the groove which separates the fasciculus cuneatus
medialis from the fasciculus cuneatus lateralis. The ventral series
arise somewhat ventrad of this groove. These rootlets are to be
distinguished from those of the ninth nerve by their smaller size.
The ninth cranial nerve (_IX_) (=N. glossopharyngeus=) arises from the
area ovalis from a line craniad of the dorsal line of origin of the
vagus roots (_X_) and between these and those of the auditory (_VIII_).
It arises by a number of rootlets which are larger than those of the
vagus (_X_), with which this nerve is closely associated.
The eighth cranial nerve (_VIII_) (=N. acusticus=) appears at the
lateral end of the trapezium (_l_). It arises from an elevation (Fig.
141, _i_) which is continued dorsomediad along the cranial border of the
area ovalis.
The seventh cranial nerve (_VII_) (=N. facialis=) leaves the lateral
border of the trapezium (_l_) near its cranial edge, between the fifth
and eighth nerves. It is much smaller than the eighth nerve.
The sixth cranial nerve (_VI_) (=N. abducens=) arises by about six
bundles from the groove between the pyramids and the trapezii and passes
craniad.
2. _Metencephalon._--The metencephalon includes the =pons= and the
=cerebellum=.
The =pons= (Fig. 138, _i_) is a mass of transverse fibres which forms
the ventral and cranial part of the primitive hindbrain. It is a
modification of the latter brought about by the development of the
cerebellum, and the degree of its development is in direct ratio to that
of the cerebellar and cerebral hemispheres. The pons forms a projecting
mass of fibres which is marked by a median longitudinal groove, the
=sulcus basilaris= (_j_), which indicates the course of the basilar
artery (Fig. 121, _c_). Laterad the fibres of the pons converge somewhat
and turning dorsad disappear in the cerebellum, forming the =brachia
pontis= (Fig. 141, _l_).
The fifth cranial nerve (Fig. 138, _V_) (=N. trigeminus=) arises by two
roots from the caudal border of the pons, near the lateral end. The
ventral root (4) is small; the dorsal one is much larger and soon forms
the large semilunar ganglion (_k_) from which three branches (1, 2, and
3) diverge. The ventral root (4) joins one of these branches (1).
The =cerebellum= (Fig. 137, _III_) is formed by an increase in size of
the cranial portion of the primitive hindbrain. This increase has
affected principally the surface of the roof, so that as the cerebellum
has grown it has been thrown into many folds, the exact form of which
varies in different specimens. The cerebellum has at the same time
increased in size and has thus extended laterad as well as caudad and
craniad. It thus touches the cerebrum in front (separated from it by the
tentorium) and aids it in concealing the midbrain and ’tween-brain in
dorsal view, while caudad in the same view it conceals the greater part
of the medulla. The connections of the cerebellum with adjacent parts of
the brain are also overhung and concealed.
The whole surface of the cerebellum is thrown up into numerous folds or
=gyri=, separated from one another by deep fissures or =sulci=, which
appear at first to render the surface wholly irregular. The entire mass
is, however, divisible into a central portion, which from its
resemblance to a segmented worm is called the =vermis= (_j_) (its
cranial part is the =superior vermis=, and its caudal part the =inferior
vermis=), and into lateral portions, the =hemispheres= (_k_). The
vermis (_j_) occupies a median longitudinal position, and its gyri and
sulci are in the main transverse. It is not directly connected with
adjacent parts, and its ventral surface extends farther caudad and
craniad than that of the hemispheres. The ventral part of the superior
vermis is fitted against the posterior corpora quadrigemina.
The =hemispheres= (_k_) may again be subdivided into groups of gyri
which have received special names. One of these, the so-called
appendicular lobe, fits into the appendicular fossa of the petrous bone.
The cerebellum is connected to the adjacent parts by three tracts of
fibres, sometimes known as crura cerebelli. The tract connecting it with
the medulla oblongata is the =corpus restiforme=; that connecting it
with the pons is the =brachium pontis= (Fig. 141, _l_); these have been
described. A third tract passes craniad to the corpora quadrigemina
(Fig. 141, _p_ and _q_); this is the =brachium conjunctivum= (Fig. 141,
_k_).
