Book - An Atlas of the Medulla and Midbrain 2: Difference between revisions

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==Chapter II. The Long Tracts==
The important As has been pointed out in describing the method of building
ace " the model, there is a central mass of fibres, which makes a foundation for the reconstruction. It consists chiefly of the long sensory
tract to the cortex, that is, the medial lemniscus. In a part of its
course it is associated with the posterior longitudinal bundle and
certain formatio reticularis tracts. This posterior longitudinal
bundle is also a long tract, for it contains fibres both from the cord,
and from the midbrain, and relates the nuclei of the eye muscles
with higher and lower centres.
In a study of sections it does not appear that the long sensory
tract has any closer form relation with other structures than the
long motor tract. The reconstruction brings out the fact that the
pyramidal tract, which is non-medullated at birth, does not mould
the shape of other structures. On the other hand, if the form of
the long sensory tract (Stratum interolivare lemnisci and lemniscus
medialis) is once mastered, all the other structures can be related
to it. For this reason the shape of the sensory tract is first described in considerable detail. It has a characteristic form in each
of the three divisions of the model; medulla, pons and midbrain.
The central fibre mass is shown best in Plate vn (Stratum interolivare lemnisci and lemniscus medialis). For convenience it may
be considered divided into two parts, a distal or medulla part
(S. i. 1.) and a proximal part (L. m.) by a cross-bar of fibres, namely,
the corpus trapezoideum. These two parts show a marked contrast
both in form and in position.
The distal part consists of a thin sheet of fibres extending the
whole length of the medulla, and reaching almost from the dorsal
to the ventral surface (Plate vi). It lies close to the middle line
in a plane parallel to the raphe (Plate in). This " medulla sheet,"
as I have named it, shows almost no evidence of being composed of
different groups of fibres.
The proximal part, on the other hand, is clearly divided into three systems, the fibres of which spread so as to occupy the region
between the raphe and the lateral surface of the pons and inidbrain.
Only the ventral portion shows in Plate vn (Lemniscus medialis),
while the dorsal and medial parts are seen in Plates v and vin
(Fasciculus longitudinalis medialis and Formatio reticularis).
==A. The Distal Subdivision of the Central Fibre Mass, or ” Medulla Sheet " (Plate VI).==
[[File:Sabin1901 plate06.jpg|thumb|(PLATE Vl]]
A cross-section of the " medulla sheet " is shown in Fig. 33
(St. i. 1. and F. 1. m.). In the model the sheet is irregularly
quadrilateral in shape. The dorsal, ventral and distal edges are
free, while the proximal edge is an imaginary line corresponding
to the position of the radix N, abducentis (Plate vi). The ventral
edge is parallel with the long axis of the medulla; the distal and
dorsal edges make a curve which follows the central canal and the
floor of the fourth ventricle. On the dorsal border is a secondary
curve corresponding to the position of the nucleus "N. hypoglossi.
At the distal ventral angle is a projection in the form of a
trough which corresponds to the ventral f uniculi of the spinal cord.
The trough makes a slight angle with the distal edge of the medulla
sheet. It is deep in the part directed toward the spinal cord, but
as it approaches the medulla oblongata it grows more shallow until
it finally opens out onto the flat surface of the sheet.
The medulla sheet is of nearly uniform thickness, except at the
ventral edge, where the fibres are more scattered. The lateral surface of the sheet is comparatively flat; in the ventral portion, however, the ends are curved, which will be understood by comparison
with Sec. 146, Fig. 20 (S. i. L). The section shows that the trough
(F. 1.) lies lateral from the main sheet owing to the position of the
decussatio pyramidum, and that this accounts for the distal curve.
The proximal curve is due to the fact that the fibres curve lateralward just as they enter the corpus trapezoideum.
Proximal to the opening of the trough, near the ventral edge of
the medulla sheet is a deep depression; dorsal to it is a second
smaller depression which lies near the middle of the distal border
of the sheet. Both of these correspond to the position of portions
of the nucleus olivaris accessorius medialis. The two spaces left
in the sheet correspond to nuclei; the smaller one (.89 x .41 mm.)
represents the so-called accessory nucleus of Roller, while the
larger (3.7 x 1.5 mm.) corresponds to the nucleus centralis inferior
of the formatio reticularis grisea.
The mesial surface of the medulla sheet is level and shows well
in Plate vn. The origin of the fibres is considered on page 36 et fol.
B. PEOXIMAL SUBDIVISION OF THE CENTEAL FIBEE MASS, OR LONG TRACTS IN PONS AND MIDBEAIN (PLATE VIl).
[[File:Sabin1901 plate07.jpg|thumb|(PLATE Vl|]]
The proximal part of the fibre mass undergoes many changes in
form, but Fig. 41 may be used as a typical section (L. m. and
F. 1. m.). It has been said that the fibres of the proximal segment
scatter and are divided into three groups: (1) a ventral (Plate vn,
L. m.), (2) a dorsal (Plate v, F. 1. m.), and (3) a middle portion
(Plate v, Formatio reticularis alba). The ventral group, which
includes all the fibres that run through the cross-bar (trapezoid
body), is in contrast with the t( medulla sheet/ 7 for instead of
being spread out in a vertical direction, the mass rotates so that the
sheet comes to lie horizontally (Plate vn). The border of this
sheet lies far out on the lateral surface of the pons. At the junction with the midbrain the mass again rotates so that it once more
assumes almost a vertical (i. e., dorsoventral) direction (Plates n
and v).
The dorsal portion of the proximal segment continues upward
in a line with the direction of the medulla sheet (Plates v and vi,
Fasciculus longitudinalis medialis). In passing through the pons
it is practically level, while just beyond it turns ventralward, taking
part in the midbrain curve (Plate vin). This curve shows also on
Plate iv. It is not labeled, but the root of the third nerve passes
through it.
The middle portion can be related as follows: while the ventral
fibres make a floor, as it were, at the junction of the pars dorsalis
with the pars ventralis pontis, the dorsal fibres remain close to the
median line, and into the area thus left between these two fibre
masses the middle fibres spread out (Plates vi, vn and vni, Formatio reticularis alba). (Fig. 41, area between L. m. and F. 1. m.)
It is now necessary to describe the various portions of the proximal subdivision of the central fibre mass more in detail. It will
be most convenient in this description to follow (a) the ventral
portion, (I) the dorsal portion, and (c) the middle portion separately
through the pons and midbrain.
(a) The Ventral Portion of the Proximal Subdivision of the Cen- Lateral
tral Fibre Mass (Lemniscus lateralis y Lemniscus medialis and
Lemniscus superior) (Plate vn). The ventral fibres of the proximal subdivision of the central fibre mass make (1) the horizontal
pontal sheet and (2) the vertical midbrain sheet. In entering the
pons the ventral fibres of the medulla sheet curve rapidly dorsalward, so that they all pass through the cross-bar (trapezoid body)
(Plate vi). As the mass of fibres leaves the cross-bar it spreads
out into " the horizontal pontal sheet " (Plates vi and vn). The
lateral portion of this horizontal sheet consists of a new mass of
fibres, the lateral lemniscus, which does not exist in the medulla
sheet, but is added on in passing through the cross-bar. It forms
a part of the pontal sheet for a short distance only, inasmuch as it
inclines rapidly dorsalward in order to reach the nucleus colliculi
inferioris in which a large part of it disappears (Plate n). The
space shown in the model between the lemniscus lateralis and the
pontal sheet is occupied at this stage of development by indifferent
substance.
The pontal sheet is best seen from its dorsomesial aspect (Plate
vn). The mesial edge is clearly defined. The sheet is comparatively thick as it emerges from the trapezoid body, but it grows
thinner as it approaches the midbrain. The dorsal surface is level
within the pons but curves ventralward in approaching the midbrain, while the ventral surface curves in crossing the pons (Plate
n). The fibres of the medial third of the sheet, its thinnest portion,
are cut off abruptly in the model as they are entering the midbrain
in order to accommodate the radix N. oculomotorii. In reality
they pass toward the nucleus ruber and appear to form a part of
its capsule (Plate vn).
The lateral portion of the pontal sheet contains the bulk of its Medial
fibres. They make a definite bundle which runs through the mid- le
brain sheet at approximately the same level as in the pontal sheet.
This bundle shows as a cut edge at the proximal end of the midbrain
sheet to signify that it passes onward beyond the limits of the
model (Plates n and v). It is the lemniscus medialis on its way
toward the cortex.
Having thus outlined the pontal sheet by itself, its form will be
best understood in connection with the midbrain sheet. From the
lateral view it will be seen that as the pontal sheet approaches the midbrain, it spreads out into a fan which is placed obliquely in a
