_1_, Dura and arachnoidea divided and reflected; _2_, spinal cord;
_3_, nerve-roots. (From Leisering’s Atlas, reduced.)
]
The surface of the spinal cord is divided into two similar halves by a
=dorsal median groove= (Sulcus medianus dorsalis) and a =ventral median
fissure= (Fissura mediana ventralis). On either side of the former is
the =dorso-lateral groove= (Sulcus dorsalis lateralis) at which the
fibers of the dorsal nerve-roots enter the cord; it is faint except at
the enlargements, and is represented by two grooves in the first
cervical segment. The ventral root-fibers as they emerge from the cord
do not form a continuous series, but arise from a zone (Area radicularis
ventralis) 3 to 5 mm. in width a little lateral to the ventral median
fissure, and no groove is found here. In the greater part of the
cervical region and the anterior part of the thoracic region there is a
shallow =dorsal paramedian groove= (Sulcus intermedius dorsalis) a short
distance lateral to the median groove. These grooves indicate the
division of the white matter of the cord into columns to be described
later.
Examination of cross-sections of the spinal cord shows that it is a
bilaterally symmetrical structure, incompletely divided into right and
left halves by a ventral fissure and a dorsal septum. The =ventral
median fissure= (Fissura mediana ventralis) is narrow and penetrates
nearly to the middle of the dorso-ventral diameter of the cord. It is
occupied by a fold of pia mater. The =dorsal median septum= (Septum
medianum dorsale) is a partition which descends from the posterior
median groove to about the middle of the cord. It apparently consists of
condensed neuroglia with an admixture of pial tissue. The two halves of
the cord are connected by commissures of gray and white matter. The
=gray commissure= (Commissura grisea) is a transverse band of gray
matter at the ventral end of the dorsal septum. It is divided into
dorsal and ventral parts by the central canal of the cord. The =white
commissure= (Commissura alba) is a bridge of white matter which connects
the ventral columns of the cord over the dorsal end of the ventral
median fissure, and constitutes a conducting path from one side to the
other.
[Illustration:
FIG. 497.—CROSS-SECTION OF SPINAL CORD _in situ_, IN PART SCHEMATIC.
_1_, Dorsal column; _2_, lateral column; _3_, ventral column; _4_,
dorsal horn; _5_, ventral horn; _6_, gray commissure; _7_, white
commissure; _8_, central canal; _9_, dorsal septum; _10_, ventral
fissure; _11_, dorsal nerve-root; _12_, ventral nerve-root; _13_,
spinal ganglion; _14_, nerve; _15_, intervertebral foramen; _16_,
arch of vertebra; _17_, epidural space; _18_, dura mater
(represented a little too thick); _19_, ligamentum denticulatum;
_20_, _20_, longitudinal venous sinuses; _21_, middle spinal artery.
The subdural and subarachnoid spaces (black) are traversed by
delicate trabeculæ. The outer part of the arachnoid and the pia
mater are not shown.
]
The =central canal= of the cord (Canalis centralis), the spinal vestige
of the lumen of the embryonal neural tube, is a minute passage which
tunnels the gray commissure. It opens at its anterior end into the
posterior part of the fourth ventricle of the brain, and its terminal
part in the conus medullaris forms a slight dilatation, the ventriculus
terminalis. It is lined by epithelium and is surrounded by a layer of
modified neuroglia (Substantia grisea centralis).
The =gray matter= of the spinal cord as seen in cross-sections resembles
roughly a capital =H= the cross-bar of the letter being formed by the
gray commissure. Each lateral part is considered as consisting of
=dorsal= and =ventral gray columns= or =horns= (Columna grisea dorsalis,
ventralis). In the greater part of the cord the dorsal column or horn is
elongated and narrow and tapers to a point which extends almost to the
surface of the cord at the attachment of the dorsal root-fibers of the
spinal nerves. Its apex or tip consists of gray matter which is lighter
in color and less opaque than that of the rest of the horn and is termed
the =substantia gelatinosa= (Rolandi). The ventral column or horn is
short, thick and rounded, and is separated from the surface of the cord
by a thick layer of white matter, through which the fibers of the
ventral roots of the spinal nerves pass. From the middle of the cervical
region to the lumbar region there is a medial projection of gray matter
on the lower part of the dorsal column; this is the =nucleus dorsalis=
or Clarke’s column. In the anterior part of the cord there is an outward
projection of the gray matter at the base of the ventral horn; this is
termed the =lateral column= or =horn=. The demarcation between the gray
and white matter is in many places indistinct; this is especially the
case laterally, where processes of gray matter extend into the white
substance, producing what is known as the =formatio reticularis=.
