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Funiculi

In order that the various fasciculi may be referred to with greater ease, the white substance of the spinal cord in section is divided into three areas known as funiculi or columns and which correspond to the funiculi already mentioned as evident upon the surface of the cord when intact. The funiculi are outlined wholly upon the basis of their position in the cord and with reference to the median line and the contour of the column of grey substance; their component fascicul are defined upon the basis of function. (1) The posterior funiculus or column is bounded by the posterior median septum and the line of the dorsal horn; (2) the lateral funiculus or column is bounded by the lateral concavity of the grey column and the lines of entrance and exit of the dorsal and ventral roots; (3) the ventral funiculus or column is bounded by the Hne of exit of the ventral roots, and by the anterior median fissure.

The posterior funiculus or column [funiculus posterior].

This funiculus is composed of two general varieties of axons arranged in five fasciculi. First, and constituting the predominant type in all the higher segments of the cord, are the afi'erent or general sensory axons, which arise in the spinal ganglia, enter the cord by the dorsal roots, assume their distribution to the neurons of the cord, and then take their ascending course toward the encephalon. The axon of the spinal ganglion neuron undergoes a T-shaped division a short distance from the cell- body, one limb of this division terminating in the peripheral organs and the other going to form the dorsal root. Upon entering the cord the dorsal root axons undergo a Y-shaped bifurcation in the neighborhood of the dorsal horn, one branch ascending and the other descending. Their ascending branches form the fasciculus gracilis (Goll's column) and the fasciculus cuneatus (Burdach's column). These fasciculi are the chief ascending or sensory spino-cerebral connections, the direct sensory path to the brain. The neurons represented in them constitute the first link in the neuron chain between the periphery of the body- and the cerebral cortex.

In threading their way toward the brain, these sensory axons tend to work toward the midline. Therefore those of longer course are to be found nearer the posterior septum, in the upper segments of the cord, than those axons which enter the cord by the dorsal roots of the upper segments. Thus it is that the fasciculus gracilis, the medial of the two fasciculi, contains the axons which arise in the spinal ganglia of the sacral and lumbar segments. In other words, it is the fasciculus bearing sensory impulses from the lower limbs to the brain, while the fasciculus cuneatus, the lateral of the two, is the corresponding pathway for the higher levels. Naturally, there is no fasciculus cuneatus as such in the lower segments of the spinal cord. The axons being much blended at first, it is only in the upper thoracic and cervical region that there is any anatomical demarcation between the two fasciculi. In this region the two become so distinct that there is in some cases an apparent connective-tissue septum between them, continuing inward from the postero-intermediate sulcus - the surface indication of the line of their junction.

Upon reaching the medulla oblongata the fibers of the fasciculus gracilis and the fasciculus cuneatus terminate about cells grouped to form the nuclei of these fasciculi. The nucleus of the fasciculus gracilis is situated medially and begins just below the point at which the central canal opens into the fourth ventricle; the nucleus of the fasciculus cuneatus is placed laterally and ex- tends somewhat higher than the other nucleus. The neurons whose cell-bodies compose these nuclei constitute the second links in the neuron chains conveying sensory impulses from the periphery to the cerebral cortex.

The descending or caudal branches of the dorsal root axons are concerned wholly with the neurons of the spinal cord. They descend varying distances, some of them as much as four segments of the cord, and give off numerous collaterals on their way to the cells of the grey column. Those terminating about cell-bodies of the ventral horn which give rise to the ventral or motor root-fibers, are responsible for certain of the so-called 'reflex activities' and thus contribute to the simplest of the reflex arcs. In descending they serve to associate different levels of the grey substance of the cord with impulses entering by way of a single dorsal root. Some of their collaterals cross the mid-line in the posterior white commissure, and thus become connected with neurons of the opposite side. The caudal branches of longer course are scattered throughout the ventral portion of the fasciculus cuneatus (middle root zone), and the longest show a tendency to collect along the border-line between the fasciculus cuneatus and the fasciculus gracilis, and thus contribute largely to the comma-shaped fasciculus. Also some of the longest of them in the lower levels course in the oval bundle or septo-marginal root zone.

The ascending branches of the dorsal root axons also give off collaterals to the grey sub- stance of the cord, thus extending the area of distribution of a given dorsal nerve-root to levels of the cord above the region at which the root enters.

The greater number of the terminations of dorsal root axons within the spinal cord are concerned first with neurons other than those contributing ventral root-fibers. The greater mass of the neurons concerned are those of the Golgi type II and those contributing the fasciculi proprii or ground bundles of the spinal cord, or the second variety of axons composing the posterior funiculus. The latter fasciculi arise from the smaller cells of the grey column.

