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Posterior median septum
medulla are made, it is seen to be a bilateral structure which is partially subdivided into a right and a left half by a median cleft (fissura mediana anterior) in front and a septum (septum medianum posterius) behind. The anterior median fissure penetrates only for a distance corresponding to somewhat less than a third of the antero-posterior diameter of the spinal medulla. The pia mater dips down into it and forms a fold or reduplication within it. The posterior median septum in the cervical and thoracic regions penetrates into the spinal medulla until it reaches a point somewhat beyond its centre. It is extremely narrow, and consists of ependymal and neuroglial elements, and is intimately connected with the adjacent sides of the two halves of the spinal medulla, between which it intervenes. The pia mater, which invests the surface of the spinal medulla. passes continuously over the posterior median septum and sends no prolongation of any kind into it. In the lumbar region of the spinal medulla the septum becomes shallower, whilst the anterior median fissure deepens, and ultimately in the inferior part of the spinal medulla the fissure and septum present a very nearly equal depth.
FIG. 465.-DIAGRAM OF THE
CVI shows the level of the 1st
The two halves of the spinal medulla may show trifling differences in the arrangement of the parts which compose them; but to all intents and purposes they are symmetrical. They are joined together by a more or less broad band or commissure, which intervenes between the median fissure and the septum.
An inspection of the surface of each half of the spinal medulla brings into view a longitudinal groove or furrow, at some little distance from the posterior median septum, which extends along the whole length of the spinal medulla. Along the bottom of this groove the fila of the posterior nerve-roots enter the spinal medulla in accurate linear order. It is called the sulcus lateralis posterior. There is no corresponding furrow on the anterior part of each half of the spinal medulla in connexion with the emergence of the fila of the anterior nerve-roots. These fila emerge irregularly over a broad strip of the surface of the spinal medulla, which corresponds in its width to the thickness of the subjacent anterior surface of the anterior column of gray matter.
The sulcus lateralis posterior subdivides each half of the spinal medulla into a small funiculus posterior and a much larger antero-lateral funiculus, and it is customary to map the latter arbitrarily off into a funiculus lateralis and a funiculus anterior by a line corresponding to the emergence of the most lateral of the fila or fascicles of the anterior nerve-roots.
In the cervical region a distinct longitudinal groove may be observed on the surface of the posterior funiculus. It is placed rather nearer to the posterior median septum than to the posterior lateral sulcus, and as it is traced down into the thoracic region it gradually becomes indistinct and finally disappears. This is called the sulcus intermedius posterior, and it marks on the surface the position of a septum of pia mater which dips into the spinal medulla and subdivides the posterior funiculus into a lateral part, termed the fasciculus cuneatus (O.T. column of Burdach), and a medial portion, which receives The name of the fasciculus gracilis (O.T. column of Goll).
INTERNAL STRUCTURE OF THE SPINAL MEDULLA.
The spinal medulla is composed of a central core of gray matter thickly coated on the outside by white matter. At only one spot does the gray matter come close to the surface, viz., at the bottom of the sulcus lateralis posterior.
Gray Matter of the Spinal Medulla.-The gray matter in the interior of the spinal medulla has the form of a fluted column, but it is customary to describe it as it appears in transverse sections. It then presents the appearance of the capital letter H. In each half of the spinal medulla there is a semilunar or crescentic mass, shaped somewhat like a comma, the concavity of which is directed laterally and the convexity medially. The two crescents of opposite sides are connected across the median plane by a transverse band, which receives the name of the commissura grisea (gray commissure). The posterior median septum extends forwards in the spinal medulla until it reaches the gray commissure. The bottom of the anterior median fissure, however, is separated from it by an intervening strip of white matter, which is termed the commissura anterior alba, or anterior white commissure. In the gray commissure may be seen the central canal of the spinal medulla (canalis centralis), which tunnels the entire length of the spinal medulla and is just visible to the naked eye as a minute speck. The portion of the gray commissure which lies behind the central canal is called the commissura posterior; whilst the portion in front receives the name of the commissura anterior grisea.