dorso ventral plane (Plate v). The gradual rotation by which the
horizontal pontal sheet becomes this obliquely vertical midbrain
sheet is best seen from the mesial view (Plate vn). The medial
lemniscus runs in the centre of this fan and determines its shape.
This can be seen in Fig. 16, for the section passes through the
central bundle and shows that in crossing the pons its fibres slant
lateralward, while at the beginning of the midbrain they turn and
run more nearly parallel to the central axis. In the midbrain the
lateral border of the bundle is curved, owing to the position of the
substantia nigra.
[[File:Sabin1901 plate05.jpg|thumb|(PLATE V]]
From the lateral view it may be seen that the midbrain sheet,
ventral to the medial lemniscus, is flat and lies more obliquely across
the midbrain (Plate v). It receives its fibres from the mesial part
of the pontal sheet, and enters into the formation of the capsule of
the nucleus ruber. One further point should be mentioned in
connection with the view of the lateral surface of the model,
namely, that just as the lemniscus medialis enters the midbrain, it
gives off from its ventrolateral portion a small bundle which enters
the substantia nigra (Plate v, M. 1. to Sn.).
Superior Having outlined the central bundle of the midbrain sheet, i. e. the lemniscus medialis, and the fibres ventral to it, namely, the
lateral capsule of the nucleus ruber, the dorsal segment of the sheet
may next be considered. This is the so-called lemniscus superior,
or dbere Schleife, of the Germans (Plates n and v). In contrast
with the central bundle, this dorsal portion has a flat surface. It
has been seen that the fibres ventral to the medial lemniscus were
derived from the mesial portion of the pontal sheet; the dorsal
fibres, on the other hand, are related to its lateral border (Plate v).
The lemniscus superior lies near the lateral surface of the midbrain
(Plate v, Figs. 12 and 44). The lines in the model show that its
fibres are inclined dorsalward. The shape of the bundle in the
model is that of a triangle with its apex at the point where the
lemniscus lateralis separates from the lemniscus medialis and its
base in a dorsoventral line near the upper end of the model. The
base is a curved line which indicates that, unlike the lemniscus
medialis, the fibres of the superior lemniscus end within the limits
of the model (Plate n). The ventral side of the triangle lies
adjacent to the medial lemniscus; the dorsal side has in part a free border, but near the base of the triangle the lemniscus superior
fuses with the lemniscus lateralis. At the point of fusion, the
lemniscus lateralis is hollowed out so as to form a capsule for the
nucleus colliculi inferioris (Plates vn and vm).
The description of the ventral portion of the central fibre mass summary of
is now complete. Its. most important element is the medial lemniscus, which enters into the formation of three great sheets
a medial, vertical sheet in the medulla; a horizontal sheet in the
pons; and a lateral obliquely vertical sheet in the midbrain.
The pontal sheet includes another group of fibres, the lemniscus
lateralis, while the midbrain sheet contains the superior lemniscus
and the lateral capsule of the nucleus ruber.
(b) The Dorsal Portion of the Proximal Subdivision of the Cen- Posterior
tral Fibre Mass (Fasciculus longitudinalis medialis). The next
bundle under consideration is the fasciculus longitudinalis medialis,
which forms the dorsal part of the proximal subdivision of the
central fibre mass, and is the continuation of the dorsal part of the
medulla sheet (Plates v and vm). As has been said, this portion
follows the course of the central canal. It passes through the pons
at the same level as in the medulla, and then turns ventralward to
conform with the midbrain curve.
In the model of the medulla sheet, a space left for a nucleus of
the formatio reticularis is the only indication that the sheet is made
of different parts (Plates vi and vii, Ku. centralis inferior). The
fibres dorsal to this space become a separate bundle in crossing the
pons. This is clear in sections where the coarse black fibres of the
fasciculus longitudinalis medialis contrast with the delicate fibres of
the formatio reticularis just ventral (Fig. 40). The fibres of the
radix X. abducentis pass directly through the edge of this dorsal
bundle, and just proximal to this nerve the bundle flattens out and
becomes triangular in cross-section (Fig. 40). At the beginning
of the midbrain the bundles of the two sides broaden, and, at the
same time, form a trough (Plate iv. Not labeled, but the root of
the third nerve passes through the side of the trough). The centre
is deep and its floor is made, not of decussating fibres, but of longitudinal fibres running in the middle line (Fig. 18). The sides of
the trough show shallow depressions, three on the right side and two
on the left. They represent the position of the nuclei of the oculomotor and trochlear nerves (Plate vin). Beyond these depressions is the space left in the wall for the exit of the main mass
of fibres of the root of the oculomotor nerve (Plate vm). This
space is artificial, the wall being in reality almost continuous. Just
proximal and lateral from this space is a long groove which opens
out into the proximal capsule of the nucleus ruber. This part of
the capsule is a sheet of fibres placed dorsoventrally across the midbrain. The groove contains the nucleus of Darkschewitsch and
marks the junction of three different fibre masses, viz., (1) the
fasciculus longitudinalis medialis, (2) the posterior commissure
and (3) the capsule of the nucleus ruber. These relations will be
more fully considered in connection with the description of the
midbrain. To return to the fasciculus longitudinalis medialis, the
sides of the trough formed by the bundles of the two sides come
into relation, in the model, with a large hollow shell which occupies
the centre of the dorsal part of the midbrain. This shell is the
stratum album profundum (Plate vni).
summary of The dorsal bundles of the two sides making up the fasciculi
dinai bundle, longitudinales mediales occupy the dorsal part of the medulla
sheets, where they form two dense bands on either side of the raphe.
In the pons each band becomes broader, and in the midbrain the
bundles of the two sides unite to form a deep trough. As a whole,
each bundle is characterized by two curves, one in the medulla, the
other in the midbrain. It is related to four nuclei, namely, the
nuclei of the !N". hypoglossus, N. abducens, ~N. trochlearis and ~N.
oculomotorius. At the distal extremity of the medulla oblongata
it is connected with the ventrolateral funiculi of the spinal cord,
while at the proximal limit of the model it is related, at least in
position, to three structures a nucleus, a decussating bundle and
a nuclear capsule. The origin of these fibres is considered on
pages 31 and 97.
Formatio (c) The Middle Portion of the Proximal Subdivision of the
Central Fibre Mass (Formatio reticularis alba). By reviewing
the shape of the two great fibre masses just described, the ventral,
with its tendency to form sheets, and the dorsal compact bundle
following the course of the central canal, a good idea of the area
left between them can be obtained. Into this area, dorsal to the
pontal sheet and medial to the midbrain sheet, the fibres of the
middle portion of the proximal subdivision spread out (Plates vi, vii and vm), Fig. 40, area between L. m. and F. 1. in.). The area
left for a nucleus in the medulla sheet lies in the course of this
fibre mass, and appears both to receive and to give rise to its fibres
(Plate vi, Nu. centralis inferior). Just proximal to this nucleus,
the fibres of the middle portion turn slightly lateralward and spread
out into the pars dorsalis pontis and the tegmentum of the midbrain (Plate vn). The part of the midbrain they enter is bounded
laterally by the lemniscus superior, medially by the stratum album
profundum, and ventrally by the capsule of the red nucleus (Plate
vm). In the model, this middle mass of fibres had to be sacrificed
somewhat on account of the representation of the brachium conjunctivum. The mass will be considered later in connection with
the formatio reticularis as a whole, and was introduced here on
account of its relations to the central fibre mass (page 107).
The general form of the central fibre mass of the model, with its
contrasting distal and proximal parts, has now been outlined. The
part distal to the cross-bar is a thin median sheet which extends
in a dorsoventral plane parallel with the raphe, and shows but little
evidence at this stage of development of being made up of different
parts; the part proximal to the cross-bar is clearly made up of three
different fibre masses which spread out to cover a wide area and
show varied connections with other structures.
==C. Relation of the Central Fibre Mass to the Serial Sections and to Other Structures in the Model==
The components of the central fibre mass must now be related to Long tracts
known bundles as seen in sections at different levels. The position of in sections.
the sections with reference to the model can be estimated by the lines
on Fig. 52. These lines are labeled with the series and section number,
I standing for the horizontal series, and n for the transverse.
In a series of sections extending from Figs. 28-36, the main character- Cross-sections
istics of the medulla sheet are well illustrated, (1) its median position, ofLemnisci.