These axons pass from the grey substance to enter the surrounding white substance, bifurcate into ascending and descending branches, which in their turn give off numerous collaterals to the cells of the grey substance of the levels through which they pass. The cell-bodies giving origin to such axons are so numerous that the entire column of grey substance is surrounded by a continuous felt-work of axons of this variety.

The dorsal fasciculus proprius (anterior root zone of posterior column) arises chiefly from cells situated in the dorsal horn (stratum zonale). Coincident with the ingrowth and arrangement of the fasciculi gracilis and cuneatus many fibers of the dorsal fasciculus proprius go to form both the oval bundle and the comma-shaped fasciculus. Thus these two bundles are mixed, being fasciculi proprii which contain caudal branches of dorsal root axons. The association fibers in the oval bundle are the longest of any belonging to the dorsal fasciculus proprius. The cephalic and caudal branches combined of some are said to extend more than half the length of the cord and it has been claimed that some even associate the cervical region with the conus medullaris. Based upon this claim, Obersteiner has called the oval bundle, the "dorso-medial sacral field" and Edinger has referred to the most dorsal part of it as the "tractus cervico-lumbalis dorsalis." The 'median triangle' is formed by the continuation of the dorsal fasciculi proprii with the oval or septo-marginal fasciculus. Some of the axons of the dorsal fasciculus proprius cross the midline to distribute impulses to the neurons of the opposite side. These commissural axons, together with certain collaterals of the dorsal root axons, which cross the mid-line out- side the dorsal white commissure, compose the so-called cornu-commissural tract at the base of the posterior septum.

The lateral funiculus or column [funiculus lateralis]

Not all the axons of the posterior or dorsal nerve-roots extend to the encephalon. Estimation shows that the sum of all the dorsal roots is greatly in excess of the sum contained in the fasciculi cuneatus and gracilis just before these enter their nuclei of termination. Therefore many of the ascending dorsal root axons are concerned with spinal- cord relations wholly.

The marginal zone of Lissauer, situated along the lateral margin of the postero-lateral sulcus is composed largely of dorsal root axons. Many of these finally work across the line of the sulcus into the posterior funiculus. Many of the dorsal root-fibers which do not reach the brain occur in Lissauer's zone. Many others of course occur throughout the posterior column. Lissauer's zone also contains some fibers arising from the small cells of the dorsal horn, and to this extent corresponds to a fasciculus proprius. Ranson has found that large numbers of the non-medullated dorsal root axons which enter the cord are contributed to Lissauer's zone.

The lateral fasciculus proprius (lateral ground bundle, lateral limiting layer) is situated in the lateral concavity of the grey column and is continuous with the other fasciculi proprii both dorsal and ventral. Beyond that it probably contains fewer commissural axons, it is of the same general significance as the others. It is frequently divided into small bundles by the reticular formation. The lateral cerebro-spinal fasciculus (crossed pyramidal tract). In contrast to the sensory fibers passing through the spinal cord conveying impulses destined to reach the cerebral cortex, axons are given off from the pyramidal cells of the cortex, which descend to terminate about the cells of the grey substance of the spinal cord, chiefly the cells which give origin to the ventral root-fibers.

Upon reaching the medulla oblongata in their descent, these axons are accumulated into two well-defined, ventrally placed bundles, the pyramids, one from each cerebral hemisphere. In passing through the brain stem the pyramids contribute many fibers which cross the mid-line to terminate in the motor nuclei of the cranial nerves of the opposite side, and thus decrease appreciably in bulk. According to the estimate of Thompson, only about 160,000 of the pyramidal fibers are destined to enter the spinal cord.

Upon reaching the lower part of the medulla, the greater mass of the fibers of each pyramid, which are destined to enter the cord, suddenly cross the mid-line in the 'decussation of the pyramids.' The remainder retain their ventral position in their descent decussating gradually in the cord itself. The pyramidal fibers which cross in the medulla course in the lateral column ventral to Lissauer's zone, and lateral to the lateral fasciculus proprius, and form the lateral cerebrospinal fasciculus (crossed pyramidal tract). It is a large fasciculus, oval shaped in transection, and since its axons terminate in the grey column of the cord all along its length, it decreases in bulk as the cord is descended.

