« PrécédentContinuer »
the arrangement of the axons of the great majority of the motor cells, it should be noted that a few cross the median plane in the anterior white commissure and emerge in the fila of origin of the opposite anterior nerve-root.
The ventral cells are not scattered uniformly throughout the anterior column of gray matter. They are aggregated more closely together in certain parts of the anterior column, and thus form sub-groups or columns more or less perfectly marked off from each other.
Thus, one sub-group or column of ventral cells occupies the medial part of the anterior column of gray matter throughout almost its whole length. In only two segments of the medulla is it absent, viz., the fifth lumbar and the first sacral; at this level in the spinal medulla alone is its continuity broken (Bruce). It is termed the antero-median column or group of ventral cells. Behind this cell-column there is another classed with it to which the name of postero-median column or group is given, but this column of cells is not continuous throughout the entire length of the medulla. It is present in the thoracic region of the spinal medulla, where the motor nuclei for the muscles of the limbs are absent; and it is seen also in two or three of the segments of the cervical region and in the first lumbar segment (Bruce); elsewhere it is not represented.
In the cervical and lumbar swellings of the medulla, where the marked lateral out
Posterior lateral furrow
FIG. 468.-SECTION THROUGH THE FIFTH CERVICAL SEGMENT OF THE SPINAL MEDULLA.
growth is added to the lateral side of the anterior column of gray matter, certain groups of large multipolar cells are visible. These are the nuclei of origin of the motor-fibres which supply the muscles of the limbs, and consequently they are not represented in the upper three cervical segments of the spinal medulla; nor in any of the thoracic segments, with the exception of the first thoracic segment; nor in the lowest two sacral segments.
These lateral cells are arranged in several columns, which extend for varying distances in the superadded lateral parts of the anterior column of gray matter. The two main columns are an antero-lateral and a postero-lateral column; in certain segments there is likewise a retro-postero-lateral column, and in a number of segments in the lumbar and sacral regions a central column of cells (Bruce).
There cannot be a doubt that the grouping of the motor cells in the anterior column of gray matter of the medulla stands in relation to the muscle groups to which their axis-cylinder processes are distributed; but from what has been said it will be apparent that sharply defined cell-clusters associated with particular muscles do not exist. Still, much can be learned regarding the localisation of the motor nuclei in the anterior column of gray matter of the medulla from the study of the changes which occur in the cell-columns after atrophy of isolated muscles or groups of muscles, and after complete or partial amputations of limbs. It has been pointed out that the long muscles of the trunk (as, for example, the different parts of the sacro-spinalis muscle) receive nerve-fibres from all the
segments of the spinal medulla. Now, we have noted that there is only one cell-column, the ventro-median column, which pursues an almost uninterrupted course throughout the
Posterior median septum
Posterior lateral furrow
Edinger states that in the anterior column of gray matter the nuclei of origin of the nerves which supply the proximal muscles are medially placed; that those for the distal muscles are in general situated laterally. If this is the case, the cells connected with the shoulder muscles will lie nearer the middle of the anterior column of gray matter than those which are connected with the handmuscles. In cases where the forearm and hand, or the leg and the foot, are amputated, it would appear that it is the pos
FIG. 469.-SECTION THROUGH THE EIGHTH THORACIC SEGMENT OF THE
Dr. Bruce's Atlas.) tero-lateral column of cells that shows changes in consequence of its separation from the muscles to which its fibres are distributed.1
Intermedio-lateral Cell-column.-The intermedio-lateral cells form a long slender column which extends throughout the entire thoracic region of the medulla in the lateral column of gray It is also promatter. longed downwards into the first and second lumbar segments, where it disappears. In transverse sections through the spinal medulla this cell-group presents a very characteristic appearance, because the cells which compose it are small and are closely packed together. Although these cells, as a continuous column, are restricted to the region. indicated, it should be noted that the same group of cells reappears above, in certain of the cervical segments, and also in the third and fourth sacral From these segments. cells very fine fibres arise and leave the spinal FIG. 470.-SECTION THROUGH THE THIRD LUMBAR SEGMENT OF THE medulla, intermingled (To a large extent founded on Plates in Dr. Bruce's Atlas.)
SPINAL MEDULLA TO SHOW THE GROUPING OF THE MOTOR CELLS.
entire length of the medulla It may be assumed, therefore, that the nerve-fibres which go to these long trunk-muscles take origin in these medial cells.
Posterior lateral furrow
Posterior column of gray matter
Postero-lateral group of cells
with the motor fibres of
the anterior nerve-roots; they pass into the sympathetic ganglia, of which they
1 Those who seek further information regarding the grouping of the ventral cells of the medulla may with advantage study Dr. Alexander Bruce's Atlas of the Spinal Cord.
constitute the white rami communicantes. They represent the splanchnic efferent fibres of the medulla spinalis.
