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and, lastly, the fibres of the cervical roots displace the thoracic fibres. The difference between the fasciculus gracilis and the fasciculus cuneatus consists simply in this, that the former is composed of the fibres of posterior nerve-roots which have entered the medulla at a lower level than those which enter into the formation of the fasciculus cuneatus. The fibres of the fasciculus gracilis, taking them as a whole, must therefore necessarily run a very much longer course.
Our knowledge of the constitution of the posterior columns of the spinal medulla is derived largely from the study of the course of degeneration in monkeys, after the medulla has been cut across-either partially or completely. But we have also a direct knowledge of the lamination of the posterior columns of the human spinal medulla (Fig. 473) that has been acquired from the examination of cases in which the medulla or its nerve-roots had been injured during life.
Numerous collateral fibrils stream into the gray matter of the posterior column both from the ascending and descending branches of the entering fibres of the posterior nerve-roots. These are classified into long and short collaterals. The long
FIG. 473.-A DIAGRAM TO ILLUSTRATE THE GROUPING OF THE VARIOUS FASCICULI IN THE SPINAL
collaterals extend forwards into the anterior column of gray matter and end in relation to the ventral nerve-cells. The short collaterals end in relation to the nerve-cells in the substantia gelatinosa, and other nerve-cells of the posterior column of gray matter.
The majority of the fibres of the posterior nerve-root enter the spinal medulla on the medial side of the apex of the posterior column of gray matter. The manner in which these are related to the fasciculus cuneatus and the fasciculus gracilis has been noticed; but a certain number of those fibres which lie most laterally take a curved course forwards on the medial side of the posterior column of gray matter and then pass into it. In the thoracic region these curved fibres end in connexion with the cells of the nucleus dorsalis (Fig. 467, B, p. 525, and Fig. 473).
Fasciculus Posterolateralis (O.T. Tract of Lissauer). The postero-lateral fasciculus is a small tract of nerve-fibres of minute calibre which assume their medullary sheaths at a comparatively late period. It is placed at the surface of the medulla close to the sulcus lateralis posterior. It is formed by some of the lateral fibres of the posterior nerve-roots which do not enter the fasciculus cuneatus, but pass upwards in the medulla close to the substantia gelatinosa, in which they ultimately end.
It must now be evident that the fibres which enter the medulla spinalis through each
posterior nerve-root have three main modes of distribution: (1) the majority take part in the formation of the fasciculus cuneatus, and pass upwards or downwards to end in the gray matter at some other level in the central nervous system; (2) some fibres, and many collaterals of fibres in the fasciculus cuneatus, lie close to the posterior column and describe a series of graceful curves as they pass forwards, prior to turning laterally into all regions of the gray matter to end at the same level as they enter the medulla spinalis ; (3) a third series form the postero-lateral fasciculus and end in connexion with the cells of the substantia gelatinosa and other cells in the posterior and anterior columns of gray matter (Fig. 473).
The fibres derived from the posterior nerve-roots which ascend in the posterior funiculi of the medulla spinalis to the medulla oblongata of the brain constitute a direct sensory tract; other fibres are described which give rise to a crossed sensory tract termed the fasciculus spinothalamicus. These latter fibres arise as the axons of certain of the cells in the posterior column in connexion with which fibres from the posterior nerveroots have ended, and crossing to the opposite side of the medulla spinalis through the anterior commissure they ascend in the antero-lateral funiculus to the brain, where they ultimately reach the thalamus. As the spino-thalamic tract ascends in the spinal medulla its fibres are not gathered into a compact strand, but are more or less loosely scattered in the lateral funiculus.
Association Fibres in the Posterior Funiculus.—But the whole of the fibres of the posterior funiculus are not derived from the posterior nerve-roots. A few fibres exist in this funiculus which have a different origin. They are derived from certain of the cells of the gray matter, and, entering the posterior funiculus, they divide into ascending and descending branches which pass upwards and downwards in the funiculus for a varying distance, before they finally turn in to end in the gray matter at higher and lower levels. These fibres, therefore, constitute links of connexion between different segments of the spinal medulla, and they constitute the fasciculus posterior proprius. Our information regarding these fibres at present is somewhat defective; but it is believed that the deepest part of the funiculus, i.e. the part next the posterior gray commissure, and the fasciculus septomarginalis of Bruce, placed in apposition with the posterior-median septum and in the adjoining part of the FIG. 474.-DIAGRAM TO SHOW THE surface, belong mainly to this category.
MANNER IN WHICH THE FIBRES OF
Funiculus Lateralis and Funiculus Anterior. -It is convenient to consider the anterior along with the lateral funiculus and to call the whole mass of white substance that is left, after eliminating the posterior funiculus, the anterolateral funiculus. In contact with the surface of the gray columns there is a broad band of white matter the parts of which are known respectively as the fasciculus proprius anterior and lateralis (O.T. the ground bundles of the antero-lateral funiculus). It is composed wholly of fibres which spring from nerve-cells in the gray columns, and, after passing for varying distances upwards or downwards, end in the gray matter of the spinal medulla. Thus they constitute an intrinsic system of fibres linking together different levels of the spinal medulla. They become medullated before any other fibres, except the root-fibres and their continuations in the posterior funiculus. When cut across some of the fibres degenerate above, others below, the injury, and the degeneration extends for varying distances upwards and downwards respectively.