Cross-sections of the spinal cord present the following gross regional
characters: (1) The cervical cord near the medulla is compressed
dorso-ventrally, its width is about 18 mm. and its greatest thickness
about 8 mm. It has dorsally a deep median sulcus and a distinct
dorso-lateral sulcus. Lateral grooves are also present. The dorsal
cornua are strongly everted. Each has an expanded head, which comes
very close to the surface of the cord, and has an extensive cap of
substantia gelatinosa. The neck is distinct. The ventral cornua are
short and blunt and diverge very little. The gray commissure is about
in the middle of the section and 2.5 mm. in length. According to
Dexler the column between the median and lateral grooves dorsally is
the funiculus cuneatus, the funiculus gracilis being very small and
not showing on the surface in this region. In the middle of the
cervical region the diameters are about 16 mm. and 10 mm.
respectively. The ventral surface is somewhat flattened. The dorsal
cornua have pointed ends and turn decidedly outward. The ventral
cornua are short and thick and are directed very slightly outward;
their ends are about 4 mm. from the ventral surface. The gray
commissure is just above the middle of the section and is about 2 mm.
long. The cervical enlargement measures about 25 mm. transversely and
12 mm. vertically. The dorsal cornua are smaller than the ventral and
have a large cap of substantia gelatinosa. The ventral cornua are
short and thick, curve strongly outward, and are about 4 mm. from the
ventral surface. Each bears a prominence on its inner side near the
base. The gray commissure is considerably above the middle of the
section and is about 4 mm. long. (2) In the middle of the thoracic
region the cross-section is biconvex, the ventral surface being the
more strongly curved. The transverse diameter is about 15 mm. and the
dorso-ventral about 10 mm. The gray columns are close together, the
gray commissure being only about 1 mm. in length, and lying
considerably above the middle of the section. The dorsal cornua are
short and have slightly enlarged ends. The ventral cornua have a
uniform diameter, turn very little outward, and end about 3 mm. from
the ventral surface. (3) The lumbar enlargement is much flattened,
especially dorsally. The transverse diameter is about 22 mm. and the
dorso-ventral 9 to 10 mm. The cornua are very large. The ventral
cornua are thick and rounded and turn sharply outward; they end about
2 mm. from the ventral surface. The dorsal cornua are smaller and
shorter and do not diverge so strongly. The gray commissure is about
in the middle of the section and is about 3 mm. long. In the third
lumbar vertebra the cord is about 3 mm. narrower and thicker, and both
surfaces are about equally convex. The dorsal cornua are smaller,
considerably everted, and constricted in the middle. The ventral
cornua are very short and do not turn outward. (4) In the first sacral
vertebra the cord is almost round and is 5 to 6 mm. in diameter; the
cornua are relatively very large and the commissure has the form of a
high intermediate mass.
The ventral horn contains large cells, the axones of which emerge as
the fibers of the ventral nerve-roots (Fila radicularia). The axones
of many cells cross to the opposite side in the white commissure and
pass out in a ventral root of that side, or enter the white matter and
pass forward and backward, associating various segments of the cord.
Some pass to the ventral horn of the opposite side at the same or at
different levels. Others pass to the periphery of the cord, join the
cerebellospinal fasciculus, and extend to the cerebellum. Scattered
through the gray matter are many smaller cells with axones which
pursue a short course and serve to connect different parts of the gray
matter.
