subdivided into several varieties. Most authors agree that there are separate
fibers for the impulses aroused by tactile and thermal stimuli; and Sherrington
(1906) has presented evidence for the existence of a separate group of fibers,
whose end organs are responsive only to agents capable of inflicting injury,
that is, to noxious or painful stimuli.

Conduction of Tactile Impulses in the Spinal Cord. The phenomena of sen-
sory dissociation, characteristic of syringomyelia, show that the intraspinal
path for the sensations of touch is rather widely separated from that for pain
and temperature sensation (Fig. 73). In that disease a cavity is developed
within the gray matter of the spinal cord; and sensations of pain and tem-
perature may be abolished over a given cutaneous area which is still sensitive
to touch. The separation of these two lines of .conduction occurs at the place
where the dorsal root fibers enter the cord. The fibers, mediating pain and
temperature sensations, end almost at once in the gray matter, while those for
touch ascend for some distance in the posterior funiculus of the same side (Head
and Thompson, 1906; Dejerine, 1914). As these fibers ascend in the posterior
funiculus they give off collaterals to the gray matter of the successive levels of
the spinal cord through which they pass. The tactile impulses from a given
root, therefore, do not enter the gray matter all at once, but filter forward through
the collaterals and terminals of these dorsal root fibers to reach the posterior
gray column in a considerable number of segments above that at which the
root enters the cord. Within the posterior gray column at these successive
levels the terminals and collaterals of the tactile fibers establish synaptic con-
nections with neurons of the second order. The axons of these neurons form the
ventral spinothalamic tract of the opposite side (Fig. 73).

The ventral spinothalamic tract is an ascending bundle of fibers found in the
anterior funiculus. It consists of fibers which take origin from cells in the pos-
terior column of the opposite side, cross the median plane in the anterior white
commissure, and ascend in the Ventral funiculus to end within the thalamus (Fig.
73). It is possible that many of the fibers do not reach the thalamus directly,
but terminate in the gray matter of the cord and medulla oblongata in rela-
tion to other neurons, whose axons continue the course to the thalamus. If



THE NERVOUS SYSTEM



this be so the path consists in part of relays of shorter neurons (Dejerine,

1914).

The uncrossed path in the posterior funiculus for tactile impulses entering
the cord through any given dorsal root overlaps by many segments the crossed
path in the ventral funiculus (Fig. 230). Some of the uncrossed fibers even
reach the nuclei of the funiculus gracilis and funiculus cuneatus in the medulla
oblongata. This extensive overlapping of the uncrossed by the crossed paths
accounts for the fact that lateral hemisection of the human spinal cord rarely
causes marked disturbance of tactile sensibility below the lesion (Petren, 1902;
Head and Thompson, 1906).



Ascending branch of dorsal root fiber

Myelinated fiber of dorsal

Spinal ganglion ,f \

Unmyelinated fiber of dorsal root




Lateral spinothalamic tract

(pain and temperature)

- Ventral spinothalamic tract



(touch)



Fig. 73. Exteroceptive pathways in the spinal cord.

Since it seems clear that the dorsal root fibers subserving tactile sensibility ascend for
some distance in the posterior funiculus, they must be included among the myelinated fibers
of the medial division of the dorsal root, because only myelinated fibers ascend in that
funiculus. This conclusion is in keeping with the facts already mentioned concerning the
termination of myelinated fibers in the supposedly tactile end organs, such as Meissner's
corpuscles and Pacinian corpuscles. It is also in keeping with facts to be mentioned in
a following paragraph concerning the structure of the median nerve.

The Lateral Spinothalamic Tract. It seems to be well established that the
dorsal root fibers, which serve as pain conductors, terminate in the gray matter
almost at once after entering the cord, and come into synaptic relations with
neurons of the second order, whose axons run in the lateral spinothalamic tract.
From cells in the posterior column fibers arise, which in man cross to the opposite
side of the cord in the anterior white commissure and ascend in the lateral spino-
thalamic tract to end in the thalamus (Figs. 73, 231). This is a tract of ascending



FIBER TRACTS OF THE SPINAL CORD 103

fibers situated in the lateral funiculus under cover of the ventral spinocerebellar
tract. Together with the spinotectal and ventral spinocerebellar tracts it forms
the fasciculus anterolateralis superficialis (of Gowers). It mediates pain and
temperature sensations.

