of the spinal cord, showing the location of the different tracts as outlined by Flechsig on the basis
of differences in time of myelination. (van Gehuchten.)
mination of the tract is unknown. The tectospinal tract, located in the ventral
funiculus, is composed of fibers which take origin in the roof (tectum) of the
mesencephalon, cross the median plane and descend into the anterior funiculus
of the spinal cord, and end in the gray matter of the anterior column. The tract
is concerned chiefly with optic reflexes. The vestibulospinal tract, also located
in the anterior funiculus, arises from the lateral nucleus of the vestibular nerve
II2 THE NERVOUS SYSTEM
in the medulla oblongata and conveys impulses concerned in the maintenance
of equilibrium. Some of its fibers can be traced as far as the lower lumbar
segments. They end in the gray matter of the anterior column.
Hemisection of the spinal cord in man produces a characteristic symptom
complex known as the Brown-Sequard's syndrome which the student is now in
position to understand. Below the level of the lesion and on the same side
there is found a paralysis of the muscles with a loss of sensation from the mus-
cles, joints, and tendons; while on the opposite side of the body, beginning two
or three segments below the level of the lesion, there is loss of sensations of
pain and temperature. Tactile sensibility is everywhere retained (Dejerine,
1914).
Order of Myelination. The fiber tracts of the spinal cord do not all become
myelinated at the same time. By a study of the fetal spinal cord at various"
developmental stages Flechsig was able to identify and trace many of these
tracts because of the difference in the time of myelination. His results agree
in general with those derived from a study of spinal cords showing ascending
and descending degeneration (Figs. 79, 80). Myelination begins during the fifth
month of intra-uterine life. The order in which the fibers of the spinal cord
acquire their myelin sheaths is as follows: (1) afferent and efferent root fibers,
(2) those of the fasciculi proprii, (3) the fasciculus cuneatus, (4) the fasciculus
gracilis, (5) the dorsal spinocerebellar tract, (6) the ventral spinocerebellar fas-
ciculus, (7) the corticospinal tracts.
CHAPTER VIII
THE GENERAL TOPOGRAPHY OF THE BRAIN. THE EXTERNAL
FORM OF THE MEDULLA OBLONGATA, PONS, AND MESEN-
CEPHALON
The General Topography of the Brain. The brain rests upon the floor of
the cranial cavity, which presents three well-marked fossae. In the posterior
cranial fossa are lodged the medulla oblongata, pons, and cerebellum, which
together constitute the rhombencephalon (Fig. 81). This fossa is roofed over
by a partition of dura mater, called the tentorium cerebelli, that separates the
cerebellum from the cerebral hemispheres. Through the notch in the ventral
Caharia
Prosen-f Telencephalon
cephalon\Diencephalon
Frontal lobe of cerebral
hemisphere in anterior
cranial fossa
Temporal lobe of cerebral
hemisphere in middle
cranialfossa
Parietal lobe of cerebral
hemisphere
Mesencephalon
Occipital lobe of cerebral
hemisphere
Tentorium cerebelli
Posterior cranial fossa
Cerebellum
Pons
Medulla oblongata
Spinal cord
Fig. 81. Median sagittal section of the head showing the relation of the brain to the cra-
nium. The sphenoid bone is shown in transparency, and through it the temporal lobe may be
seen.
border of the tentorium projects the mesencephalon, connecting the rhomben-
cephalon below with the prosencephalon above that partition. The cerebral
hemispheres form the largest part of the prosencephalon, occupy the anterior
and middle cranial fossae, and extend to the occiput on the upper surface of the
tentorium.
The dorsal aspect of the human brain presents an ovoid figure, the large
cerebral hemispheres, covering the other parts from view. In the sheep's brain the
8 113
THE NERVOUS SYSTEM
hemispheres are smaller and fail to hide the cerebellum and medulla oblongata
(Fig. 82). The cerebral hemispheres, which are separated by a deep cleft called
the longitudinal fissure of the cerebrum, together present a broad convex surface
which lies in close relation to the internal aspect of the calvaria. From the
latter it is separated only by the investing membranes or meninges of the brain.