Nucleus Dorsalis (O.T. Clarke's Column).-This occupies the posterior column of gray matter and is the most conspicuous of all the cell-groups in the medulla. It does not, however, extend along the whole length of the medulla; indeed it is almost entirely confined to the "dorsal" region, which is the reason for the designation "nucleus dorsalis." (When, in the recent revision of nomenclature, the term "thoracic" was substituted for "dorsal" the revisers omitted to change the name of this structure to "thoracic "). Above, it begins opposite the seventh or eighth cervical nerve, whilst below, it may be traced to the level of the second lumbar nerve, where it disappears. In transverse section of the medulla it presents an oval outline, and is seen in the median part of the cervix of the posterior column of gray matter, immediately behind the gray commissure (Fig. 469, p. 530). On the lateral side it is circumscribed by numerous curved fibres from the entering posterior nerve-root, and in the lower thoracic region of the spinal medulla (opposite the eleventh and twelfth thoracic nerves) it becomes so marked that it forms a bulging on the median aspect of the posterior gray column.
The cells of the nucleus dorsalis are large, and possess several dendritic processes. The axons enter the lateral funiculus of white matter and there form a strand of fibres, which will be described later under the name
of the fasciculus spino- Anterior median
Nerve-fibres in the
Central group of cells
Posterior-lateral furrow Posterior column of gray matter
Retro-posterolateral group of cells
Antero-lateral group of cells
medullated variety pervade every part of the
gray matter. They are of three kinds, viz., (1) collaterals, (2) terminations of nervefibres, (3) axons given off by the cells. Many of the nerve-fibres which compose the funiculi of the medulla give off numerous fine collateral branches, which pass into the gray matter from all sides and finally end in relation with the nervecells. The majority of the nerve-fibres themselves, which thus give off collaterals, finally enter the gray matter, and end similarly. The axons of the majority of the cells leave the gray matter and emerge either for the purpose of entering a peripheral nerve or for the purpose of entering a strand of fibres in the white matter of the spinal medulla.
The nerve-fibres thus derived are interwoven together in the gray matter in a dense inextricable interlacement.
COMPONENT PARTS OF THE WHITE MATTER OF THE SPINAL MEDULLA.
The white matter of the spinal medulla is composed of medullated nerve-fibres, embedded in neuroglia. The fibres, for the most part, pursue a longitudinal course; and, from the deep surface of the pia mater which surrounds the medulla, fibrous septa or partitions are carried in along vertical planes between the fibres so as
to form an irregular and very imperfect fibrous framework of support. The neuroglia is disposed in a layer of varying thickness around the medulla, subjacent to the pia mater, and is carried into the medulla so as to give a coating to both sides of the various pial septa. The neuroglia is disposed also around the various nerve-fibres, so that each of these may be said to lie in a canal or tunnel of this substance. The nerve-fibres are all medullated, but they are not provided with primitive sheaths. It is the medullary substance of the nerve-fibres which gives to the white matter its opaque, milky-white appearance. When a thin transverse section of the medulla is stained in carmine and examined under the microscope the white matter presents the appearance of a series of closely applied circles, each with a dot in the centre. The dot is the transversely divided axis-cylinder of a nerve-fibre, and the dark ring which forms the circumference of the circle represents the wall of the neuroglial canal which is occupied by the fibre. The medullary substance is very faintly seen. It presents a filmy or cloudy appearance between the axis-cylinder and the neuroglial ring.
Arrangement of the Nerve-fibres of the White Matter in Fasciculi or Tracts.-When the white matter of a healthy adult spinal medulla is examined, the fibres which compose it are seen to vary considerably in point of size; and although there are special places where large fibres-or it may be small fibres -are present in greater numbers than elsewhere, yet, as a rule, both great and small fibres are mixed up together. No conclusive evidence can be obtained in such a spinal medulla, by any means at our disposal, of the fact that the longitudinally arranged fibres are grouped together in more or less definite tracts or fasciculi, the fibres of which run a definite course and present definite connexions. Yet this is known to be the case, and the existence of these separate tracts has been proved both by embryological investigation, as well as by the examination of the effects of injuries produced experimentally or accidentally on the nervous system in living beings.
By the experimental method it has been shown that when a nerve-fibre is severed the part which is detached from the nerve-cell from which it is an offshoot degenerates, whilst the part which remains connected with the nerve-cell undergoes little or no change. This is called the law of "Wallerian " degeneration. Thus, if in a living animal one-half of the spinal medulla is cut across, and after a few weeks the animal is killed and the medulla examined, it will be seen that there are degenerated tracts of fibres in the white matter, both above
and below the plane of division; but, still further, it will also be manifest that the tracts which are degenerated above the plane of division are not the same as those which are degenerated in the part of the medulla which lies below this level. The interpretation of this is obvious. The nerve-tracts which have degenerated above the plane of section are the offshoots of nerve-cells which lie in lower segments of the medulla or in spinal ganglia below the plane of section. Severed from these nerve-cells, they undergo what is called ascending degeneration. The nervetracts, on the other hand, which have degenerated in the portion of the medulla below the plane of division are the axons of cells which lie at a higher level than the plane of section, either in higher segments of the spinal medulla or in the brain itself. Cut off from the nerve-cells from which they proceed, they present an example of descending degeneration.