The cerebellum is composed of white and gray matter, the latter on the
surface (Fig. 143, _III_). The folds of its surface present thus a
contrivance for increasing the amount of gray matter. The white matter
forms a central mass from which tracts extend into the folds. The whole
mass of white matter has thus in section (more particularly in a
longitudinal section of the vermis) the appearance of a tree, whence the
name =arbor vitæ= (Fig. 143).
The =fourth ventricle= (Fig. 141, _h_; Fig. 143, _m_) is the cavity of
the original hindbrain. It begins caudad at the clava (Fig. 141, _d_) as
a widening and continuation of the central canal of the spinal cord and
extends craniad, becoming wider and passing ventrad of the cerebellum.
It becomes narrower craniad and ends at the posterior corpora
quadrigemina (_p_), where it becomes continuous with the aqueductus
cerebri (Fig. 141, _o_; Fig. 143, _j_). The cavity is shallow and is
encroached upon dorsally by the vermis of the cerebellum (Fig. 143).
The floor of the cavity is known as the =fossa rhomboidea= (Fig. 141,
_h_). It is formed by the continuation of the gray matter which
surrounds the central canal of the cord. It is marked by a median
longitudinal groove. At its widest part are seen two considerable tracts
of white fibres (=striæ medullares=) which pass from near the median
line laterad and extend into the auditory (eighth) nerves. The floor
caudad and craniad of these striæ is marked by a number of elevations
and depressions. Similarly situated elevations in the human brain differ
from one another slightly in color, are made up of gray matter, and are
the centres of origin of most of the cranial nerves. Their homologues in
the cat appear not to have been determined.
The side walls of the fourth ventricle (Fig. 141) are formed by the
following in order, beginning caudad: the =clava= (_d_), the =corpus
restiforme=, =brachium pontis= (_l_), =brachium conjunctivum= (_k_), and
=caudal corpora quadrigemina= (_p_) (=colliculi inferiores=).
The roof (Fig. 143) is formed caudad by a thin layer of non-nervous
matter which is closely associated with the pia mater. This thin layer
is known as the =velum medullare posterius= (_n_). It connects the
dorsal surface of the medulla with the caudal border of the cerebellum.
The pia mater covering this portion of the roof is vascular and is
folded in toward the floor of the ventricle, forming the =choroid
plexus= of the fourth ventricle. In the middle the roof of the fourth
ventricle is the cerebellum, while craniad the roof is the =velum
medullare anterius= (Fig. 143, _l_). This is a thin layer just craniad
of the cerebellum, connecting it with the corpora quadrigemina, and
attached laterally to the brachia conjunctiva. Here the fourth ventricle
narrows craniad and becomes continuous with the slender =aqueductus
cerebri= (Fig. 143, _j_) (aqueduct of Sylvius). The narrowed portion of
the brain is frequently known as the =isthmus rhombencephali=.
[Illustration: FIG. 141.--DORSAL SURFACE OF MYELENCEPHALON,
MESENCEPHALON, AND DIENCEPHALON.
The cerebellum and the greater part of the cerebral hemispheres have
been removed. _a_, posterior sulcus of cord; _b_, posterior lateral
sulcus; _c_, fasciculus gracilis; _d_, clava; _e_, fasciculus
cuneatus; _f_, area ovalis; _g_, fasciculus cuneatus lateralis; _h_,
fossa rhomboidea or floor of fourth ventricle; _i_, projection formed
by origin of auditory nerve; _j_, facial nerve; _k_, cut end of
brachium conjunctivum; _l_, cut end of brachium pontis (_l′_, brachium
pontis); _m_, velum medullare anterius; _n_, fourth nerve; _o_,
depression marking caudal end of aqueductus cerebri (aqueduct of
Sylvius); _p_, caudal corpora quadrigemina (colliculi posteriores);
_q_, cranial corpora quadrigemina (colliculi anteriores); _r_,
brachium quadrigeminum inferius; _s_, corpus geniculatum mediale; _t_,
thalamus; _u_, striæ medullares; _v_, third ventricle; _w_, pulvinar;
_x_, corpus geniculatum laterale; _y_, corpus striatum; _z_, outlines
of olfactory bulbs. 1, boundary between hemispheres and ’tween-brain;
2, pineal body.]
The fourth cranial nerve (Fig. 141, _n_; Fig. 138, _IV_) (=N.
trochlearis=) arises from the brain at the craniolateral angle of the
velum medullare anterius.