The best-known long or extrinsic systems of fibres in the antero-lateral funiculus are those known as the fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract), the fasciculus cerebrospinalis anterior (O.T. direct pyramidal tract), the fasciculus cerebellospinalis (O.T. direct cerebellar tract) (which goes from the spinal medulla to the cerebellum, and ought therefore to be called spinocerebellaris, as it will be subsequently named in this account), and the fasciculus anterolateralis superficialis (O.T. Gowers' tract).
There are, however, many other fasciculi at least as important as these, but there is as yet no close agreement as to their precise limits or connexions. One reason for this is that some of the elements of one tract may become intermingled with those of another; moreover, the position and relations of certain of them
- posteri funicula
Spin thai tract
FIG. 475.-DIAGRAM REPRESENTING THE CONNEXIONS OF SOME IMPORTANT SENSORY AND MOTOR TRACTS
vary considerably at different levels of the spinal medulla. In Fig. 473 an attempt has been made to present the present state of our knowledge of these great strands of white fibres. This diagram is not intended to represent any definite level of the spinal medulla, though certain features are shown which occur only in the cervical region; and in respect of other features, the arrangement found in lower regions of the spinal medulla has been introduced to render the diagram more serviceable.
Much of the apparent complexity of this chart will disappear if the reader recalls some general statements (p. 512) made with regard to the outstanding features of the brain. It was then explained that when sensory nerves, coming from the skin and muscles, enter the spinal medulla, they not only establish relations with the motor nuclei and other spinal structures in the neighbourhood of their insertion, but also give rise, directly or indirectly (see Fig. 475) to many
fasciculi which pass upwards in the spinal medulla to reach the medulla oblongata, the pons and cerebellum, the mesencephalon (corpora quadrigemina), I the thalamus, and the cerebral hemisphere. In the neighbourhood of each level where these ascending sensory tracts end, such as for example the region of the vestibular nucleus and cerebellum, the tectum mesencephali, the corpus striatum, and the cerebral hemisphere, great descending tracts originate and pass downwards in the spinal medulla (Fig. 475-the red lines). Thus we have cerebro-spinal, rubro-spinal, tecto-spinal, vestibulo-spinal, and bulbo-spinal fasciculi passing down the spinal medulla; and each system eventually ends around the series of motor nuclei (Fig. 475), many of them in the spinal medulla.
In the anterolateral funiculus the various fasciculi will be found to be grouped roughly into three bands:-Next to the gray columns is the fasciculus proprius; then comes a band of descending (motor) fasciculi; and then, upon the surface, a series of ascending (sensory) fasciculi. This arrangement, however, is not maintained with any degree of exactitude in the anterior funiculus, where the sharp demarcation between ascending and descending fasciculi is in great part destroyed by the intermingling of fibres passing in opposite directions.
The fibres of the posterior nerve-root have already been studied so far as their relation to the posterior funiculus is concerned. No clear conception of the nature and significance of the ascending fasciculi in the anterolateral funiculus can be obtained unless they also are studied in relationship with the fibres of the posterior root.
It has already been explained that of the fibres which enter the spinal medulla in the posterior root the great majority enter the posterior funiculus, where they bifurcate (Fig. 473, a); one branch of each fibre passes upwards either in the funiculus gracilis or in the funiculus cuneatus, or it may pass from the latter into the former; the other descends in the fasciculus interfascicularis (O.T. comma tract). Other fibres perhaps enter the posterolateral fasciculus (O.T. Lissauer's bundle). But all the other fibres of the posterior root, together with the majority of the fibres of the fasciculus cuneatus, sooner or later enter the gray matter (Fig. 473, b to h) of the spinal medulla.
Some of them (b) pass directly to end in the nucleus dorsalis of their own side, and from its cells fresh fibres arise, which pass laterally through the posterior column and lateral funiculus to reach the surface, where they bend upwards as constituent fibres of the spino-cerebellar fasciculus. These pass upwards throughout the whole length of the spinal medulla (above their place of origin), into the medulla oblongata, thence into the cerebellum through the restiform body.
Other fibres on the same side (c), and perhaps also on the other side (d), end amidst cells of the gray matter, the axis-cylinder processes of which pass into the antero-lateral superficial fasciculus (O.T. Gowers' tract). In this tract they ascend throughout the spinal medulla, medulla oblongata, and pons, to enter the cerebellum alongside the brachium conjunctivum (superior peduncle). This element in the antero-lateral fasciculus is sometimes designated the fasciculus spinocerebellaris anterior, to distinguish it both from the non-cerebellar fibres of the parent fasciculus and from the fasciculus spinocerebellaris [posterior] (O.T. the direct cerebellar tract). These two spino-cerebellar tracts convey to the cerebellum information from the muscles and overlying skin which assists it to co-ordinate the muscles for carrying on precisely adjusted movements.