In addition to the three dispositions of the dorsal root axons given above, certain of them, either by collaterals or terminal twigs, form telodendria about the cells of the dorsal nucleus (Clarke's column), which nucleus extends from about the seventh cervical to the third lumbar segment of the cord. The axons given off by these cells pass to the dorso-lateral periphery of the lateral funiculus, and there collect to form the dorsal spino-cerebellar fasciculus (direct cerebellar tract of Flechsig). As such they ascend without interruption, and in the upper level of the medulla oblongata pass into the cerebellum by way of the inferior cerebellar peduncle or restiform body. Necessarily, this fasciculus is not evident in levels below the extent of the nucleus dorsalis.

Also situated superficially in the lateral funiculus is another ascending con- duction path, and, like the dorsal spino-cerebellar fasciculus, to which it is adjacent, it is also in part at least a cerebellar connection. Its position suggests its name, superficial ventro-lateral spino-cerebellar fasciculus (Gowers' tract).

This tract at 'present' does not include as great an area in transverse section as when originally described. The more internal portion of the original Gowers' tract is now given a separate significance, and will be considered separately. While the exact location in the grey column of all the cell-bodies giving origin to the superficial ventro-lateral spino-cerebellar fasciculus is un-certain, it is known that certain ventral horn cells contribute their axons to it. Many of its cells of origin are scattered in the area immediately ventral to the nucleus dorsalis, others in the intermediate and mesial portion of the lateral group of ventral horn cells. In the lumbar region these cells are quite numerous, and, therefore, the fasciculus begins at a lower level in the spinal cord than does the direct cerebellar tract. In degenerations it becomes visible in the upper segments of the lumbar region, and has been proved to increase notably in volume as the cord is ascended. Its axons arise for the most part directly from cell-bodies of the same side of the cord, though it has been shown by several investigators that many of its axons come from the grey substance of the opposite side by way of the ventral white commissure. Terminal twigs and collaterals of the dorsal root-fibers, mostly of the same side, but occasionally from the opposite side, terminate about its cells of origin. At one time Gowers' tract was considered an entity, but now, even in the more Umited area it occupies, it must be considered a mixture of axons of several terminal destinations or distinct neuron systems. The destination of some of its axons has not been determined with certainty. A portion, the spino-cerebellar fasciculus proper, go to the cerebellum, and there have been traced to the cortex of the superior vermis. Most of these reach the cerebellum not by way of the restiform body, as does the dorsal spino-cerebellar tract, but pass on in the brain-stem to the level of the inferior corpora quadrigemina, and there turn back to join the brachium conjunctivum or superior cerebellar peduncle. (Auerbach, Mott, Hoche.) Only a few of its axons leave the fasciculus lower down in the medulla, to enter the cerebellum by way of the restiform body, in company with the dorsal spino-cerebellar tract. (Rossolimo, Tschermak.) Another portion of its axons are thought to reach the cerebrum, probably the nucleus lentiformis, though it has not been positively traced further than the superior corpora quadrigemina. Many axons in Gowers' tract of the cord correspond to those of the fasciculi proprii, and merely run varying distances in the cord, to turn again into its grey substance. Schaeffer followed some of these from the lumbar region up to the level of the second cervical nerve.

In the ventro-medial border of Gowers' tract and immediately upon the periphery, near the antero-lateral sulcus (exit of ventral nerve-roots), there is found in the higher segments of the cord a small oval bundle, the spino-olivary fasciculus or Helweg's (Bechterew's) bundle. The functional direction of its fibers has not been settled.

It is asserted to arise from cell-bodies of the ohve in the medulla oblongata, and in the cord is believed to be associated with the cells of the ventral column of grey substance, probably those of the lateral; horn. More recent claims assert that it arises from cell bodies in the cord and thus is spino-olivary. By some observers it has been traced as far down as the mid-thoracic region; by others, however, only as far as the third cervical segment. The olives being nuclei largely concerned with cerebellar connections, Helweg's fasciculus is probably an indirect cerebellar association with the spinal cord neurons. It is composed of axons of relatively very small diameter, and is one of the last fasciculi of the spinal cord to become medullated.

Situated between the superficial ventro-lateral spino-cerebellar fasciculus and the lateral fasciculus proprius is an area which, in transverse sections, may be, by position, referred to collectively as the intermediate fasciculus. So intermingled are the axons comprising it that it has been called the mixed lateral zone. It contains fibers of at least five functional varieties:

(1) Fibers belonging to the lateral fasciculus proprius which are of longer extent gradually course farther away from the grey substance of the cord and such mix into the intermediate fasciculus.

(2) It is said to contain fibers descending from the cerebellum to associate with the neurons of spinal cord, probably directly with the ventral root or motor neurons.

