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segments; followed in a downward direction it blends with the anterior column in the lumbar swelling and contributes to the thickening of that column.
The gray matter is for the most part mapped off from the surrounding white matter with a considerable degree of sharpness; but in the cervical region, on the lateral aspect of the crescentic mass and in the angle between the anterior and posterior columns, fine bands of gray matter penetrate the white matter, and, joining with each other, form a network, the meshes of which enclose small islands of white matter. This constitutes what is called the formatio reticularis. Although best marked in the cervical region, traces of the same reticular formation may be detected in lower segments of the spinal medulla.
Characters presented by the Gray Matter in Different Regions of the Spinal Medulla.-The gray matter is not present in equal quantity nor does it exhibit the same form in all regions of the spinal medulla. Indeed, each segment presents its own special characters in both of these respects. It is not necessary, however, in the present instance, to enter into this matter with any degree of minute detail. It will be sufficient if the broad distinctions which are evident in the different regions are pointed out.
It may be regarded as a general law that, wherever there is an increase in the 'size of the nerves attached to a particular part of the spinal medulla, a corresponding increase in the amount of gray matter will be observed. It follows from this that the regions where the gray matter bulks most largely are the lumbar and the cervical swellings. The great nerve-roots which go to form the nerves of the large limb-plexuses enter and pass out from those portions of the spinal medulla. In the thoracic region there is a reduction in the quantity of gray matter in correspondence with the smaller size of the thoracic nerves.
In the thoracic region (Fig. 467, B) both columns of gray matter are narrow, although the distinction between the anterior column and the still more attenuated posterior column is sufficiently manifest. In this region the lateral column of gray matter also is characteristic, and the substantia gelatinosa in transverse section is pointed and spear-shaped.
In the upper three segments of the cervical region the anterior columns of gray matter are not large and they resemble the corresponding columns in the thoracic region. A lateral column also is present. But in these segments (and more especially in the first and second) there is a marked attenuation of the neck of the posterior column, and the posterior commissure is very broad.
In the cervical swelling the contrast between the two columns is most striking; the anterior column is of great size and presents a very broad surface towards the anterior aspect of the spinal medulla, whilst the posterior column remains narrow. This great increase in the bulk of the anterior column is due to a marked addition of gray matter on the lateral side of the column, and seeing that this additional matter is traversed by a greater number of fibres, it stands out, in well-prepared specimens, more or less distinctly from the part of the column which lies to the medial side, and which may be considered to represent the entire anterior column in the thoracic and upper cervical segments. Within this lateral addition to the anterior column are placed those collections of cells which constitute the nuclei of origin of the motor nerves of the muscles of the upper limb. The characteristic thickening of the anterior column of gray matter is evident, therefore, in those segments of the spinal medulla to which the nerves which enter the brachial plexus are attached, viz., the lower five cervical segments and the first thoracic segment.
In the lumbar swelling the anterior columns again broaden out, and for the same reason as in the case of the corresponding columns in the cervical swelling. The nuclear masses which contain the cells from which the motor fibres which supply the muscles of the lower limbs take origin are added to the lateral aspect of the columns and give them a very characteristic appearance. In this region of the spinal medulla, however, the posterior columns also are broad and are capped by substantia gelatinosa which in transverse section presents a semilunar outline. There is consequently no difficulty in distinguishing, from an inspection of the gray matter alone, between transverse sections of the spinal medulla taken from the cervical and lumbar swellings of the spinal medulla.
In the lower part of the conus medullaris the gray matter in each half of the spinal medulla assumes the form of an oval mass joined to its fellow of the opposite side by a thick gray commissure. Here, almost the entire bulk of the spinal medulla consists of gray matter, seeing that the white matter is reduced to such an extent that it forms only a thin coating on the outside.