Other fibres of the posterior nerve-root (e, f, g, and h) terminate in relationship with cells in the gray columns of their own side of the spinal medulla, the axons of which cross the median plane in the anterior commissure to pass respectively (e) into the anterolateral superficial fasciculus [not to be confused with the cerebellar constituents of this bundle]; (f) into the real fasciculus spinothalamicus [posterior], of which the last-mentioned fibres are merely outlying members; (g) into the fasciculus spinotectalis, to ascend to the mesencephalon; and (h) into the marginal area of the anterior funiculus to form a group which may be called the fasciculus spinothalamicus anterior.
The careful investigations of the late Dr. Page May led him to attach a definite physiological significance to this grouping of the ascending paths. The fasciculus spinothalamicus [posterior] is supposed to convey upwards to the thalamus (for
transmission to the cerebral cortex, which is concerned with the conscious appreciation of sensations) all impulses of pain, heat, and cold coming from the skin upon the opposite side of the body. The fasciculus spinothalamicus anterior conveys impulses of touch and pressure from the opposite side.
The spino-cerebellar fasciculi [anterior and posterior] convey to the cerebellum respectively homolateral and bilateral unconscious afferent impulses underlying muscular co-ordination and reflex tone.
Among the descending tracts that establish connexions between various parts of the brain (see Fig. 475) and the motor nerve-cells in the anterior column may be mentioned the cerebrospinal, the rubro-spinal (from the red nucleus), the tectospinal (from the corpora quadrigemina), the vestibulo-spinal (from the terminal nucleus of the vestibular nerve), and the bulbo-spinal tracts. The last-mentioned forms a peculiar triangular area upon the surface immediately to the lateral side of the anterior nerve-roots (Fig. 473), but there is great uncertainty as to its mode of origin: it is often called the fasciculus olivospinalis, from the fact that its discoverer, Helweg, believed it to originate from the olivary nucleus in the bulb or medulla oblongata. It may be regarded as an outlying part of the vestibular (or cerebellar) tract to the motor nuclei of the spinal medulla.
The fasciculus cerebrospinalis lateralis (O.T. crossed pyramidal tract) is a large well-defined descending tract which lies immediately in front of the posterior column of gray matter, and subjacent to the posterior spino-cerebellar fasciculus, which shuts it out from the surface. Below the point where the posterior spino-cerebellar fasciculus begins the cerebrospinal fasciculus becomes superficial, and in this position it can be traced as low as the fourth sacral nerve, at which level it ceases to exist as a distinct strand. The cerebro-spinal fasciculus is composed of an admixture of both large and small fibres. These arise in the brain from the large pyramidal cells of the motor or precentral area of the cerebral cortex, and pass downwards through various subdivisions of the brain to gain the spinal medulla. As they enter the spinal medulla they cross the median plane from one side to the other, and it thus happens that the cerebro-spinal tract in the right lateral funiculus of the spinal medulla has its origin in the cortex of the left cerebral hemisphere, and vice versa. As the tract descends in the spinal medulla it gradually diminishes in size; and this is due to the fact that, as it traverses each spinal segment, numerous fibres leave it to enter the anterior column of gray matter, and end in connexion with the anterior motor cells from which the fibres of the anterior nerve-roots arise. The entire strand is ultimately exhausted in this way. Numerous collateral fibrils spring from the cerebro-spinal fibres, and, entering the gray matter, end in a similar manner. In this way a single cerebro-spinal fibre may be connected with several spinal segments before it finally ends. The lateral cerebro-spinal fasciculus must be regarded as a great motor strand which brings the spinal motor apparatus under the control of the will.
Schäfer believes that many of the fibres of the cerebro-spinal fasciculus end in connexion with the cells of the nucleus dorsalis.
In many marsupials, rodents, and ungulates the lateral cerebro-spinal fasciculus lies in the posterior funiculus of the spinal medulla.
The fasciculus lateralis proprius represents the remainder of the lateral funiculus. Its fibres are largely derived from the cells situated in all parts of the gray matter, and also from the nerve-cells of the opposite side of the spinal medulla. After a course of very varying length in the fasciculus lateralis, these fibres turn medially and re-enter the gray matter. Such fibres may thus be regarded as inter-segmental association fibres binding two or more segments of the spinal medulla together. It may be mentioned that the association fibres which link together segments of the spinal medulla which are near to each other lie close to the gray matter, whilst those which connect the more distant segments are situated further out in the lateral funiculus.
Funiculus Anterior.-One well-defined tract is situated in the funiculus anterior. This is termed the fasciculus cerebrospinalis anterior. The remainder of the funiculus receives the name of the fasciculus anterior proprius.