The thin convoluted layer of gray matter upon the surface of the hemispheres is
known as the cerebral cortex.
The ventral aspect or base of the brain presents an irregular surface adapted
to the uneven floor of the cranial cavity (Figs. 83, 86). The medulla oblongata,
Face and tongue
Head and eyes
Fore limb
Hind limb
Cyrus sylviactis (arcuatus)
Cyrus lateralis
Gyri mediates
Cyrus intermix -
Vermis cerebelli
Hemispluerium cerebelli
Medulla oblongata
Medulla spinalis
Cyrus frontalis medialis
Cyrus frontalis superior
Sulcus coronal is
Sulcus splenialis
Fissura ansata (cruciata)
Fissura lateralis (Sylvii)
Fissura suprasylvia
Fisstira longitudinalis
Sulcus lateralis
Sulcus intermedius
Sulcus medialis
Flocculus
Nervus accessorius
Nervus spinalis I
Fig. 82. Dorsal view of the sheep's brain. The motor cortex is shaded on the left side. (Herrick
and Crosby.)
which is continuous through the foramen magnum with the spinal cord, lies on
the ventral aspect of the cerebellum in the vallecula between the two cere-
bellar hemispheres. Rostral to the medulla oblongata and separated from it
only by a transverse groove is a broad elevated band of fibers, which plunges
into the cerebellum on either side and is known as the pons. The cerebellum
can be seen occupying a position dorsal to the pons and medulla oblongata, and
can easily be recognized by its grayish color and many parallel fissures. A
pair of large rope-like strands are seen to emerge from the rostral border of
the pons and to diverge from each other as they run toward the under surface
THE GENERAL TOPOGRAPHY OF THE BRAIN
of the cerebral hemispheres. These are the cerebral peduncles and they form
the ventral part of the mesencephalon. At its rostral extremity each peduncle
is partially encircled by a flattened band, known as the optic tract, which is con-
tinuous through the optic chiasma with the optic nerves. A lozenge-shaped
depression, known as the inter peduncular fossa, is outlined by the diverging
cerebral peduncles and by the optic chiasma and tracts. Within the area thus
outlined and beginning at its caudal angle may be distinguished the following
parts: the inter peduncular nucleus, which is very large in the sheep and occu-
Longitudinal fissure of cerebrum
Optic nerve^
Optic chiasma
Rhinal fissure
Insula
Lateral fissure-
Optic tract .
Infundibulum -
Mammillary body -
Cerebral peduncle -^gr
Inter peduncular fossa and
nucleus
Trigeminal nerve
f ' Olfactory bulb
Medial olfactory gyrus
Jt\*- Anterior perforated substance
,-V" Lateral olfactory stria
.__ jfc - ' Lateral olfactory gyrus
L- Diagonal band
tA ..- Amygdaloid nucleus
~"\?T Pyriform area
V - - Hippocampal gyrus
- - Trochlear nerve
A bducens nerve - - ~
Acoustic{ Vestibular '*
nerve (Cochlearn.
Glossopharyngeal nerve - '"
Vagus nerve''
Hypoglossal nerve-?''
Anterior median fissure''
II - -' Abducens nerve
',, - Facial nerve
r Trapezoid body
Cerebellum
-Olive
^^Chorioid plexus
"Accessory nerve
Tractus later alis minor
Fig. 83. Ventral view of the sheep's brain.
pies an area designated in man as the substantia perforata posterior; the corpus
mammillare, which in man is divided by a longitudinal groove into two mam-
millary bodies; and also the tuber cinereum, infundibulum, and hypophysis.
Rostral to the optic tract there is on either side a triangular field of gray matter,
studded with minute pit-like depressions and known as the anterior -perforated
substance.