(3) The rubro-spinal fasciculus. - This arises from cell-bodies in the red nucleus of the tegmentum (in the mesencephalon) and is a crossed fasciculus. Axons arising from the red nucleus of one side cross the mid-Une while yet in the mesencephalon and descend in the lateral funiculus of the cord to terminate gradually about cell-bodies of the ventral horn, both those which give rise of ventral root fibers and those which contribute to the fasciculi proprii. Its fibers are more thickly bundled in a crescentic area fitting onto the ventral side of the lateral cerebro-spinal fasciculus, and some are said to mix into the area of this latter.

(4) The vestibulo-spinal fasciculus. - This is sometimes called the lateral vestibulo-spinal fasciculus from the fact that there is a tract of similar significance in the ventral funiculus of the cord. It arises from some of the cell-bodies comprising Deiter's nucleus, the lateral nucleus of termination of the vestibular nerve, and from some of those of the spinal nucleus (nucleus of the descending root) of this nerve, all of which is in the medulla. It descends the cord, un- crossed, to terminate gradually about ventral horn cells, thus comprising a part of the apparatus for the equilibration of the body. . Its fibers are thought to be more closely collected in the area immediately ventral to the rubro-spinal fasciculus, but of course commingle with the latter.

(5) The corpora-quadrigemina-thalamus path. The most lateral portion of the intermediate fasciculus, a small area once included in Gower's tract, contains fibers both ascending and descending, connecting the spinal cord with the thalamus (diencephalon) and the quadrigeminate bodies of the mesencephalon. These are crossed paths. The ascending fibers arise from cell-bodies in the ventral horn of one side, cross in the ventral white commissure (commissural neurons) and course upward in the intermediate fasciculus to their termination in the opposite side. Those terminating about cell-bodies in the thalamus form what is known as the spino- thalamic tract, while those terminating in the nuclei of the quadrigeminate bodies are called the spino-mesencephalic or spino-tectal tract (Iradus spino-tectalis) . It is not known in which region of the cord most of these fibers arise but it is quite probably the cervical region. The fibers which arise from cell-bodies of the thalamus and nuclei of the quadrigeminate bodies cross the mid-line in the mesencephalon and descend the cord to terminate gradually about cell-bodies in the ventral horn of the opposite side. Those from the thalamus are known as the thalamo-spinal tract and those from the quadrigemina, as the mesencephalo- or tecto-spinal tract. The latter is thought to be the larger.

By the fibers of the above tracts general sensory impulses from the body (skin, etc.) are carried to the central portion of the optic apparatus, and the descending fibers give a simple anatomical possibility for the movements of the body in response to visual and auditory im- pulses. The descending fibers are thought to terminate chiefly in contact with association neurons of the fasciculi proprii, these transferring the impulses to the neurons giving origin to the ventral or motor root fibers, but some are thought to terminate directly about the cell- bodies of ventral-root neurons. A portion of the intermediate fasciculus, most adjacent to Gower's tract, has been designated as Loewenthal's tract.

The anterior funiculus or column [funiculus anterior]

The intermediate fasciculus is continued ventrally and medially across the line of exit of the ventral root axons, and thus into the anterior funiculus. This portion is also mixed, but its axons of long course associate somewhat different portions of the nerve axis from those connected by the more lateral portion.

According to the studies of Flechsig, von Bechterew, and Held, this mesial portion contains fibers, both ascending and descending, which associate the various levels of the grey substance of the spinal cord with the neurons in the reticular formation of the medulla oblongata. The levels to which they have been traced comprise the olivary nuclei, which are largely concerned in cerebellar connections, and the nuclei of the vagus, glosso-pharyngeal, auditory, facial and the spinal tract of the trigeminus. Also some of the ascending fibers are probably associated with the nuclei of the eye-moving nerves. This portion of the intermediate fasciculus also grades into and is mixed with the axons of the ventral fasciculus proprius, as is its lateral portion with the lateral fasciculus proprius. In other words, the fasciculi proprii proper, the axons nearest the grey substance, serve for the intersegmental association of the different levels of the grey substance of the cord, while the intermediate fasciculus contains axons of longer course which serve to associate more distant levels of the grey substance of the nerve axis - that of the spinal cord with its upward continuation into the medulla oblongata, pons and mesencephalon.

The anterior marginal fasciculus, ventral vestibulo-spinal tract (Loewenthal's tract)

It forms the superficial boundary of the mesial portion of the intermediate fasciculus. It is a narrow band, parallel with the surface of the cord, and extends medially from the medial extremity of Gowers' tract (from Helweg's bundle) to the beginning of the anterior median fissure.