(2) the gradual increase in its dorsoventral diameter due to the cervical
curve, (3) the lack of differentiation of its fibres, except in the last
section, which shows the middle fibres disappearing in a nucleus.
The ventral portion of the sheet will be recognized in the sections of s. i. 1.
the new-born child as the stratum interolivare lemnisci. In a section of
adult tissue so many more fibres are crowded into this portion that it
forms a triangle in cross-section, with the base against the pyramid.
A series of transverse sections corresponding to the proximal portion L. m.
of the central fibre mass illustrate the region well (Figs. 38 to 49). The
first two sections pass through the trapezoid body; the second, taken
where that body is widest, shows that all the fibres of the lemniscus
L. 1. pass through it. The next five sections pass through the pontal sheet.
Fig. 43 shows the separation of the lateral from the medial lemniscus
(Plate n). The area between the two lemnisci is wider in proportion in
the transverse series owing to the obliquity of the sections; moreover,
it contains a few medullated fibres, whereas there are none in the same
area in the longitudinal series.
L. s. The lemniscus superior is shown in Figs. 46 to 48. It will be considered later in connection with the midbrain. Fig. 47 passes through the
midbrain sheet just distal to the red nucleus. The oblique position of
the sheet is evident as well as its relation to the substantia nigra.
Fig. 48 shows the relation of the medial lemniscus to the red nucleus
and its capsule.
Longitudinal The longitudinal series gives a far more comprehensive view of the
(frontal) ^secttona me dial lemniscus than the transverse (Figs. 12 to 22). As has been said,
the section shown in Fig. 16 was used as a starting point in building the
L. m. model, for it contained a bundle, the lemniscus medialis, which ran the
entire length of the section. This bundle includes the stratum interolivare, the pontal sheet and the midbrain bundle. The shape and the
curves of the tract are evident. The points of especial interest are (1)
the radiation of the fibres in passing through the corpus trapezoideum,
(2) the distribution of the fibres in the pontal sheet, the main mass
lying in the lateral part, and (3) the midbrain curve in relation to the
substantia nigra. The junction of the pons and midbrain is plain, and
many scattered fibres can be seen entering the midbrain from the
medial part of the sheet.
L. 1. In considering the lemniscus medialis, it is necessary to relate it (1) to
the lemniscus lateralis, (2) to the lemniscus superior and (3) to the
capsule of the red nucleus. The beginning of the lateral lemniscus is
seen in Fig. 16, but the main mass of it is seen at a level farther dorsal
in Fig. 13. This section is taken just at the junction of the medial
L. s. lemniscus with the lemniscus superior, while in Fig. 12, the medial lemniscus disappears and the superior lemniscus becomes visible by itself.
This description will perhaps suffice for the structures that lie dorsal
to the level of the pontal sheet. Fig. 19 is farther ventral, and, owing to
the arch formed by the pontal sheet (Plate v), it shows the medulla
sheet and midbrain sheets but not the poutal sheet.
In this section two points are well shown, (1) the radiation of the
lemniscus fibres as they emerge from the trapezoid body, and (2) the
curves which the lemniscus forms in the midbrain. It will be noted
that by means of two curves the fibres of the lemniscus adjust themselves to the substantia nigra and the red nucleus. Passing still more
ventralward the lateral capsule of the red nucleus becomes visible (Fig.
20). It consists at this stage of very few fibres. The rest of the sections show simply the stratum interolivare lemnisci.
Origin of fibres The fibres of the stratum interolivare lemnisci, or ventral part
f '' of the medulla sheet, have been traced in the model from this sheet
(1) through the corpus trapezoideum, (2) through the pontal sheet
and (3) through the central part of the midbrain sheet. Here the
lemniscus medialis, now deflected far to the side, lies lateral to the nucleus ruber. The model takes the bundle only as far as the
hypothalamic region (Plate 11).
Thus far I have not discussed the origin of these fibres. There
are three groups of fibres to be considered with reference to the
stratum interolivare lemnisci. (1) In Plate v will be seen a small
bundle of fibres connected with the lateral funiculus of the cord.
It passes toward the middle line and appears to end blindly against
the root of the hypoglossal nerve. In reality, its fibres pass on
between the root bundles of the nerve adjacent to the stratum
interolivare lemnisci. From the Weigert specimens alone I cannot
say that these fibres enter the stratum interolivare lemnisci, but
the model shows the possibility of an uncrossed path from the
lateral funiculus of the cord to the main sensory path toward the
cortex (Fig. 19 F. 1.). (2) By far the thickest bundles of fibres entering the lemniscus in the medulla are the bundles of internal arcuate
fibres from the nucleus funiculi gracilis and the nucleus funiculi
cuneati. These arcuates form two principal groups, one of which
lies just distal to the root of the N". hypoglossus, while the other
lies opposite the proximal third of the nucleus funiculi cuneati
(Plate vn). The distal bundle is labeled decussatio lemniscorum,
the proximal bundle is not labeled but the line marked nucleus funiculi cuneati points toward it. In sections the two groups show in
Figs. 28, F. a. i., and 31, F. a. i. These bundles can be described best in connection with the dorsal funiculi of the spinal cord, sections. F. a
(3) As a further source of fibres for the lemniscus are the arcuates
from the nuclei of termination of the cerebral sensory nerves.
These arcuates are too diffuse and scattered to be modeled as
bundles.
The sensory path to the cortex is called stratum interolivare summary of
lemnisci in the medulla and lemniscus medialis in the pons and
midbrain. It makes the ventral part of the medulla sheet, the
medial part of the pontal sheet and the central part of the midbrain sheet. It carries (1) sensory fibres from the cord both
directly from the ventrolateral columns (Plate vi), and indirectly
from the dorsal columns by means of two bundles of internal arcuate
fibres. (2) It carries fibres from all the nuclei of the sensory
cerebral nerves. (Scattered arcuate fibres not seen in the reconstruction.) (3) A few descending fibres not yet medullated.
Posterior The fasciculus longitudinal! s medialis can be seen in the transverse
D bimdle n fn sections from Figs. 28 to 51. In the model it will be remembered that
sections, the ventrolateral funiculi of the cord are represented as a trough in
which the ventral horn cells lie, and that this trough opens out upon the
surface of the medulla sheet (Plate vi). The walls of this trough are
the ground bundles, the fasciculus ventralis et lateralis proprius. In
the medulla oblongata the fibres curve dorsalward into the border of
the medulla sheet. This is best made out in sagittal section. In Fig.
28 the trough is cut in cross-section and its fibres can be traced to a
point just ventral to the central canal. A single section will suffice to
show the relations of the fasciculus longitudinalis medialis in the pons
(Fig. 42). In the midbrain the trough described before in connection
with this bundle is seen in Figs. 46 to 51. Its shape, its relations to the
nucleus N. oculomotorii and the nucleus N. trochlearis, and especially
the superior connections of its walls deserve notice. In the last section, Fig. 51, the relation to the nucleus of Darks chewitsch is evident.
These connections will be considered in detail in the study of the midbrain.
The longitudinal series, as in the case of the lemniscus medialis, gives
the best idea of the medial longitudinal bundle (Figs. 7 to 20). A view
of the model from the lateral aspect shows that this bundle reaches its
most dorsal point opposite the nucleus N. abducentis (Plate vi). The
section in Fig. 7 passes through this point. Fig. 9 gives the best view
of the bundle, both as to its shape and its relations. The two extremities of the bundle in this section represent the positions of the cervical
and midbrain curves.
By tracing the series ventral ward the following points can be studied:
(1) the ventral course of the fibres at either end, (2) the widening of
the bundle at the proximal end, (3) the formation of the midbrain
trough, (4) the relation of the latter to the nuclei of the oculomotor
and trochlear nerves, and (5) the relation to the nucleus of Darkschewitsch, the posterior commissure and the capsule of the red nucleus.
Relations of Since the medial longitudinal bundle is especially related to a
Tu^dinlTbundie group of cerebral nerves, it may be well to consider these relations
cerebral nerves! ^ n detail. In the model there are four depressions in the bundle,
one in the medulla oblongata, one in the pons, and two in the midbrain. These four depressions correspond to the nuclei of (1) the
N". hypoglossus, (2) the N. abducens, (3) the N. trochlearis and
(4) the N. oculomotorius. The nucleus "N. hypoglossi corresponds
to the shallow depression just proximal to the distal dorsal angle of
the medulla sheet (Plates vi and vn, and Fig. 31). The nucleus is
a long one and lies in the gray matter ventral to the central canal.
The nucleus of the N. abducens lies in this same central gray