The Rhinencephalon. The olfactory bulb is situated near the rostral end
of the hemisphere, to the ventral surface of which it is attached by the olfactory
n6
THE NERVOUS SYSTEM
peduncle (and in man by the long olfactory tract). In the sheep's brain there
diverge from the olfactory peduncle two well-defined gray bands, the medial
and lateral olfactory gyri, which are less evident in man; and furthermore, the
lateral olfactory gyrus is obviously continuous with the hippocampal gyrus,
forming the pyriform area (Fig. 83). All of these structures are closely asso-
ciated in function and belong to the rhinencephalon, or olfactory part of the
brain, which, because of the greater importance of the sense of smell in the
sheep, is better developed in that animal than in man. A prominent longi-
tudinal fissure separates this part of the brain from the rest of the hemisphere.
Inter-ventricular foramen Body of corpus callosum
Body of fornix .
\ Hippocampal com. Roofs of third ventricle or tela chonoidea
\ \ Stria med. ,'Haben. com.
\\\\ Habcnular / 1 > S plenium Suprapineal recess
\ \ \ \ \ Trigone / //Pineal /', Superior colliculus
/Primary fissure
White center of vermis
Anterior commissure
Septum pellucidum^ '
Rostral lamina \
Rostrum of corpus callosum .^ \ \
Genu of corpus callosum , ', \
Olfactory bulb /
Medial olfactory gyrus /
Anterior perf. substance'/
Lamina terminalis'r
Diagonal band
'/ ! j / | Infundib. "
I ; : Third vent.
i < Massa intermedia
i Optic chiasma
Preoptic recess
^ \ fans
\ \ 'Aqueduct
J\Lamina quad.
\ 'Posterior com.
^Hypophysis
Mammillary body
Central canal
. ', Medulla
\ \ Medial aperture of
( \ \ fourth ventricle
\ \^ \Tela chorioidea
\ ' Fourth ventricle
x Anterior medullary
velum
Fig. 84. Medial sagittal section of the sheep's brain.
This is known as the rhinal fissure; and all that portion of the cerebral cortex
which lies dorsal to it is the new or non-olfactory cortex, the neopallium. In
contrast to the older olfactory cortex or archipallium, which includes the pyri-
form area, the neopallium is of recent phyletic development. It first forms a
prominent part of the brain in mammals and is by far the most highly developed
in man.
Interrelation of the Various Parts of the Brain. An examination of a medial
sagittal section of the brain will make clear the relation which the various parts
bear to each other (Fig. 84). The medulla oblongata, pons, and cerebellum are
seen surrounding the fourth ventricle, and are intimately connected with one
THE GENERAL TOPOGRAPHY OF THE BRAIN IIJ
another. The medulla oblongata is directly continuous with the pons, and on
either side a large bundle of fibers from the dorsal aspect of the former runs into
the cerebellum. These two strands, which are known as the restiform bodies
or inferior cerebellar peduncles, constitute the chief avenues of communication
between the spinal cord and medulla oblongata on the one hand and the cere-
bellum on the other. The ventral prominence of the pons is produced in large
part by transverse bundles of fibers, which when traced lateralward are seen to
form a large strand, the brachium pontis or middle cerebellar peduncle, that
enters the corresponding cerebellar hemisphere (Figs. 83, 86). The brachium
conjunctivum or superior cerebellar peduncle can be traced rostrally from the
cerebellum to the mesencephalon. The three peduncles are paired structures,
symmetrically placed on the two sides of the brain (Figs. 87, 88).
The Cerebrum. The mesencephalon surrounds the cerebral aqueduct and
consists of the ventrally placed cerebral peduncles, and a dorsal plate with four
rounded elevations, the lamina and corpora quadrigemina (superior and inferior
colliculi). The cerebral hemispheres form the most prominent part of the
cerebrum and are separated from each other by the longitudinal fissure (Fig.
82), at the bottom of which is a broad commissural band, the corpus callosum,
which joins the two hemispheres together (Fig. 85). Under cover of the cere-
bral hemispheres and concealed by them, except on the ventral aspect of the
brain, is the diencephalon. This includes most of the parts which help to form
the walls of the third ventricle. These are from above downward, the epithal-
amus, including the habenular trigone and pineal body near the roof of the
ventricle; the thalamus, which forms most of the lateral wall of the ventricle,
and is united with its fellow across the cavity by a short bar of gray substance,
the massa intermedia; and the hypothalamus, including the mammillary bodies,
infundibulum, and part of the hypophysis (Figs. 84, 85).