The embryological method was first employed by Flechsig, and it is often referred to as Flechsig's method. It is based upon the fact that nerve-fibres in the earliest stages of their development consist of naked axis-cylinders, and are not provided with medullary sheaths. Further, the nerve-fibres of different strands assume the medullary sheaths at different periods. If the foetal central nervous system is examined at different stages of its development, it is a comparatively easy matter to locate the different tracts of fibres by evidence of this kind. Speaking broadly, the tracts which myelinate first are those which bring the central nervous system into relation with the peripheral parts (skin, muscles, etc.); then those fibres which bind the various segments of the central nervous system together; next, those which connect the spinal medulla with the cerebellum; and, lastly, the tracts which connect the spinal medulla with the cerebral hemispheres. The nervous apparatus for the performance of automatic movements is fully
provided, therefore, before this is put under the control and direction of the higher centres. It by no means follows that in all the higher animals corresponding strands myelinate at relatively corresponding periods. Take the case of a young animal which from the time of its birth is able to move about and perform voluntary movements of various kinds in a more or less perfect manner, and compare it with the helpless new-born infant which is capable of exhibiting automatic movements only. In the former, the cerebro-spinal tracts, or motor tracts, which descend from the cerebrum into the spinal medulla, and which are the paths along which the mandates of the will travel, myelinate at an early period; whilst in the infant the corresponding fibres do not obtain their medullary sheaths until after birth. The study of the dates, therefore, at which the various strands of nerve-fibres myelinate not only gives the anatomist a means of locating their position in the white matter of the central nervous system, but it also affords the physiologist most important information regarding their functions, and also the periods at which these functions are called into play.
It is a matter of interest to note that influences which either accelerate or retard the periods at which nerve-fibres are brought into functional activity have also an effect in determining the dates at which these fibres assume their sheaths of myelin. Thus, when a child is prematurely born the whole process of myelinisation is, as it were, hurried up; and further, when in newborn animals light is freely admitted to one eye whilst it is carefully excluded from the other, the fibres of the optic nerve of the former myelinate more rapidly than those of the opposite nerve.
Study of the minute structure (Anatomical method) of the central nervous system, especially of material that has been stained by the methods of Golgi and Ramon y Cajal or by the use of methylene blue, completes the results attained by these other methods, by demonstrating the precise mode of origin and termination of the various fasciculi.
Posterior Funiculus and the Posterior Roots of the Spinal Nerves.-In the cervical and upper thoracic regions of the spinal medulla the posterior funiculus is divided by the posterior intermediate sulcus and septum into the fasciculus cuneatus, which lies laterally and next to the posterior column of gray matter, and the fasciculus gracilis, which lies medially and next to the posteriormedian septum. The fasciculus cuneatus is composed of nerve-fibres which are for the most part larger than those entering into the formation of the fasciculus gracilis, and both tracts have a most intimate relation to the posterior nerve-roots; indeed, they are both composed almost entirely of fibres which enter the medulla by these roots, and then pursue a longitudinal course.
The nerve-fibres which form the posterior nerve-roots, on entering the medulla along the sulcus lateralis posterior, divide within the fasciculus cuneatus into ascending and descending branches which diverge abruptly as they pass respectively upwards and downwards. The descending fibres are, as a rule, short, and soon end in the gray matter of the spinal medulla. These descending fibres occupy an 'area in the posterior funiculus along the line of separation of the fasciculus gracilis and the fasciculus cuneatus, and, hence, may be called the fasciculus interfascicularis (comma tract of Schultze). This area, when the spinal medulla is divided, undergoes descending degeneration and then presents a comma-shaped outline. (Fig. 473).
The ascending fibres vary greatly in length, and at differing distances from the point where the parent fibres enter the medulla they end in the gray matter. A small contribution, however, of ascending fibres, from each posterior nerve-root, extends upwards to the upper end of the spinal medulla, to end in the medulla oblongata (Figs. 474 and 475).
As each posterior nerve-root enters, its fibres range themselves in the lateral part of the posterior funiculus close up against the posterior column of gray matter. The nervefibres of the nerve-root next above take the same position, and consequently those which entered from the nerve immediately below are displaced medially, and come to lie in the posterior funiculus nearer to the median plane. This process goes on as each nerveroot enters, and the result is that the fibres of the lower nerves are gradually pushed nearer and nearer to the posterior median septum in a successive series of lamellar tracts. Of course, the greater proportion of the fibres which are thus carried upwards from the posterior nerve-roots sooner or later leave the posterior funiculus and enter the gray matter, to end there in relation to some of its cells; but, as we have said, every posterior nerve-root sends a few fibres up the whole length of that portion of the spinal medulla which lies above, and thus the posterior funiculus gradually increases in bulk as it is traced upwards, and in all except the lowest part of the spinal medulla, the posterior funiculus is separable into a fasciculus gracilis and a fasciculus cuneatus. The fasciculus gracilis is composed of the long ascending fibres of the posterior nerve-roots, which have entered the lower segments of the spinal medulla. To put the matter differently, the fibres of the sacral roots are displaced medially by the entering lumbar fibres, while the fibres of the lumbar roots are in their turn pushed medially by the entering thoracic fibres,