White Matter of the Spinal Medulla. In transverse sections of the spinal medulla the three funiculi into which the white matter is subdivided become very
FIG. 467.-SECTION THROUGH EACH OF THE FOUR REGIONS OF THE MEDULLA SPINALIS. (From specimens prepared by the Weigert-Pal method; therefore the white matter is rendered dark in colour whilst the gray matter is bleached.)
apparent. The posterior funiculus is wedge-shaped, and lies between the posterior median septum and the posterior column of gray matter. The lateral funiculus occupies the concavity of the gray crescent. Behind, it is bounded by the posterior column of gray matter and the sulcus lateralis posterior, whilst in front it extends as far as the most lateral fasciculi of the anterior nerve-roots as they pass out from the anterior column. The anterior funiculus includes the white matter between the anterior median fissure and the anterior column of gray matter, and also the white
matter which separates the broad extremity of the anterior column from the surface of the spinal medulla. This latter portion of the anterior funiculus is traversed by the emerging fila of the anterior nerve-roots.
In cross-sections of the spinal medulla the partition of pia mater, which dips in at the sulcus intermedius posterior and divides the posterior funiculus into the medial fasciculus gracilis and the lateral fasciculus cuneatus, is very strongly marked in the cervical regions, but as it is traced downwards into the thoracic region it becomes shorter and fainter, and finally disappears altogether at the level of the eighth thoracic nerve. Below this point there is no visible demarcation of the posterior funiculus into two parts.
The white matter is not present in equal quantity throughout the entire length of the spinal medulla. It increases steadily from below upwards, and this increase is most noticeable in the lateral and posterior funiculi. In the lower part of the conus medullaris the amount of gray matter is actually greater than that of the white matter: but very soon this state of affairs is changed, and in the lumbar region the proportion of gray to white matter is approximately as 1:21; in the thoracic region as 1:5; and in the cervical region as 1:51. When it is remembered how the gray matter expands in the lumbar and cervical regions, and how greatly it becomes reduced in the thoracic region, the significance of these figures will become more apparent.
Canalis Centralis. As previously stated, the central canal is found in the gray commissure. It is a very minute tunnel, barely visible to the naked eye when seen in transverse section, and it traverses the entire length of the spinal medulla. Above, it passes into the medulla oblongata, and finally opens into the fourth ventricle of the brain; below, it is continued for a variable distance into the filum terminale, and in this it ends blindly. Only in the lumbar region does! the central canal occupy the centre of the spinal medulla. Above this level, in the thoracic and cervical regions, it lies much nearer the anterior than the posterior aspect of the spinal medulla; whilst below the lumbar region, as it is traced down into the conus medullaris, it inclines backwards and approaches the posterior aspect of the spinal medulla. The calibre of the canal also varies somewhat in different parts of the spinal medulla. It is narrowest in the thoracic region; and in the lower part of the conus medullaris it expands into a distinct fusiform dilatation (very nearly 1 mm. in transverse diameter), which is termed the ventriculus terminalis (Krause).
The central canal is lined with a layer of ciliated columnar cells, the deep tapering ends of which are prolonged into slender processes which penetrate into the substance of the spinal medulla. These cells constitute the lining ependymal cells of the canal. The cilia of the epithelial cells are very early lost, and it is not uncommon to find the canal blocked up by epithelial debris.
The central canal is of interest because it represents in the adult the relatively wide lumen of the early ectodermal neural tube from which the spinal medulla is developed.
Filum Terminale. The delicate thread to which this name is applied is continuous with the inferior tapered end of the conus medullaris. It is easily distinguished, by its silvery and glistening appearance, from the numerous long nerve-roots (cauda equina) amidst which it lies. It is about six inches long, and down to the level of the second sacral vertebra it is enclosed with the surrounding nerve-roots within the dura mater. Below this point the dura mater is applied directly to the surface of the filum terminale and is called filum duræ matris spinalis. The filum terminale proceeds downwards in the sacral canal, and finally receives attachment to the periosteum on the posterior aspect of the coccyx (Fig. 460, p. 518). It is customary to speak of the filum as consisting of two parts, viz., the filum terminale internum and the filum terminale externum, or the part inside and the part outside the tube of dura mater.
The filum terminale externum is simply a fibrous thread, strengthened by the prolongation it receives from the dura mater. The filum terminale internum is composed largely of pia mater; but in its superior half it encloses the terminal part of the central canal, and around this a variable amount of the gray substance of the spinal medulla is prolonged downwards into the filum. When transverse sections are made through
the superior part of the filum terminale internum some bundles of medullated nerve-fibres are observed clinging to its sides, and with these are associated some nerve-cells identical with those in the spinal ganglia. These represent rudimentary or aborted caudal nerves (Rauber).