Conduction of Painful Afferent Impulses in the Spinal Cord. Not all of the fibers of
the lateral spinothalamic tract reach the thalamus. According to May (1906), "Some of
these fibers certainly pass directly to the thalamus, while others terminate in the inter-
mediate gray matter, and thus, by means of a series of short chains, afford secondary paths
to the same end station, which may supplement the direct path, or be made available after
interruption of the direct path." It has been shown in many cases in man and animals that,
after a complete hemisection of the spinal cord, the loss of sensibility to pain on the op-
posite side of the body below the lesion was only temporary. In time there may occur a
more or less perfect restoration of pain conduction, showing that the homolateral side of
the cord is able to supplement or replace the heterolateral path. According to the researches
of Karplus and Kreidl (1914) and Ranson and Billingsley (1916) these short chains, which are
of secondary importance in man, are much better developed in the cat. In this animal
pain conduction through the spinal cord is bilateral and is effected to a large extent through
a series of short relays.

According to Head and Thompson (1906) the path for pain in the spinal cord is the same
whether the impulses arise in the skin or in the deeper parts, such as the muscles and joints.
But Dejerine (1914) is of the opinion that painful impulses from the muscles may be trans-
mitted in the posterior funiculus and remain uncrossed as far as the medulla oblongata.

Until recently we possessed no information as to which dorsal root fibers served as pain
conductors. But in the last few years evidence has been presented which points toward the
unmyelinated fibers of the spinal nerves and dorsal roots as the pain fibers (Ranson, 1915).
Space does not permit a detailed presentation of the evidence here. It should be noted,
however, that the unmyelinated fibers of the lateral division of the dorsal root terminate in
the gray matter almost immediately after their entrance into the spinal cord, and in this
respect correspond to the known course of the fibers carrying painful impulses. The un-
myelinated fibers are chiefly distributed in the cutaneous nerves, although a few run in the
muscular branches. This coincides with the much greater sensitiveness to pain of the
skin than of the deeper tissues. Furthermore, the median nerve at the wrist, a large nerve
supplying a relatively small area of skin richly endowed with the sense of touch, contains
relatively few unmyelinated fibers. On the other hand, nerves like the lateral cutaneous
of the thigh and the medial cutaneous of the forearm, which supply relatively large cutaneous
areas of low tactile sensibility, but not inferior to the fingers in sensitiveness to pain, are com-
posed in large part of unmyelinated fibers. This difference between the composition of the
median nerve and the medial cutaneous nerve of the forearm is just what should be expected
if the touch fibers are myelinated and the pain fibers unmyelinated. Head and his co-workers
(1905, 1906, 1908) have regarded the group of sensations (protopathic) , to which according
to their classification cutaneous pain belongs, as primitive in character and the first to appear
in the phylogenetic series. It is well known that nerve-fibers in their earliest phylogenesis
are unmyelinated. If our conception is correct, a great many of the afferent fibers of mam-
mals remain in this primitive undifferentiated state and mediate a relatively primitive
form of sensation. In this connection it is interesting to note that Dejerine (1914) believes
that pain is conducted by the "sympathetic" fibers contained in the cutaneous and muscular
nerves. He does not state the evidence on which this belief is based; but if by "sympathetic"
he means to designate the unmyelinated fibers his view agrees perfectly with that presented
in the preceding paragraphs.



104



NERVOUS SYSTEM



The problem can be approached from the experimental standpoint. The seventh lum-
bar dorsal root of the cat is especially adapted for such a test. This root as it approaches
the cord breaks up into a number of filaments which spread out in a longitudinal direction
and enter the cord along the posterolateral sulcus. Within each root filament, as it ap-
proaches this sulcus, the unmyelinated separate out from among the myelinated fibers and
take up a position around the circumference of the filament and along septa that divide it
into smaller bundles. As the root enters the cord, these unmyelinated fibers turn laterally
into the dorsolateral fasciculus, constituting together with a few fine myelinated fibers the
lateral division of the root (Fig. 74). Almost all of the myelinated fibers run through the
medial division of the root into the cuneate fasciculus. A slight cut in the direction of the



Posteric



ticulu:








Fig. 74. From a section of the seventh lumbar segment of the spinal cord of the cat, showing the
unmyelinated fibers of the dorsal root entering the tract of Lissauer.



arrow, which as shown by subsequent microscopic examination divided the lateral without
injury to the medial division of the root, at once eliminated the pain reflexes obtainable
from this root in the anesthetized cat, such as struggling, acceleration of respiration, and
rise of blood-pressure. On the other hand, a long deep cut in the plane indicated by B,
Fig. 74, which severed the medial division of the root as it entered the cord, had little or no
effect on the pain reflexes. This series of experiments, the details of which are given else-
where (Ranson and Billingsley, 1916), furnishes strong evidence that painful afferent im-
pulses are carried by the unmyelinated fibers of the lateral division of the dorsal root.