The Brain Ventricles. The central canal of the spinal cord is prolonged
through the caudal portion of the medulla oblongata and finally opens out into
the broad rhomboidal fourth "ventricle of the rhombencephalon. At its pointed
rostral extremity this ventricle is continuous with the cerebral aqueduct, the
elongated slender cavity of the mesencephalon. This, in turn, opens into the
third "ventricle, which is a narrow vertical cleft between the two laterally sym-
metric halves of the diencephalon. It is bridged by the massa intermedia.
Near the dorsal part of the rostral border of the ventricle is a small opening in
each lateral wall, the interventricular foramen or foramen of Monro. This
leads into the lateral -ventricle, the cavity of the cerebral hemisphere.
T lS THE NERVOUS SYSTEM
THE ANATOMY OF THE MEDULLA OBLONGATA
At its rostral end the spinal cord increases in size and goes over without
sharp line of demarcation into the medulla oblongata, or myelencephalon, which,
as we learned in Chapter II, is derived from the posterior part of the third brain
vesicle. The medulla oblongata may be said to begin just rostral to the high-
est rootlet of the first cervical nerve at about the level of the foramen magnum;
Marginal part of sulcus cinguli
Sukus of corpus callosum
Splenium of corpus callosum \
Precuneus
Subparietal sulcus\ ]
Parieto-occipital fissure'-..
Lamina quadrigemina y
Cuneus
Superior vermis^
Calcarine fissure
Occipital
Lingual gyrus
Transverse fissure
Cerebettar hem.
Medullary substance
of vermis
Inferior vermis
Calamus scriptorius^-
Central canal
Spinal cord
Tela chorioidea of fourth ventricle
Fourth ventricle
Medulla oblongata
Anterior medullary velum
Cerebral aqueduct
Pons
Posterior perforated substance \
Oculomotor nerve
Central sidcus in paracentral lobule
Pineal body
Pineal recess
! Posterior commissure
i I Tela chorioidea of third ventricle
I Massa inter media
. Cyrus cinguli
Thalamus
Body of corpus callosum
Body of fornix
''Septum pellucidum
Sulcus cinguli
'Interventric. foramen
- Column of fornix
Anterior commis-
sure
. Superior frontal
gyrus
Frontal pole
Genu of corpus callosum
^Rostrum of cor p. callosum
" v Parolfactory area and sulci
\ \ " Subcallosal gyrus
^ \ Hypothalamic sulcus
\ ^'Lamina terminalis
\ Optic recess
Optic nerve
Optic chiasma
Infundibulum
^MammiUary body
Fig. 85. Medial sagittal section of the human brain. (Sobotta-McMurrich.)
and at the opposite extremity it is separated from the pons by a horizontal groove
(Figs. 81, 85). Its ventral surface rests upon the basilar portion of the occipital
bone; while its dorsal surface is in large part covered by the cerebellum. The
shape of the medulla oblongata is roughly that of a truncated cone, the smaller
end of which is directed caudally and is continuous with the spinal cord. In
man it measures about 3 cm., or a little more than 1 inch, in length (Fig. 86).
Like the spinal cord, the medulla oblongata presents a number of more or
ANATOMY OF THE MEDULLA OBLONGATA IIQ
less parallel longitudinal grooves. These are the anterior and posterior median
fissures, and a pair each of anterior lateral and posterior lateral suki (Figs. 86,
89). By means of the fissures it is divided symmetrically into right and left
halves; while these, in turn, are marked off by the sulci into ventral, lateral, and
dorsal areas, which as seen from the surface appear to be the direct upward con-
tinuation of the anterior, lateral, and posterior funiculi of the spinal cord.