matter close to the floor of the ventricle (Plates vi and vn). It
lies farther lateral than any other nucleus of this group, as though
it had been pulled from the middle line by the knee of the root of
the facial nerve (Plate in). The fibres of the root of the N. abducens pass through the edge of the medial longitudinal bundle.
The relations of the nucleus JST. abducentis are shown in Fig. 7.
In the midbrain trough of the medial longitudinal bundle are embedded the nuclei of the oculomotor and trochlear nerves (Plate
vm). Since the nucleus E". trochlearis is asymmetrical, there are
two depressions in the fasciculus longitudinalis medialis on the
right side and one on the left (Figs. 44 and 46, and Figs. 10 and 12).
Proximal and ventral to the position of the nucleus "N. trochlearis
are the depressions for the nucleus N. oculomotorii (Fig. 47 and
Fig. 13). ^
The main features of the medial longitudinal bundle have thus Summary^!
been illustrated in the model and proved in the sections. In brief, tudin"ib
they are as follows: It is a long tract running through the entire
area covered by the model. 1 As to position, it lies just ventral to
the central canal save at its superior limit; as to form, its striking
features are (1) the two curves, (2) the depressions for the nuclei,
(3) the midbrain trough; as to origin, it is known to consist in large
part of the continuation of the ventrolateral funiculi of the spinal
cord. It receives, however, many fibres from adjacent gray masses
on its course, and gives off fibres to these. It has undoubted relations at the superior limit with the nucleus of Darkschewitsch.
Indeed, this nucleus has been called by at least one writer the
nucleus fasciculi longitudinalis medialis. The question of higher
connections will be discussed later. There are fibres in the fasciculus longitudinalis medialis running in both directions, as Golgi
studies by various investigators have demonstrated.
The middle portion of the central fibre mass can be studied to better
advantage in connection with the formatio reticularis as a whole. I
shall refer, therefore, in this place merely to one section (Fig. 13),
which will show the essential points, viz.: (1) that the fibres spread out
into the pars dorsalis pontis and the tegmentum; and (2) that the fibres
come into relation with several groups of cells.
1 The description given diif ers from that found in many books where
the term is applied only to the bundle in the pons and midbrain. The
authors of these books usually refer to the portion in the medulla
oblongata as the " remains of the ventrolateral funiculi of the spinal
cord" (Verderseitenstranffreste of the Germans). It seems to me more
satisfactory to include in Ihe fasciculus longitudinalis medialis the
fibres of the medulla sheet which are continuous with the bundle in the
pons.
It remains now to show how the central fibre mass, so fundamental in the construction of the model, is, in fact, a foundation
central Fibre f or the entire structure. It is clear that in each of the three prinfoundation for cipal regions of the model, namely, the medulla oblongata, the pons
(see page 16). and the midbrain, this central bundle, at least the main part of
it, has a characteristic shape; in the medulla oblongata it is a median vertical sheet, in the pons, a horizontal sheet, in the midbrain,
a lateral obliquely vertical sheet. If now the medulla is considered
in the light of its development from the comparatively simple structure of the cord, it will be seen that, as the central canal moves dorsalward to make room for the intrinsic structures of the medulla,
the ventral ground bundle, which underlies the canal, becomes
enormously developed so as to form the medulla sheet. This medulla sheet, indeed, represents far more than the ventral funiculus,
for it is made up of fibres from the lateral and dorsal funiculi as
well. Indeed, at the beginning of the medulla oblongata, almost
the entire dorsal funiculus passes by means of secondary neurones,
arcuate fibres, either dorsally to the cerebellum or ventrally into
the medulla sheet. This change is interesting in connection with
the fact that the cerebellum develops from the dorsal wall of the
neural tube at this position according to the work of His.
The grouping About this vertical sheet can be grouped all the other structures
in the Meduiia of the medulla oblongata. Just ventral to it, lies the pyramidal
tract; while the region lateral to it can be divided into two levels,
a ventral and a dorsal. The ventral level is characterized by the
nucleus olivaris inferior; the dorsal level corresponds to a formatio
reticularis area which is bordered by the roots of the cerebral
nerves and their nuclei.
in the pons. In the pons, the central fibre mass makes a complete floor for the
pars dorsalis pontis. The whole of the structure which characterizes the pons, namely, the pontal nuclei, lies ventral to the sheet;
while on the other hand, all the structures, which more or less correspond to the upward continuation of the spinal cord, lie dorsal to
it This pars dorsalis pontis is composed chiefly of formatio reticularis. It receives (1) the middle portion of the proximal subdivision of the central fibre mass, and (2) a mass of fibres from the
formatio reticularis of the medulla oblongata. The brachium conjunctivum, or superior cerebellar peduncle, passes through it.
in the midbrain. The midbrain structures in turn can be grouped around the central fibre mass. The lateral course of the lemniscus medialis above
the medulla is interesting in connection with the size and central position of the red nucleus. In the ventral region of the midbrain
are the substantia nigra and nucleus ruber separated from one
another by the lateral capsule of the red nucleus and the lemniscus
medialis, while dorsal to the red nucleus is a formatio reticularis
area. This area is bounded laterally by the lemniscus superior
and medially by the fasciculus longitudinalis medialis and stratum
album profundum (Plate vm).
The cerebral nerves can be grouped in part with relation to the Relations of
foundation of the model. Plates v and vi of the model, is shown
the trough which contains the ventral column of gray substance of fi
the spinal cord opening out onto the medulla sheet. From this
point can be traced two separate groups of nuclei pertaining to the
cerebral nerves, a median, motor group, and a lateral, mixed group.
The median, motor group includes the nucleus E". hypoglossi, nucleus ."N". abducentis, nucleus N". trochlearis and nucleus N. oculoinotorii, which have already been related to the fasciculus longitudinalis medialis. The lateral group, on the other hand, is not
directly related to the central fibre mass, but rather lies within or
bordering upon the area of the formatio reticularis. This lateral
group can be subdivided into two parts, a ventral, motor group and
a dorsal, sensory group. The ventral group includes the nucleus
N. accessorii and the motor nuclei of the !N". glossopharyngeus, N.
vagus, N. facialis and !N". trigeminus. They all lie within the formatio reticularis. The sensory group includes the nuclei of termination of the sensory portions of the N. glossopharyngeus, N.
vagus, N. facialis and 1ST. trigeminus together with all the nuclei of
the cochlear and vestibular nerves. This group lies far dorsalward
and its nuclei and nerve tracts border upon the formatio reticularis.
In the positions of these groups as here outlined is a striking contrast. On the one hand, the motor groups are both placed in close
relation to association fibres, the median group to a definite compact
bundle (the fasciculus longitudinalis medialis), the lateral to diffuse
association paths (formatio reticularis); the sensory group of nuclei, on the other hand, lies not actually in but rather bordering
upon the formatio reticularis. This grouping of the cerebral nuclei
which will be used in connection with the model has been noted in
comparative anatomy as well as in connection with the human brain.
Besides these special. form relations the model illustrates certain
general relations. A view from the lateral surface (Plate n) shows General relations that the ventral regions of the model are occupied largely by nue ' clear masses: the olive, the pontal nuclei, the substantia nigra and
the red nucleus. The absence of medullated fibres in this region
is due to two facts, (1) that the pyramidal tracts and the brachium
pontis which belong here become medullated at a later period, and
(2) that, at the cervical curve in the medulla, most of the fibres of
the spinal cord curve dorsalward, and thus leave the olivary region
comparatively free. At this stage of the development the ventral
level of the medulla oblongata, pons and midbrain is a nuclear
region. Dorsal to this level is the area corresponding more or less
to the continuation of the spinal cord. In the views of the model,
one is struck by the number of fibre, bundles this dorsal level
contains, long and short paths, that is, tracts to the cerebrum and
cerebellum as well as fibres to and from many way-stations. These
fibres represent in part the funiculi of the cord. The region is not
wanting in nuclei, for the dorsal and ventral columns of the cord
are represented by the nuclei of the cerebral nerves. The ground
bundles of the cord correspond to the extensive f ormatio reticularis
alba and grisea, while the central gray matter of the cord becomes
much enlarged to form the substantia grisea centralis of the medulla, pons and midbrain.