These fibers probably terminate in the substantia gelatinosa Rolandi, and, if so, it is
not unlikely that intermediate neurons are intercalated between them and the neurons
whose axons run in the ventral spinothalamic tra.ct.



FIBER TRACTS OF THE SPINAL CORD 10$

The Conduction of Sensations of Pain, of Heat, and of Cold. It is well estab-
lished on the basis of clinical observations that the paths for sensations of heat
and cold follow closely those for pain. They pass through the gray matter im-
mediately after entering the cord, cross to the opposite side, and ascend in the
lateral spinothalamic tract.

According to May (1906) "it is clear that there are distinct and separate
paths for the impulses of pain, of heat, or of cold in the spinal cord, and that
these different and specific qualities of sensation may be dissociated in an affec-
tion of the spinal cord." That is, one of these forms of sensibility may be lost,
although the other two are retained. "But as these paths are anatomically
very closely associated from origin to termination these three forms of sensa-
tion are usually affected to a like degree."

From what has been said above it will be apparent that the paths, mediating
pain and temperature sensibility, cross promptly to the opposite side of the
cord and ascend in the lateral spinothalamic tract. The path for touch crosses
more gradually, but finally comes to lie in the ventral spinothalamic tract of
the opposite side; while the sensory impulses from the muscles, joints, and
tendons, as well as some elements of tactile sensibility, are carried upward on
the same side of the cord by the long ascending branches of the dorsal root fibers,
which terminate in the nuclei of the funiculus gracilis and the funiculus cuneatus.
The connections established within the brain by the fibers of these various paths
cannot profitably be discussed at this point, but will be considered in Chapter XIX.

Other afferent paths besides those already mentioned exist in the spinal
cord. These include the spino-olivary and spinotectal tracts (Fig. 78). The
former consists of fibers which arise from cells in the posterior gray column,
cross to the opposite side of the cord, and ascend in the ventral funiculus, to
end in the inferior olivary nucleus of the medulla oblongata. The spinotectal
tract consists of fibers which arise from cells in the posterior gray column and
which, after crossing, ascend in the late r il funiculus in company with those of
the lateral spinothalamic path to end in the roof (tectum) of the mesencephalon,
i. e., in the corpora quadrigemina.

ASCENDING AND DESCENDING DEGENERATION OF THE SPINAL CORD

When as a result of an injury a nerve-fiber is divided, that part which is
severed from its cell of origin degenerates, while the part still connected with
that cell usually remains intact. This is known as Wallerian degeneration, and,
as will be readily understood, gives valuable information concerning the course



io6



THE NERVOUS SYSTEM



of the fiber tracts. In case of a complete transection of the spinal cord all the
ascending fibers whose cells are located below the cut will degenerate in the
segments above; while those descending fibers whose cells of origin are located
above will degenerate below the lesion (Fig. 75). Injury to the dorsal roots
proximal to the spinal ganglia causes a degeneration of the dorsal root fibers

Dorsal spinocerebellar tract
f ./ Corticospinal tract

'Ascending branches of dorsal root fibers




Fasciculus proprius

Descending branch of dorsal root fiber



Fig. 75. Diagram of the spinal cord to illustrate the principle of Wallerian degeneration.
The broken lines represent the degeneration resulting from 1, section of the ventral root; 2,
section of the spinal nerve distal to the spinal ganglion ; 3, section of the dorsal root proximal to
the spinal ganglion, and 4, a lesion in the lateral funiculus.

throughout their length in the spinal cord. Brain injuries may, according to
their location, result in the degeneration of one or more of the tracts which
descend into the spinal cord from above.

By the study of a great many cases of injury to the central nervous system
in man and of experimentally produced lesions in animals a very considerable



FIBER TRACTS OF THE SPINAL CORD



I0 7



amount of information has been obtained concerning the fiber tracts of the
spinal cord (Collier and Buzzard, 1901, 1903; Stewart, 1901; Thiele and Horsley,
1901; Batten and Holmes, 1913). This is summarized in the accompanying table
and in Fig. 78.