But, as we shall see in the following chapter, this continuity is not as perfect
as it appears from the surface; because the tracts of the cord undergo a rear-
rangement as they enter the medulla oblongata. The posterior median fissure
does not extend beyond the middle of the oblongata, at which point its lips
separate to form the lateral boundaries of the caudal portion of the fourth ven-
tricle. The caudal half of the medulla oblongata contains a canal, the direct
continuation of the central canal of the spinal cord, and is known as the dosed
portion of the medulla oblongata (Fig. 85). This canal opens out into the fourth
ventricle in the rostral half, which helps to form the ventricular floor, and which
is often spoken of as the open part of the medulla oblongata.
Fissures and Sulci. The posterior median fissure represents the continua-
tion of the posterior median sulcus of the spinal cord and, as noted above, ends
near the middle of the medulla oblongata. The anterior median fissure is con-
tinued from the spinal cord to the border of the pons, where it ends abruptly
in a pit known as the foramen cczcum. Near the caudal extremity of the medulla
oblongata this fissure is interrupted by interdigitating bundles of fibers which
pass obliquely across the median plane. These are the fibers of the lateral
corticospinal tract, which undergo a decussation on passing from the medulla
oblongata into the spinal cord, known as the decussation of the pyramids. The
anterior lateral sulcus also extends throughout the length of the medulla ob-
longata and represents the upward continuation of a much more indefinite groove
bearing the same name in the spinal cord. From it emerge the root filaments
of the hypoglossal nerve. From the posterior lateral sulcus emerge the rootlets
of the glossopharyngeal, vagus, and accessory nerves (Figs. 86, 88, 89).
The ventral area of the medulla oblongata is included between the anterior
median fissure and the anterior lateral sulcus, and has the false appearance of
being a direct continuation of the anterior funiculus of the spinal cord. On
either side of the anterior median fissure there is an elongated eminence, taper-
ing toward the spinal cord, and known as the pyramid (pyramis Fig. 86). It
is formed by the fibers of the corticospinal or pyramidal tract. Just before the
fibers of this tract enter the spinal cord they undergo a more or less complete
120
THE NERVOUS SYSTEM
decussation, crossing the median plane in large obliquely interdigitating bundles,
which fill up and almost obliterate the anterior median fissure in the caudal
part of the medulla oblongata. This is known as the decussation of the pyra-
mids (decussatio pyramidum). In the sheep these fibers pass into the opposite
posterior funiculus of the spinal cord. In man the crossing is incomplete, a
Orbital sulci of frontal lobe*
Orbital gyri of frontal lobe
Hypophysis *
Temporal pole .Jf
Anterior perfor. substance ^fln
Oculomotor nerve x 9
Un
Mammillary body
Cerebral peduncle
PO "
Trigeminal nerve ..
Temporal lobe '-
Facial nerve
Infundibulum . Frontal pole
Nervus inter medius-
Acouslic nerve
Flocculus of cerebellum
Cerebellum '
Clwrioid plexus of ventricle IV
Glossopharyngeal nerve
Olfactory sulcus
Olfactory bulb
, Olfactory tract
Optic nerve
.-Optic chiasma
Lateral olfactory stria
Tuber cinereum
Maxillary nerve
^.Ophthalmic nerve
Portia minor of trigem.
nerve
Mandibular nerve
Semilunar ganglion
' Trochlcar nerve
Inter peduncular fossa
Abducens nerve
'Olive
Pyramid
\ Medulla oblongata
, \ Tonsil of cerebellum
\ Occipital pole
Spinal cord
Vermis of cerebellum
Fig. 86. Ventral view of the human brain. (Sobotta-McMurrich.)
Vagus nerve
Hypoglossal nerve
Accessory nerve
Root filaments of cervical nerve I i
Decussation of pyramids
majority of the fibers descending into the lateral funiculus of the opposite side,
a minority into the anterior funiculus of the same side (Fig. 77). We are al-
ready acquainted with these bundles in the spinal cord as the ventral and lateral
corticospinal tracts (direct and crossed pyramidal tracts). In addition to the
pyramid the ventral area of the medulla also contains a bundle of fibers, the
ANATOMY OF THE MEDULLA OBLONGATA 121
medial longitudinal fasciculus, which is continuous with the anterior fasciculus
proprius of the spinal cord.