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Sabin FR. and Knower H. An atlas of the medulla and midbrain, a laboratory manual (1901) Baltimore: Friedenwald.

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This 1901 book by Florence Rena Sabin (1871 - 1953) and her collaborator presents one of the very earliest atlases of the human central nervous system, describing the midbrain and brainstem. This atlas was extremely useful for later researchers attempting to both understand the development and mapping of the midbrain and medulla. Florence Sabin later work was as a key historic researcher in early 1900's establishing our early understanding of both vascular and lymphatic development in the embryo.



Modern Notes: Medulla | Mesencephalon | Florence Sabin

Neural Links: ectoderm | neural | neural crest | ventricular | sensory | Stage 22 | gliogenesis | neural fetal | Medicine Lecture - Neural | Lecture - Ectoderm | Lecture - Neural Crest | Lab - Early Neural | neural abnormalities | folic acid | iodine deficiency | Fetal Alcohol Syndrome | neural postnatal | neural examination | Histology | Historic Neural | Category:Neural


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 prosencephalon (forebrain) telencephalon Rhinencephalon, Amygdala, hippocampus, cerebrum (cortex), hypothalamus‎, pituitary | Basal Ganglia, lateral ventricles
diencephalon epithalamus, thalamus, Subthalamus, pineal, posterior commissure, pretectum, third ventricle
mesencephalon (midbrain) mesencephalon tectum, Cerebral peduncle, cerebral aqueduct, pons
rhombencephalon (hindbrain) metencephalon cerebellum
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Chapter II. The Long Tracts

The important As has been pointed out in describing the method of building ace " the model, there is a central mass of fibres, which makes a foundation for the reconstruction. It consists chiefly of the long sensory tract to the cortex, that is, the medial lemniscus. In a part of its course it is associated with the posterior longitudinal bundle and certain formatio reticularis tracts. This posterior longitudinal bundle is also a long tract, for it contains fibres both from the cord, and from the midbrain, and relates the nuclei of the eye muscles with higher and lower centres.

In a study of sections it does not appear that the long sensory tract has any closer form relation with other structures than the long motor tract. The reconstruction brings out the fact that the pyramidal tract, which is non-medullated at birth, does not mould the shape of other structures. On the other hand, if the form of the long sensory tract (Stratum interolivare lemnisci and lemniscus medialis) is once mastered, all the other structures can be related to it. For this reason the shape of the sensory tract is first described in considerable detail. It has a characteristic form in each of the three divisions of the model; medulla, pons and midbrain.

The central fibre mass is shown best in Plate vn (Stratum interolivare lemnisci and lemniscus medialis). For convenience it may be considered divided into two parts, a distal or medulla part (S. i. 1.) and a proximal part (L. m.) by a cross-bar of fibres, namely, the corpus trapezoideum. These two parts show a marked contrast both in form and in position.

The distal part consists of a thin sheet of fibres extending the whole length of the medulla, and reaching almost from the dorsal to the ventral surface (Plate vi). It lies close to the middle line in a plane parallel to the raphe (Plate in). This " medulla sheet," as I have named it, shows almost no evidence of being composed of different groups of fibres.

The proximal part, on the other hand, is clearly divided into three systems, the fibres of which spread so as to occupy the region between the raphe and the lateral surface of the pons and inidbrain. Only the ventral portion shows in Plate vn (Lemniscus medialis), while the dorsal and medial parts are seen in Plates v and vin (Fasciculus longitudinalis medialis and Formatio reticularis).

A. The Distal Subdivision of the Central Fibre Mass, or ” Medulla Sheet " (Plate VI).

(PLATE Vl

A cross-section of the " medulla sheet " is shown in Fig. 33 (St. i. 1. and F. 1. m.). In the model the sheet is irregularly quadrilateral in shape. The dorsal, ventral and distal edges are free, while the proximal edge is an imaginary line corresponding to the position of the radix N, abducentis (Plate vi). The ventral edge is parallel with the long axis of the medulla; the distal and dorsal edges make a curve which follows the central canal and the floor of the fourth ventricle. On the dorsal border is a secondary curve corresponding to the position of the nucleus "N. hypoglossi.

At the distal ventral angle is a projection in the form of a trough which corresponds to the ventral f uniculi of the spinal cord. The trough makes a slight angle with the distal edge of the medulla sheet. It is deep in the part directed toward the spinal cord, but as it approaches the medulla oblongata it grows more shallow until it finally opens out onto the flat surface of the sheet.

The medulla sheet is of nearly uniform thickness, except at the ventral edge, where the fibres are more scattered. The lateral surface of the sheet is comparatively flat; in the ventral portion, however, the ends are curved, which will be understood by comparison with Sec. 146, Fig. 20 (S. i. L). The section shows that the trough (F. 1.) lies lateral from the main sheet owing to the position of the decussatio pyramidum, and that this accounts for the distal curve. The proximal curve is due to the fact that the fibres curve lateralward just as they enter the corpus trapezoideum.

Proximal to the opening of the trough, near the ventral edge of the medulla sheet is a deep depression; dorsal to it is a second smaller depression which lies near the middle of the distal border of the sheet. Both of these correspond to the position of portions of the nucleus olivaris accessorius medialis. The two spaces left in the sheet correspond to nuclei; the smaller one (.89 x .41 mm.) represents the so-called accessory nucleus of Roller, while the larger (3.7 x 1.5 mm.) corresponds to the nucleus centralis inferior of the formatio reticularis grisea.

The mesial surface of the medulla sheet is level and shows well in Plate vn. The origin of the fibres is considered on page 36 et fol.

B. PEOXIMAL SUBDIVISION OF THE CENTEAL FIBEE MASS, OR LONG TRACTS IN PONS AND MIDBEAIN (PLATE VIl).

Sabin1901 plate07.jpg

The proximal part of the fibre mass undergoes many changes in form, but Fig. 41 may be used as a typical section (L. m. and F. 1. m.). It has been said that the fibres of the proximal segment scatter and are divided into three groups: (1) a ventral (Plate vn, L. m.), (2) a dorsal (Plate v, F. 1. m.), and (3) a middle portion (Plate v, Formatio reticularis alba). The ventral group, which includes all the fibres that run through the cross-bar (trapezoid body), is in contrast with the t( medulla sheet/ 7 for instead of being spread out in a vertical direction, the mass rotates so that the sheet comes to lie horizontally (Plate vn). The border of this sheet lies far out on the lateral surface of the pons. At the junction with the midbrain the mass again rotates so that it once more assumes almost a vertical (i. e., dorsoventral) direction (Plates n and v).

The dorsal portion of the proximal segment continues upward in a line with the direction of the medulla sheet (Plates v and vi, Fasciculus longitudinalis medialis). In passing through the pons it is practically level, while just beyond it turns ventralward, taking part in the midbrain curve (Plate vin). This curve shows also on Plate iv. It is not labeled, but the root of the third nerve passes through it.

The middle portion can be related as follows: while the ventral fibres make a floor, as it were, at the junction of the pars dorsalis with the pars ventralis pontis, the dorsal fibres remain close to the median line, and into the area thus left between these two fibre masses the middle fibres spread out (Plates vi, vn and vni, Formatio reticularis alba). (Fig. 41, area between L. m. and F. 1. m.)

It is now necessary to describe the various portions of the proximal subdivision of the central fibre mass more in detail. It will be most convenient in this description to follow (a) the ventral portion, (I) the dorsal portion, and (c) the middle portion separately through the pons and midbrain.

(a) The Ventral Portion of the Proximal Subdivision of the Cen- Lateral tral Fibre Mass (Lemniscus lateralis y Lemniscus medialis and Lemniscus superior) (Plate vn). The ventral fibres of the proximal subdivision of the central fibre mass make (1) the horizontal pontal sheet and (2) the vertical midbrain sheet. In entering the pons the ventral fibres of the medulla sheet curve rapidly dorsalward, so that they all pass through the cross-bar (trapezoid body) (Plate vi). As the mass of fibres leaves the cross-bar it spreads out into " the horizontal pontal sheet " (Plates vi and vn). The lateral portion of this horizontal sheet consists of a new mass of fibres, the lateral lemniscus, which does not exist in the medulla sheet, but is added on in passing through the cross-bar. It forms a part of the pontal sheet for a short distance only, inasmuch as it inclines rapidly dorsalward in order to reach the nucleus colliculi inferioris in which a large part of it disappears (Plate n). The space shown in the model between the lemniscus lateralis and the pontal sheet is occupied at this stage of development by indifferent substance.

The pontal sheet is best seen from its dorsomesial aspect (Plate vn). The mesial edge is clearly defined. The sheet is comparatively thick as it emerges from the trapezoid body, but it grows thinner as it approaches the midbrain. The dorsal surface is level within the pons but curves ventralward in approaching the midbrain, while the ventral surface curves in crossing the pons (Plate n). The fibres of the medial third of the sheet, its thinnest portion, are cut off abruptly in the model as they are entering the midbrain in order to accommodate the radix N. oculomotorii. In reality they pass toward the nucleus ruber and appear to form a part of its capsule (Plate vn).