SUMMARY OF THE CHIEF CHARACTERS PRESENTED BY THE SPINAL MEDULLA IN ITS DIFFERENT REGIONS.
COMPONENT PARTS OF THE GRAY MATTER OF THE SPINAL MEDULLA.
Neuroglia enters largely into the constitution of the gray matter of the spinal medulla. It forms a bed within which the nervous elements are distributed. These nervous elements consist of (1) nerve- cells and (2) nerve-fibres-both medullated and non-medullated. The nerve-cells lie in small spaces in the
neuroglia, whilst the nerve-fibres traverse fine passages the walls of which are formed of the same substance. The neuroglia is thus an all-pervading basis substance which isolates the nervous elements one from the other more or less completely, and at the same time binds them together into a consistent solid mass. In two situations the gray matter presents peculiar features, viz., the apex of the posterior column and the tissue surrounding the central canal. In both situations the gray matter stains more deeply with carmine and presents a more translucent appearance; in other respects the substantia grisea centralis and the substantia gelatinosa are very different.
The substantia grisea centralis forms a thick ring around the central canal. It is traversed by the fine processes which proceed from the deep ends of the ependymal cells which line the canal. It is composed almost entirely of neuroglia.
In transverse sections of the spinal medulla the substantia gelatinosa, in the cervical and thoracic regions, presents the appearance of a V-shaped mass, embracing the extremity of the posterior column of gray matter; in the lumbar region this cap assumes a semilunar outline.
In the substantia gelatinosa the neuroglia is present in small quantity, and small nerve-cells are developed within it in considerable numbers.
Nerve-Cells. The nerve-cells are scattered plentifully throughout the gray matter, but perhaps not in such great numbers as might be expected when we note the enormous number of nerve-fibres with which they stand in relation. They are all, without exception, multipolar, and send off from their various aspects several branching protoplasmic processes or dendrites, and one axon, which becomes the axis-cylinder of a nerve-fibre. In size they vary considerably, and as a rule (to which, however, there are many exceptions) the bulk of a nerve-cell has a more or less definite relation to the length of the axis-cylinder which proceeds from it.
When the nerve-cells are studied in a series of transverse sections of the spinal medulla, it will be noticed that a large proportion of them are grouped in clusters in certain districts of the gray matter; and as these groups are seen in very much the same position in successive sections, it is clear that these cells are arranged in longitudinal columns of greater or less length. Thus we recognise (1) a ventral group or column of cells in the anterior column of gray matter; (2) an intermedio-lateral group or column in the lateral column of gray matter, where this exists; and (3) a posterior vesicular column of cells (nucleus dorsalis), forming a most conspicuous group in the medial part of the neck of the posterior column in the thoracic region of the spinal medulla.
Other cells, besides those forming these columns, are scattered somewhat irregularly throughout the gray matter of the posterior column and the part of the gray crescent which lies between the two columns; and although these also in some measure may be classified into groups, the arrangement thus effected is not of so definite a character as to justify us in dwelling upon it in the present instance.
Ventral Cell-Column and the Origin of the Fibres of the Anterior Nerveroots. The ventral cell-group occupies the anterior column of gray matter, and in it are found the largest and most conspicuous cells in the spinal medulla. It extends from one end of the spinal medulla to the other. These ventral nerve-cells have numerous wide-spreading dendritic processes, and it is to be noticed that certain of these dendrites do not confine their ramifications to the gray matter. Thus, some of the cells along the medial border of the anterior column of gray matter send dendrites across the median plane in the anterior commissure to end in the anterior gray column of the opposite side; whilst others, lying along the lateral margin of the anterior column of gray matter, send dendrites in amongst the nerve-fibres of the adjoining white matter.
The axons or axis-cylinder processes of a large proportion of the ventral cells converge together; and, becoming medullated, they form bundles which pass out from the gray matter, and through the white matter which separates the thick end of the anterior column from the surface of the spinal medulla, to emerge finally as the fila of the anterior nerve-roots. These cells, then, are the sources from which the nerve-fibres of the anterior nerve-roots proceed, and in consequence they are frequently spoken of as the "motor cells" of the spinal medulla. Whilst this is