The =white matter= of the spinal cord is divided into three pairs of
=columns=. The =dorsal columns= (Funiculi dorsales) lie on either side
of the dorsal median septum and extend outward to the dorso-lateral
groove and the dorsal gray column. The =ventral columns= (Funiculi
ventrales) are situated on either side between the median fissure and
the ventral gray columns. They are connected above the fissure by the
white commissure. The =lateral columns= (Funiculi laterales) lie
external to the gray columns on either side; their limits are indicated
superficially by the dorso-lateral groove and the emergence of the
ventral root-fibers. The paramedian groove (where present) indicates a
subdivision of the dorsal column into two fasciculi or tracts; the inner
of these is the =fasciculus gracilis= (Goll’s column); the outer, the
=fasciculus cuneatus= (Burdach’s column).
The amounts of gray and white matter vary greatly in different parts of
the cord both absolutely and relatively. In cross-section the absolute
areas of both are greatest in the enlargements. The relative area of
gray matter is smallest in the thoracic region (except at its anterior
end), and increases from the lumbar enlargement backward.
Investigations have shown that in man the columns of white matter are
subdivided into fasciculi or tracts, which constitute definite
conducting paths of greater or less length. Our knowledge of the
tracts in the domesticated animals is very limited, and it is quite
unsafe to make inferences from the arrangement in man. As evidence of
this it may be noted that the ventral cerebrospinal or direct
pyramidal tract of man cannot be recognized as such.
The dorsal white columns consist essentially of two sets of axones.
The afferent or sensory axones which come from the cells of the spinal
ganglia enter as the dorsal roots of the spinal nerves and divide into
two branches in the vicinity of the dorsal gray column. The anterior
(ascending) branches form the direct sensory path to the brain and
extend in the fasciculus cuneatus and fasciculus gracilis or
corresponding tracts to nuclei in the medulla oblongata. The posterior
(descending) branches extend backward for varying distances and give
off numerous collaterals to cells of the gray column, thus forming
part of the mechanism for the mediation of reflex action. Some
collaterals cross in the white commissure to the opposite side. Many
of these fibers are collected in the comma-shaped tract between the
fasciculus gracilis and cuneatus. The second set of axones arises from
the smaller cells of the gray column. They enter the white matter,
divide into anterior and posterior branches, forming the fasciculi
proprii or ground bundles of the cord. Some branches cross to the
opposite side. The function of this set of axones is chiefly to
associate various levels of the cord.
The lateral columns contain some axones of the dorsal nerve-roots,
which (in man) are grouped in the marginal tract of Lissauer, situated
just dorsal to the apex of the dorsal horn. The cerebellospinal
fasciculus (direct cerebellar tract of Flechsig) extends along the
periphery of the lateral column. It contains the axones of the cells
of the nucleus dorsalis (Clarke’s column), which proceed to the
medulla oblongata and enter the cerebellum by the restiform body. The
rubrospinal tract of Monakow appears to take the place of the lateral
cerebrospinal fasciculus or crossed pyramidal tract of man. It lies at
the inner side of the cerebellospinal fasciculus. Its fibers arise in
the nucleus ruber of the mid-brain, cross the median plane
(decussation of Forel), and pass backward in the tegmentum and medulla
oblongata to the lateral column of the cord. It is a path for motor
impulses coming from the cerebral cortex and the cerebellum. The
lateral fasciculus proprius or ground-bundle is deeply situated at the
side of the gray columns. The bulk of its fibers are axones of cells
of the posterior cornu which divide into anterior and posterior
branches. They are intersegmental paths which associate different
levels of the gray matter of the cord. The significance of the
remaining fibers is not yet known.
The ventral white columns do not contain a ventral cerebrospinal or
direct pyramidal tract, as in man. There is a small tract (Fasciculus
intracommissuralis ventralis) above the white commissure, which
separates it from the rest of the ventral column. It extends to the
middle of the thoracic region. It consists of intersegmental fibers,
and contains in the anterior part of the cervical cord in the sheep
and goat both crossed and direct pyramidal fibers. The descending
cerebellospinal fasciculus extends from the cerebellum to the lumbar
region. In the cervical region it occupies a semilunar area which
reaches almost to the surface ventro-laterally. Scattered fibers
belonging to it lie also in the medial part of the ventral column.
Posteriorly it diminishes in size and comes to occupy a position next
to the ventral median fissure, corresponding to the sulco-marginal
fasciculus of man (Dexler).