The axons belonging to it proper are descending from the recipient nuclei of the vestibular nerve. Of these nuclei it has been held by some investigators that only Deiters' nucleus (the lateral nucleus of termination in the upper extremity of the medulla oblongata) gives origin to the axons of the anterior marginal fasciculus. Others agree with Tschermak that the superior and more laterally situated Bechterew's nucleus of the vestibular nerve also contributes axons to it, and quite probably the nucleus of the spinal root of the vestibular adds further axons. Still other investigations have shown that a part at least of the fasciculus comes from the nucleus fastigius (roof nucleus) of the cerebellum. Since many axons from both Deiters' and Beehterew's nucleus terminate in the nucleus fastigius, the ventral vestibulo-spinal fasciculus is, in any case, a conduction path from the nerve connections for equilibration to the grey sub- stance of the spinal cord. The fasciculus is said to extend as far as the sacral region of the cord, its axons terminating about the cells of the ventral horns. The term "ventral" is added to its name to distinguish it from the vestibulo-spinal tract described above as coursing in the lateral funiculus. It is considered an uncrossed pathway.

The ventral cerebro-spinal fasciculus (anterior or direct pyramidal tract), as stated above, is the uncrossed portion of the descending cerebro-spinal system of neurons. It is a small, oblong bundle, situated medially in the anterior funiculus, parallel with the anterior median fissure. Like the lateral cerebro-spinal fasciculus (crossed pyramidal tract), its axons arise from the large pyramidal cells of the motor area of the cerebral cortex, and transmit their impulses to the neurons of the ventral horns of the grey substance of the spinal cord, and almost entirely to those neurons which give origin to the ventral or motor root fibers.

It represents merely a delayed decussation of the pyramidal fibers, for instead of crossing to the opposite side in the lower portion of the medulla oblongata, as do the fibers of the lateral fasciculus, its fibers decussate all along its course, crossing in the ventral white commissure and in the commissural bundle of the cord to terminate about the ventral horn cells of the opposite side. Hoohe, employing Marchi's method, found that a few of its fibers terminate in the ventral horn of the same side. This conforms to the pathological and experimental evidence that there are homolateral or uncrossed fibers in the crossed pyramidal tracts also. Like the crossed tract, the ventral pyramidal tract diminishes rapidly in volume as it descends the cord. Its loss is greatest in the cervical enlargement, and it is entirely exhausted in the thoracic cord. With the exception of the anthropoid apes and certain monkeys, none of the mammalia below man, which have been investigated, possess this ventral pyramidal tract

Lying between the ventral cerebro-spinal fasciculus and the pia mater of the anterior median fissure is a thin tract of descending axons continuous ventrally with the anterior marginal fasciculus. From its position it is known as the sulco-marginal fasciculus; functionally it is the ventral mesencephalo-spinal (tecto- spinal) tract.

The extent of its course in the spinal cord is uncertain. It arises from the cells of the grey substance of the superior pair of the quadrigeminate bodies, and there, in largest part at least, it crosses the mid-line, and in the so-called 'optic acoustic reflex path' descends through the medulla oblongata into the spinal cord of the opposite side. The superior quadrigeminate bodies having to do with sight, this tract forms a second path conveying visual impulses to the neurons of the spinal cord.

The commissural bundle

It is situated about the floor of the anterior median fissure, and is the most dorsal tract of the anterior funiculus. It contains decussating or commissural axons of three varieties.

(1) It contains the decussating axons of the ventral cerebro-spinal fasciculus throughout the extent of that fasciculus;

(2) it is chiefly composed of the axons of the ventral fasciculus proprius which arise in the grey substance (ventral horn) of one side, cross the mid-line as com- missural fibers, and course both upward and downward to be distributed to the neurons of different levels of the grey substance of the opposite side;

(3) it contains decussating axons which arise from cell-bodies in the grey substance of one side and cross the mid-line to terminate about cell-bodies in practically the same level of the opposite side. The latter are merely axons belonging to the ventral white commissure which course without the confines of the grey figure. The commissural bundle is present throughout the length of the spinal cord, and is largest in the enlargements, i. e., where the association and commissural neurons occur in greater number generally. In its two last-mentioned varieties of axons it corresponds to the commissural portion of the dorsal fasciculus proprius (the cornu-commissural bundle).

The ventral fasciculus proprius is but a continuation of the lateral fasciculus proprius, and is composed of ascending and descending association fibers of the same general significance.

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