The lateral portion of the pontal sheet contains the bulk of its Medial fibres. They make a definite bundle which runs through the mid- le brain sheet at approximately the same level as in the pontal sheet. This bundle shows as a cut edge at the proximal end of the midbrain sheet to signify that it passes onward beyond the limits of the model (Plates n and v). It is the lemniscus medialis on its way toward the cortex.

Having thus outlined the pontal sheet by itself, its form will be best understood in connection with the midbrain sheet. From the lateral view it will be seen that as the pontal sheet approaches the midbrain, it spreads out into a fan which is placed obliquely in a dorso ventral plane (Plate v). The gradual rotation by which the horizontal pontal sheet becomes this obliquely vertical midbrain sheet is best seen from the mesial view (Plate vn). The medial lemniscus runs in the centre of this fan and determines its shape. This can be seen in Fig. 16, for the section passes through the central bundle and shows that in crossing the pons its fibres slant lateralward, while at the beginning of the midbrain they turn and run more nearly parallel to the central axis. In the midbrain the lateral border of the bundle is curved, owing to the position of the substantia nigra.

(PLATE V

From the lateral view it may be seen that the midbrain sheet, ventral to the medial lemniscus, is flat and lies more obliquely across the midbrain (Plate v). It receives its fibres from the mesial part of the pontal sheet, and enters into the formation of the capsule of the nucleus ruber. One further point should be mentioned in connection with the view of the lateral surface of the model, namely, that just as the lemniscus medialis enters the midbrain, it gives off from its ventrolateral portion a small bundle which enters the substantia nigra (Plate v, M. 1. to Sn.).

Superior Having outlined the central bundle of the midbrain sheet, i. e. the lemniscus medialis, and the fibres ventral to it, namely, the lateral capsule of the nucleus ruber, the dorsal segment of the sheet may next be considered. This is the so-called lemniscus superior, or dbere Schleife, of the Germans (Plates n and v). In contrast with the central bundle, this dorsal portion has a flat surface. It has been seen that the fibres ventral to the medial lemniscus were derived from the mesial portion of the pontal sheet; the dorsal fibres, on the other hand, are related to its lateral border (Plate v). The lemniscus superior lies near the lateral surface of the midbrain (Plate v, Figs. 12 and 44). The lines in the model show that its fibres are inclined dorsalward. The shape of the bundle in the model is that of a triangle with its apex at the point where the lemniscus lateralis separates from the lemniscus medialis and its base in a dorsoventral line near the upper end of the model. The base is a curved line which indicates that, unlike the lemniscus medialis, the fibres of the superior lemniscus end within the limits of the model (Plate n). The ventral side of the triangle lies adjacent to the medial lemniscus; the dorsal side has in part a free border, but near the base of the triangle the lemniscus superior fuses with the lemniscus lateralis. At the point of fusion, the lemniscus lateralis is hollowed out so as to form a capsule for the nucleus colliculi inferioris (Plates vn and vm).

The description of the ventral portion of the central fibre mass summary of is now complete. Its. most important element is the medial lemniscus, which enters into the formation of three great sheets a medial, vertical sheet in the medulla; a horizontal sheet in the pons; and a lateral obliquely vertical sheet in the midbrain.

The pontal sheet includes another group of fibres, the lemniscus lateralis, while the midbrain sheet contains the superior lemniscus and the lateral capsule of the nucleus ruber.

(b) The Dorsal Portion of the Proximal Subdivision of the Cen- Posterior tral Fibre Mass (Fasciculus longitudinalis medialis). The next bundle under consideration is the fasciculus longitudinalis medialis, which forms the dorsal part of the proximal subdivision of the central fibre mass, and is the continuation of the dorsal part of the medulla sheet (Plates v and vm). As has been said, this portion follows the course of the central canal. It passes through the pons at the same level as in the medulla, and then turns ventralward to conform with the midbrain curve.

In the model of the medulla sheet, a space left for a nucleus of the formatio reticularis is the only indication that the sheet is made of different parts (Plates vi and vii, Ku. centralis inferior). The fibres dorsal to this space become a separate bundle in crossing the pons. This is clear in sections where the coarse black fibres of the fasciculus longitudinalis medialis contrast with the delicate fibres of the formatio reticularis just ventral (Fig. 40). The fibres of the radix X. abducentis pass directly through the edge of this dorsal bundle, and just proximal to this nerve the bundle flattens out and becomes triangular in cross-section (Fig. 40). At the beginning of the midbrain the bundles of the two sides broaden, and, at the same time, form a trough (Plate iv. Not labeled, but the root of the third nerve passes through the side of the trough). The centre is deep and its floor is made, not of decussating fibres, but of longitudinal fibres running in the middle line (Fig. 18). The sides of the trough show shallow depressions, three on the right side and two on the left. They represent the position of the nuclei of the oculomotor and trochlear nerves (Plate vin). Beyond these depressions is the space left in the wall for the exit of the main mass of fibres of the root of the oculomotor nerve (Plate vm). This space is artificial, the wall being in reality almost continuous. Just proximal and lateral from this space is a long groove which opens out into the proximal capsule of the nucleus ruber. This part of the capsule is a sheet of fibres placed dorsoventrally across the midbrain. The groove contains the nucleus of Darkschewitsch and marks the junction of three different fibre masses, viz., (1) the fasciculus longitudinalis medialis, (2) the posterior commissure and (3) the capsule of the nucleus ruber. These relations will be more fully considered in connection with the description of the midbrain. To return to the fasciculus longitudinalis medialis, the sides of the trough formed by the bundles of the two sides come into relation, in the model, with a large hollow shell which occupies the centre of the dorsal part of the midbrain. This shell is the stratum album profundum (Plate vni).

summary of The dorsal bundles of the two sides making up the fasciculi dinai bundle, longitudinales mediales occupy the dorsal part of the medulla sheets, where they form two dense bands on either side of the raphe. In the pons each band becomes broader, and in the midbrain the bundles of the two sides unite to form a deep trough. As a whole, each bundle is characterized by two curves, one in the medulla, the other in the midbrain. It is related to four nuclei, namely, the nuclei of the !N". hypoglossus, N. abducens, ~N. trochlearis and ~N. oculomotorius. At the distal extremity of the medulla oblongata it is connected with the ventrolateral funiculi of the spinal cord, while at the proximal limit of the model it is related, at least in position, to three structures a nucleus, a decussating bundle and a nuclear capsule. The origin of these fibres is considered on pages 31 and 97.

Formatio (c) The Middle Portion of the Proximal Subdivision of the Central Fibre Mass (Formatio reticularis alba). By reviewing the shape of the two great fibre masses just described, the ventral, with its tendency to form sheets, and the dorsal compact bundle following the course of the central canal, a good idea of the area left between them can be obtained. Into this area, dorsal to the pontal sheet and medial to the midbrain sheet, the fibres of the middle portion of the proximal subdivision spread out (Plates vi, vii and vm), Fig. 40, area between L. m. and F. 1. in.). The area left for a nucleus in the medulla sheet lies in the course of this fibre mass, and appears both to receive and to give rise to its fibres (Plate vi, Nu. centralis inferior). Just proximal to this nucleus, the fibres of the middle portion turn slightly lateralward and spread out into the pars dorsalis pontis and the tegmentum of the midbrain (Plate vn). The part of the midbrain they enter is bounded laterally by the lemniscus superior, medially by the stratum album profundum, and ventrally by the capsule of the red nucleus (Plate vm). In the model, this middle mass of fibres had to be sacrificed somewhat on account of the representation of the brachium conjunctivum. The mass will be considered later in connection with the formatio reticularis as a whole, and was introduced here on account of its relations to the central fibre mass (page 107).

The general form of the central fibre mass of the model, with its contrasting distal and proximal parts, has now been outlined. The part distal to the cross-bar is a thin median sheet which extends in a dorsoventral plane parallel with the raphe, and shows but little evidence at this stage of development of being made up of different parts; the part proximal to the cross-bar is clearly made up of three different fibre masses which spread out to cover a wide area and show varied connections with other structures.

C. Relation of the Central Fibre Mass to the Serial Sections and to Other Structures in the Model

The components of the central fibre mass must now be related to Long tracts known bundles as seen in sections at different levels. The position of in sections. the sections with reference to the model can be estimated by the lines on Fig. 52. These lines are labeled with the series and section number, I standing for the horizontal series, and n for the transverse.