TABLE SHOWING THE LOCATION OF THE CHIEF FIBER TRACTS OF THE SPINAL CORD AND THE
DIRECTION IN WHICH THEY DEGENERATE





Ascending degeneration.


Descending degeneration.


Anterior funiculus


Ventral spinothalamic tract


Ventral corticospinal tract, w
Vestibulospinal tract, o. C
Tectospinaltaict i^A ^


Lateral funiculus


Dorsal spinocerebellar tract,
Ventral spinocerebellar tract,
Lateral spinothalamic tract,
Spinotectal tract


Lateral corticospinal tract, ^
Rubrospinal tract, ^
Bulbospinal tract,
Tectospinal tract 1 A L.4


Posterior funiculus


Ascending branches of the
dorsal root fibers


Fasciculus interfascicularis,
Septomarginal tract

^lX*LiMi^Lw.T



The fasciculi proprii or ground bundles are composed of short ascending
and descending fibers, which arise and terminate within the gray matter of the
spinal cord and link together the various segments of the cord. These fascicles,
one of which is present in each of the three funiculi, immediately surround
the gray columns. After a transection of the spinal cord the fasciculi proprii
undergo an incomplete degeneration for some distance both above and below
the lesion (Figs. 75, 76). In cross-section the ground bundle of the posterior
funiculus has the form of a narrow band upon the surface of the posterior column
and posterior commissure, and was once called the cornu-commissural bundle
(Fig. 78). In addition to this fascicle there are in the posterior funiculus two
other tracts which in part belong to the same system the septomarginal tract
and the fasciculus interfascicularis, or comma tract of Schultze. These are
both composed of descending fibers, in part of intraspinal origin and in part
representing the descending branches of the dorsal root fibers. The septomar-
ginal tract is situated along the dorsal periphery of the posterior funiculus in
the thoracic region; it takes up a position along the septum in the lumbar segments
(oval area of Flechsig) ; and in the sacral region it forms a triangular field at the
dorsomedial angle of the posterior funiculus (triangle of Gombault and Philippe)
(Fig. 76). The fasciculus interfascicularis is best developed in the thoracic
segments, where it occupies a position near the center of the posterior funiculus.



io8



THE NERVOUS SYSTEM



In the anterior funiculus, in addition to the fasciculus proprius which imme-
diately surrounds the gray matter, there is a thin layer of similar fibers spread
out along the border of the anterior fissure and known as the sulcomarginal
fasciculus. This tract also contains the fibers which descend into the cord from
the medial longitudinal bundle of the medulla oblongata.

As a general rule the short fibers of the fasciculus proprius lie nearer the
gray substance than the fibers of greater length; and the long tracts, which



Fasciculus gracilis..^



Spinocerebellar, spinotectal, and lateral
spinothalamic tracts



Fasciculus inter fasciculari.



Septomarginal fasciculus -.
Lateral corticospinal tract



Septomarginal fasciculus, oval area of Flechsig ..
Lateral corticospinal tract*.




Cervical enlargement
ascending degeneration



Upper thoracic
ascending degeneration



Middle thoracic
site of compression



Lower thoracic

descending degeneration



Upper lumbar
descending degeneration



Lower lumbar

descending degeneration



Fig. 76. Ascending and descending degeneration resulting from a compression of the thoracic
spinal cord in man. Marchi method. (Hoche.)

connect the spinal cord with the brain, occupy the most peripheral position.
But the fact must not be overlooked that many fibers of the fasciculus proprius
are intermingled with those of the long tracts.




LONG DESCENDING TRACTS OF THE SPINAL CORD

Fibers which arise from cells in various parts of the brain descend into the
spinal cord, where they form several well-defined tracts. The most important



FIBER TRACTS OF THE SPINAL CORD



I0 9



and most conspicuous of these are the cerebrospinal fasciculi, which are more
properly called the cor tico spinal tracts. There are two in each lateral half of
the cord, the lateral and the ventral corticospinal tracts. Their constituent
fibers take origin from the large pyramidal cells of the precentral gyrus or motor
region of the cerebral cortex and pass through the subjacent levels of the brain
to reach the spinal cord (Fig. 77). Just before they enter the spinal cord they
undergo an incomplete decussation in the medulla oblongata, giving rise to a
ventral and a lateral corticospinal tract.