The lateral area of the medulla oblongata, included between the antero-
lateral and posterolateral sulci, appears as a direct continuation of the lateral
funiculus of the spinal cord; but, as a matter of fact, many of the fibers of that
funiculus find their way into the anterior area (as, for example, the lateral cor-
ticospinal tract) or into the posterior area (dorsal spinocerebellar tract). In
the rostral part of the lateral area, between the root filaments of the glosso-
pharyngeal and vagus nerves, on the one hand, and those of the hypoglossal,
on the other, is an oval eminence, the olive (oliva, olivary body), which is pro-
duced by a large irregular mass of gray substance, the inferior olivary nucleus,
located just beneath the surface (Figs. 87, 88). By a careful inspection of the
surface of the medulla oblongata it is possible to distinguish numerous fine
bundles of fibers, which emerge from the anterior median fissure or from the
groove between the pyramid and the olive and run dorsally upon the surface
of the medulla to enter the restiform bodies. These are the ventral external
arcuate fibers and are most conspicuous on the surface of the olive (Fig. 88).
In the sheep there are two superficial bands of fibers not seen in the human
brain. Placed transversely near the caudal border of the pons is a belt-like
elevation, known as the trapezoid body, through which emerge the roots of the
abducens and facial nerves (Figs. 83, 87). In man the much larger pons covers
this band from view and the sixth and seventh nerves emerge from under the
caudal border of the pons. Another bundle, beginning on the ventral sur-
face of the trapezoid body near the seventh nerve, describes a graceful curve
around the ventral border of the olive and becomes lost in the lateral area of
the medulla oblongata. This has been called the fasciculus lateralis minor.
The dorsal area of the medulla oblongata is bounded ventrally by the pos-
terolateral sulcus and emergent root filaments of the glossopharyngeal, vagus,
and accessory nerves. In the closed part of the medulla oblongata it extends
to the posterior median fissure, while in the open part its dorsal boundary is
formed by the lateral margin of the floor of the fourth ventricle. The caudal
portion of this area is, in reality, as it appears, the direct continuation of the
posterior funiculus of the spinal cord. On the dorsal .aspect of the medulla
oblongata the fasciculus cuneatus and fasciculus gracilis of the cord are con-
tinued as the funiculus cuneatus and funiculus gracilis, which soon enlarge into
elongated eminences, known respectively as the cuneate tubercle and the clava
(Figs. 89, 91). These enlargements are produced by gray masses, the nucleus
I22 THE NERVOUS SYSTEM
gracilis and nucleus cuneatus, within which end the fibers of the corresponding
fasciculi of the spinal cord. The clava and cuneate tubercle are displaced lat-
erally by the caudal angle of the fourth ventricle. Somewhat rostral to the mid-
dle of the medulla oblongata they gradually give place to the restiform body.
More laterally, between the cuneate funiculus and tubercle on the one hand
and the roots of the glossopharyngeal, vagus, and accessory nerves on the other,
is a third longitudinal club-shaped elevation called the tuberculum cinereum.
It is produced by a tract of descending fibers, derived from the sensory root of
the trigeminal nerve, and by an elongated mass of substantia gelatinosa which
Corona radiata-
Lentiform nucleus
Lateral geniculate body ^
Medial geniculate body
Optic radiation
Corona radiata
Pulvinar''
Inferior quadrigeminal brachium"
Superior cotticulus
Trochlear nerve ..
Inferior colliculus-
Brachium pontis
Brachium conjunctivum'
Restiform body
^>{ res! S;:,- n -::_.
Dorsal cochlear nucleus -^
Glossopharyngeal nerve "',
Vagus nerve and restiform body*".
Accessory nerve- -~ _ -
Cuneate tubercle'
Anterior perforated substance
Optic tract
^- Optic nerve
Infundibulum
/ ,-Mammillary body
Hypophysis
^"Oculomotor nerve
- Transverse peduncular tract
' Cerebral peduncle