In a series of sections extending from Figs. 28-36, the main character- Cross-sections istics of the medulla sheet are well illustrated, (1) its median position, ofLemnisci. (2) the gradual increase in its dorsoventral diameter due to the cervical curve, (3) the lack of differentiation of its fibres, except in the last section, which shows the middle fibres disappearing in a nucleus.

The ventral portion of the sheet will be recognized in the sections of s. i. 1. the new-born child as the stratum interolivare lemnisci. In a section of adult tissue so many more fibres are crowded into this portion that it forms a triangle in cross-section, with the base against the pyramid.

A series of transverse sections corresponding to the proximal portion L. m. of the central fibre mass illustrate the region well (Figs. 38 to 49). The first two sections pass through the trapezoid body; the second, taken where that body is widest, shows that all the fibres of the lemniscus

L. 1. pass through it. The next five sections pass through the pontal sheet. Fig. 43 shows the separation of the lateral from the medial lemniscus (Plate n). The area between the two lemnisci is wider in proportion in the transverse series owing to the obliquity of the sections; moreover, it contains a few medullated fibres, whereas there are none in the same area in the longitudinal series.

L. s. The lemniscus superior is shown in Figs. 46 to 48. It will be considered later in connection with the midbrain. Fig. 47 passes through the midbrain sheet just distal to the red nucleus. The oblique position of the sheet is evident as well as its relation to the substantia nigra.

Fig. 48 shows the relation of the medial lemniscus to the red nucleus and its capsule.

Longitudinal The longitudinal series gives a far more comprehensive view of the (frontal) ^secttona me dial lemniscus than the transverse (Figs. 12 to 22). As has been said, the section shown in Fig. 16 was used as a starting point in building the L. m. model, for it contained a bundle, the lemniscus medialis, which ran the entire length of the section. This bundle includes the stratum interolivare, the pontal sheet and the midbrain bundle. The shape and the curves of the tract are evident. The points of especial interest are (1) the radiation of the fibres in passing through the corpus trapezoideum, (2) the distribution of the fibres in the pontal sheet, the main mass lying in the lateral part, and (3) the midbrain curve in relation to the substantia nigra. The junction of the pons and midbrain is plain, and many scattered fibres can be seen entering the midbrain from the medial part of the sheet.

L. 1. In considering the lemniscus medialis, it is necessary to relate it (1) to the lemniscus lateralis, (2) to the lemniscus superior and (3) to the capsule of the red nucleus. The beginning of the lateral lemniscus is seen in Fig. 16, but the main mass of it is seen at a level farther dorsal in Fig. 13. This section is taken just at the junction of the medial L. s. lemniscus with the lemniscus superior, while in Fig. 12, the medial lemniscus disappears and the superior lemniscus becomes visible by itself.

This description will perhaps suffice for the structures that lie dorsal to the level of the pontal sheet. Fig. 19 is farther ventral, and, owing to the arch formed by the pontal sheet (Plate v), it shows the medulla sheet and midbrain sheets but not the poutal sheet.

In this section two points are well shown, (1) the radiation of the lemniscus fibres as they emerge from the trapezoid body, and (2) the curves which the lemniscus forms in the midbrain. It will be noted that by means of two curves the fibres of the lemniscus adjust themselves to the substantia nigra and the red nucleus. Passing still more ventralward the lateral capsule of the red nucleus becomes visible (Fig. 20). It consists at this stage of very few fibres. The rest of the sections show simply the stratum interolivare lemnisci.

Origin of fibres The fibres of the stratum interolivare lemnisci, or ventral part f of the medulla sheet, have been traced in the model from this sheet (1) through the corpus trapezoideum, (2) through the pontal sheet and (3) through the central part of the midbrain sheet. Here the lemniscus medialis, now deflected far to the side, lies lateral to the nucleus ruber. The model takes the bundle only as far as the hypothalamic region (Plate 11).

Thus far I have not discussed the origin of these fibres. There are three groups of fibres to be considered with reference to the stratum interolivare lemnisci. (1) In Plate v will be seen a small bundle of fibres connected with the lateral funiculus of the cord. It passes toward the middle line and appears to end blindly against the root of the hypoglossal nerve. In reality, its fibres pass on between the root bundles of the nerve adjacent to the stratum interolivare lemnisci. From the Weigert specimens alone I cannot say that these fibres enter the stratum interolivare lemnisci, but the model shows the possibility of an uncrossed path from the lateral funiculus of the cord to the main sensory path toward the cortex (Fig. 19 F. 1.). (2) By far the thickest bundles of fibres entering the lemniscus in the medulla are the bundles of internal arcuate fibres from the nucleus funiculi gracilis and the nucleus funiculi cuneati. These arcuates form two principal groups, one of which lies just distal to the root of the N". hypoglossus, while the other lies opposite the proximal third of the nucleus funiculi cuneati (Plate vn). The distal bundle is labeled decussatio lemniscorum, the proximal bundle is not labeled but the line marked nucleus funiculi cuneati points toward it. In sections the two groups show in Figs. 28, F. a. i., and 31, F. a. i. These bundles can be described best in connection with the dorsal funiculi of the spinal cord, sections. F. a (3) As a further source of fibres for the lemniscus are the arcuates from the nuclei of termination of the cerebral sensory nerves. These arcuates are too diffuse and scattered to be modeled as bundles.

The sensory path to the cortex is called stratum interolivare summary of lemnisci in the medulla and lemniscus medialis in the pons and midbrain. It makes the ventral part of the medulla sheet, the medial part of the pontal sheet and the central part of the midbrain sheet. It carries (1) sensory fibres from the cord both directly from the ventrolateral columns (Plate vi), and indirectly from the dorsal columns by means of two bundles of internal arcuate fibres. (2) It carries fibres from all the nuclei of the sensory cerebral nerves. (Scattered arcuate fibres not seen in the reconstruction.) (3) A few descending fibres not yet medullated.

Posterior The fasciculus longitudinal! s medialis can be seen in the transverse D bimdle n fn sections from Figs. 28 to 51. In the model it will be remembered that sections, the ventrolateral funiculi of the cord are represented as a trough in which the ventral horn cells lie, and that this trough opens out upon the surface of the medulla sheet (Plate vi). The walls of this trough are the ground bundles, the fasciculus ventralis et lateralis proprius. In the medulla oblongata the fibres curve dorsalward into the border of the medulla sheet. This is best made out in sagittal section. In Fig. 28 the trough is cut in cross-section and its fibres can be traced to a point just ventral to the central canal. A single section will suffice to show the relations of the fasciculus longitudinalis medialis in the pons (Fig. 42). In the midbrain the trough described before in connection with this bundle is seen in Figs. 46 to 51. Its shape, its relations to the nucleus N. oculomotorii and the nucleus N. trochlearis, and especially the superior connections of its walls deserve notice. In the last section, Fig. 51, the relation to the nucleus of Darks chewitsch is evident. These connections will be considered in detail in the study of the midbrain.

The longitudinal series, as in the case of the lemniscus medialis, gives the best idea of the medial longitudinal bundle (Figs. 7 to 20). A view of the model from the lateral aspect shows that this bundle reaches its most dorsal point opposite the nucleus N. abducentis (Plate vi). The section in Fig. 7 passes through this point. Fig. 9 gives the best view of the bundle, both as to its shape and its relations. The two extremities of the bundle in this section represent the positions of the cervical and midbrain curves.

By tracing the series ventral ward the following points can be studied: (1) the ventral course of the fibres at either end, (2) the widening of the bundle at the proximal end, (3) the formation of the midbrain trough, (4) the relation of the latter to the nuclei of the oculomotor and trochlear nerves, and (5) the relation to the nucleus of Darkschewitsch, the posterior commissure and the capsule of the red nucleus.