The Lateral Corticospinal Tract (Crossed Pyramidal Tract, Fasciculus
Cerebrospinalis Lateralis). The majority of the pyramidal fibers, after cross-
ing the median plane in the decussation of the pyramids, enter the lateral fu-



Cerebral hemisphere




Spinal
cord



Fig. 77. Diagram of the corticospinal tracts.

niculus of the spinal cord as the lateral corticospinal tract, which occupies a posi-
tion between the dorsal spinocerebellar tract and the lateral fasciculus proprius
(Fig. 78). In the lumbar and sacral regions, below the origin of the dorsal
spinocerebellar tract, the lateral corticospinal tract is more superficial. It can
be traced as a distinct strand as far as the fourth sacral segment; and as it
descends in the spinal cord it gradually decreases in size. Throughout its
course in the spinal cord it gives off collateral and terminal fibers which end in
the gray matter.

The ventral corticospinal tract (fasciculus cerebrospinalis anterior or direct
pyramidal tract) is formed by the smaller part of the corticospinal fibers, which
do not cross in the medulla, but pass directly into the ventral funiculus of the



THE NERVOUS SYSTEM



same side of the cord. They form a tract of small size, which lies near the
anterior median fissure and which can be traced as a distinct strand as far as the
middle of the thoracic region of the spinal cord. Just before terminating these
fibers cross in the anterior white commissure. They end like those of the lateral
corticospinal tract, either directly or perhaps through an intercalated neuron,
in relation to the motor cells in the anterior column. The crossing of these
fibers is only delayed, and it will be apparent that all of the corticospinal fibers
arising in the right cerebral hemisphere terminate in the anterior column of the
left side of the cord, and conversely, those from the left hemisphere end on the
right side. It is along these fibers that impulses from the motor portion of the
cerebral cortex reach the cord and bring the spinal motor apparatus under
voluntary control.



Fasciculus septomarginalis

Fasciculus inlerfascicularis N

Fasciculus proprius ,^
Sensory fibers of the
second order '
Lateral corticospinal _
tract



Rubrospinal tract-
Tectospinal tract -
Fasciculus proprius-

Bulbospinal tract
Vestibulospinal tract




Fasciculus gracilis

-Fasciculus cuneatus

- - Dorsolateral fasciculus

_ Dorsal spinocerebellar
tract

"Fasciculus proprius

...Ventral spinocere-
bellar tract
..Lateral spinottialamic

tract
Spinotectal tract

- Ventral root

* Ventral spinothalamic tract
" Sulcomarginal fasciculus



Ventral corticospinal tract

Fig. 78. Diagram showing the location of the principal fiber tracts in the spinal cord of man.
Ascending tracts on the right side, descending tracts on the left.

It is stated by some authors, although on the basis of rather unsatisfactory evidence,
that the fibers of the lateral corticospinal tract ramify in the formatio reticularis (Mona-
kow, 1895) and the nucleus dorsalis (Schafer, 1899). The corticospinal path is from the
standpoint of phylogenesis a relatively new system and varies a great deal in different
mammals. It is found in the ventral funiculus in the mole, while in the rat it occupies the
posterior funiculus. In the mole it is almost completely unmyelinated, in the rat largely so.
It contains many unmyelinated fibers in the cat, fewer in the monkey (Linowiecki, 1914).
In man it does not become fully myelinated before the second year. An uncrossed ventral
corticospinal tract seems to be present only in man and the anthropoid apes, and this tract
varies greatly in size in different individuals.

The rubrospinal tract (tract of Monakow) is situated near the center of the
lateral funiculus just ventral to the lateral corticospinal tract (Fig. 78). Its
fibers come from the red nucleus of the mesencephalon, cross the median plane,



FIBER TRACTS OF THE SPINAL CORD



and descend into the spinal cord, within which some of them can be traced to
the sacral region. Their collateral and terminal branches end within the an-
terior column in relation to the primary motor neurons.

Other Descending Tracts. The bulbospinal tract (olivospinal tract, tract of
Helweg) is a small bundle of fibers found in the cervical region near the surface
of the lateral funiculus opposite the anterior column. The fibers arise from
cells in the medulla oblongata, possibly in the inferior olivary nucleus, and end
somewhere in the gray matter of the spinal cord. The exact origin and ter-



Fasciculus cuneatus
\



Fasciculus gracilis



Lateral corticospinal tract -

Fasciculi proprii
Ventral corticospinal tract



Dorsal spinocerebellar tract




Oval area of Flechsig



III



L. IV



Fig. 80.

Figs. 79 and 80. Diagrams of the sixth cervical, third thoracic, and fourth lumbar segments