Relations of Since the medial longitudinal bundle is especially related to a Tu^dinlTbundie group of cerebral nerves, it may be well to consider these relations cerebral nerves! ^ n detail. In the model there are four depressions in the bundle, one in the medulla oblongata, one in the pons, and two in the midbrain. These four depressions correspond to the nuclei of (1) the N". hypoglossus, (2) the N. abducens, (3) the N. trochlearis and (4) the N. oculomotorius. The nucleus "N. hypoglossi corresponds to the shallow depression just proximal to the distal dorsal angle of the medulla sheet (Plates vi and vn, and Fig. 31). The nucleus is a long one and lies in the gray matter ventral to the central canal. The nucleus of the N. abducens lies in this same central gray matter close to the floor of the ventricle (Plates vi and vn). It lies farther lateral than any other nucleus of this group, as though it had been pulled from the middle line by the knee of the root of the facial nerve (Plate in). The fibres of the root of the N. abducens pass through the edge of the medial longitudinal bundle. The relations of the nucleus JST. abducentis are shown in Fig. 7. In the midbrain trough of the medial longitudinal bundle are embedded the nuclei of the oculomotor and trochlear nerves (Plate vm). Since the nucleus E". trochlearis is asymmetrical, there are two depressions in the fasciculus longitudinalis medialis on the right side and one on the left (Figs. 44 and 46, and Figs. 10 and 12). Proximal and ventral to the position of the nucleus "N. trochlearis are the depressions for the nucleus N. oculomotorii (Fig. 47 and Fig. 13). ^

The main features of the medial longitudinal bundle have thus Summary^! been illustrated in the model and proved in the sections. In brief, tudin"ib they are as follows: It is a long tract running through the entire area covered by the model. 1 As to position, it lies just ventral to the central canal save at its superior limit; as to form, its striking features are (1) the two curves, (2) the depressions for the nuclei, (3) the midbrain trough; as to origin, it is known to consist in large part of the continuation of the ventrolateral funiculi of the spinal cord. It receives, however, many fibres from adjacent gray masses on its course, and gives off fibres to these. It has undoubted relations at the superior limit with the nucleus of Darkschewitsch. Indeed, this nucleus has been called by at least one writer the nucleus fasciculi longitudinalis medialis. The question of higher connections will be discussed later. There are fibres in the fasciculus longitudinalis medialis running in both directions, as Golgi studies by various investigators have demonstrated.

The middle portion of the central fibre mass can be studied to better advantage in connection with the formatio reticularis as a whole. I shall refer, therefore, in this place merely to one section (Fig. 13), which will show the essential points, viz.: (1) that the fibres spread out into the pars dorsalis pontis and the tegmentum; and (2) that the fibres come into relation with several groups of cells.


1 The description given diif ers from that found in many books where the term is applied only to the bundle in the pons and midbrain. The authors of these books usually refer to the portion in the medulla oblongata as the " remains of the ventrolateral funiculi of the spinal cord" (Verderseitenstranffreste of the Germans). It seems to me more satisfactory to include in Ihe fasciculus longitudinalis medialis the fibres of the medulla sheet which are continuous with the bundle in the pons.


It remains now to show how the central fibre mass, so fundamental in the construction of the model, is, in fact, a foundation 

central Fibre f or the entire structure. It is clear that in each of the three prinfoundation for cipal regions of the model, namely, the medulla oblongata, the pons

(see page 16). and the midbrain, this central bundle, at least the main part of it, has a characteristic shape; in the medulla oblongata it is a median vertical sheet, in the pons, a horizontal sheet, in the midbrain, a lateral obliquely vertical sheet. If now the medulla is considered in the light of its development from the comparatively simple structure of the cord, it will be seen that, as the central canal moves dorsalward to make room for the intrinsic structures of the medulla, the ventral ground bundle, which underlies the canal, becomes enormously developed so as to form the medulla sheet. This medulla sheet, indeed, represents far more than the ventral funiculus, for it is made up of fibres from the lateral and dorsal funiculi as well. Indeed, at the beginning of the medulla oblongata, almost the entire dorsal funiculus passes by means of secondary neurones, arcuate fibres, either dorsally to the cerebellum or ventrally into the medulla sheet. This change is interesting in connection with the fact that the cerebellum develops from the dorsal wall of the neural tube at this position according to the work of His. The grouping About this vertical sheet can be grouped all the other structures in the Meduiia of the medulla oblongata. Just ventral to it, lies the pyramidal tract; while the region lateral to it can be divided into two levels, a ventral and a dorsal. The ventral level is characterized by the nucleus olivaris inferior; the dorsal level corresponds to a formatio reticularis area which is bordered by the roots of the cerebral nerves and their nuclei.

in the pons. In the pons, the central fibre mass makes a complete floor for the pars dorsalis pontis. The whole of the structure which characterizes the pons, namely, the pontal nuclei, lies ventral to the sheet; while on the other hand, all the structures, which more or less correspond to the upward continuation of the spinal cord, lie dorsal to it This pars dorsalis pontis is composed chiefly of formatio reticularis. It receives (1) the middle portion of the proximal subdivision of the central fibre mass, and (2) a mass of fibres from the formatio reticularis of the medulla oblongata. The brachium conjunctivum, or superior cerebellar peduncle, passes through it. in the midbrain. The midbrain structures in turn can be grouped around the central fibre mass. The lateral course of the lemniscus medialis above the medulla is interesting in connection with the size and central position of the red nucleus. In the ventral region of the midbrain are the substantia nigra and nucleus ruber separated from one another by the lateral capsule of the red nucleus and the lemniscus medialis, while dorsal to the red nucleus is a formatio reticularis area. This area is bounded laterally by the lemniscus superior and medially by the fasciculus longitudinalis medialis and stratum album profundum (Plate vm).

The cerebral nerves can be grouped in part with relation to the Relations of foundation of the model. Plates v and vi of the model, is shown the trough which contains the ventral column of gray substance of fi the spinal cord opening out onto the medulla sheet. From this point can be traced two separate groups of nuclei pertaining to the cerebral nerves, a median, motor group, and a lateral, mixed group. The median, motor group includes the nucleus E". hypoglossi, nucleus ."N". abducentis, nucleus N". trochlearis and nucleus N. oculoinotorii, which have already been related to the fasciculus longitudinalis medialis. The lateral group, on the other hand, is not directly related to the central fibre mass, but rather lies within or bordering upon the area of the formatio reticularis. This lateral group can be subdivided into two parts, a ventral, motor group and a dorsal, sensory group. The ventral group includes the nucleus N. accessorii and the motor nuclei of the !N". glossopharyngeus, N. vagus, N. facialis and !N". trigeminus. They all lie within the formatio reticularis. The sensory group includes the nuclei of termination of the sensory portions of the N. glossopharyngeus, N. vagus, N. facialis and 1ST. trigeminus together with all the nuclei of the cochlear and vestibular nerves. This group lies far dorsalward and its nuclei and nerve tracts border upon the formatio reticularis. In the positions of these groups as here outlined is a striking contrast. On the one hand, the motor groups are both placed in close relation to association fibres, the median group to a definite compact bundle (the fasciculus longitudinalis medialis), the lateral to diffuse association paths (formatio reticularis); the sensory group of nuclei, on the other hand, lies not actually in but rather bordering upon the formatio reticularis. This grouping of the cerebral nuclei which will be used in connection with the model has been noted in comparative anatomy as well as in connection with the human brain.

Besides these special. form relations the model illustrates certain general relations. A view from the lateral surface (Plate n) shows General relations that the ventral regions of the model are occupied largely by nue ' clear masses: the olive, the pontal nuclei, the substantia nigra and the red nucleus. The absence of medullated fibres in this region is due to two facts, (1) that the pyramidal tracts and the brachium pontis which belong here become medullated at a later period, and (2) that, at the cervical curve in the medulla, most of the fibres of the spinal cord curve dorsalward, and thus leave the olivary region comparatively free. At this stage of the development the ventral level of the medulla oblongata, pons and midbrain is a nuclear region. Dorsal to this level is the area corresponding more or less to the continuation of the spinal cord. In the views of the model, one is struck by the number of fibre, bundles this dorsal level contains, long and short paths, that is, tracts to the cerebrum and cerebellum as well as fibres to and from many way-stations. These fibres represent in part the funiculi of the cord. The region is not wanting in nuclei, for the dorsal and ventral columns of the cord are represented by the nuclei of the cerebral nerves. The ground bundles of the cord correspond to the extensive f ormatio reticularis alba and grisea, while the central gray matter of the cord becomes much enlarged to form the substantia grisea centralis of the medulla, pons and midbrain.


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An Atlas of the Medulla and Midbrain (1901): Chapter I. Introductory | Chapter II. The Long Tracts | Chapter III. The Columns Of The Spinal Cord | Chapter IV. Cerebellar Peduncles | Chapter V. The Cerebral Nerves And Their Nuclei | Chapter VI. The Cerebral Nerves And Their Nuclei (Continued). Lateral Group | Chapter VII. The Inferior And Accessory Olives | Chapter VIII. The Midbrain | Chapter IX. The Formatio Reticularis Alba And Grisea | General Summary of what Is shown In Reconstruction | References To Literature | Abbreviations | Description of Figures and Plates

Cite this page: Hill, M.A. (2024, April 23) Embryology Book - An Atlas of the Medulla and Midbrain 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_An_Atlas_of_the_Medulla_and_Midbrain_2

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