Each crescentic mass of
gray matter presents certain well-defined parts.
which extend behind and FIG. 466.-TRANSVERSE SECTION THROUGH THE SUPERIOR PART OF THE
The projecting portions
in front of the connecting
transverse gray commissure
are termed respectively the
CERVICAL REGION OF THE SPINAL MEDULLA OF AN ORANG. (From a specimen prepared by the Weigert-Pal method, by which the white matter is rendered dark whilst the gray matter is bleached.)
posterior and the anterior columns of gray matter (columnæ grisex). These stand out in marked contrast to each other. In section the columna anterior is short, thick, and very blunt at its extremity. Further, its extremity falls considerably short of the surface of the spinal medulla and is separated from it by a moderately thick coating of white matter. Through this the fila of the anterior nerve-roots, as they emerge from the gray matter of the anterior column, pass on their way to the surface. Throughout the greater part of the spinal medulla the columna posterior (O.T. posterior cornu) is elongated and narrow, and is drawn out to a fine point, which almost reaches the bottom of the posterior lateral sulcus. This pointed extremity receives the name of the apex columnæ posterioris; the slightly swollen part which succeeds it is the caput columnæ; whilst the slightly constricted part adjoining the gray commissure goes under the name of the cervix columnæ posterioris.
The apex or tip of the posterior column differs considerably in appearance from the general mass of the gray matter. It is composed of a material which presents a lighter hue and has a somewhat translucent look. It is called the substantia gelatinosa [Rolandi], and, when seen in transverse section, it exhibits a V-shaped outline and fits on the posterior column like a cap.
A pointed and prominent triangular projection juts out from the lateral aspect of the gray matter nearly opposite the gray commissure. This is the columna lateralis (O.T. lateral cornu), and it is best marked in the upper thoracic region. (Fig. 467, B). Traced upwards it becomes absorbed in the greatly expanded anterior column of the cervical swelling, but it reappears again in the upper part of the spinal medulla, and is particularly noticeable in the second and third cervios
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
At the margin of the foramen magnum the spinal medulla becomes continuous with the medulla oblongata of the brain, whilst below, it tapers rapidly to a point and forms a conical extremity termed the conus medullaris. From the end of the conus medullaris a slender glistening thread is prolonged downwards within the vertebral canal, and finally anchors the spinal medulla to the back of the coccyx. This prolongation receives the name of the filum terminale.
The diameter of the spinal medulla is very much shorter than that of the vertebral canal within which it lies. A wide interval is left between its surface and the walls of its canal, and this excess of space is clearly a provision for allowing free movement of the vertebral column without producing any jarring contact between the delicate spinal medulla and the surrounding bones.
Three protective membranes are wrapped around the spinal medulla. From within outwards these are termed (1) the pia mater, (2) the arachnoid, and (3) the
dura mater. The pia mater is a fibrous membrane which forms the immediate investment. It is closely applied to the spinal medulla, and from its deep
FIG. 460.-THE CONUS MEDULLARIS AND THE
FIG. 461. THE ROOTS OF ORIGIN OF THE SEVENTH THORACIC NERVE (semi-diagrammatic).
surface numerous fine septa penetrate into the substance of the spinal medulla. The arachnoid is an exceedingly delicate transparent membrane which is loosely wrapped around the spinal medulla so as to leave a considerable interval, between itself and the pia mater, termed the subarachnoid space, in which there is always a varying amount of cerebro-spinal fluid. Outside the arachnoid, the dura mater forms a wide, dense, fibrous, tubular sheath, which extends downwards within the vertebral canal for a considerable distance beyond the conical extremity of the spinal medulla. The spinal medulla is suspended within its sheath or theca of dura mater by two lateral wing-like ligaments, termed the ligamenta denticulata. These extend laterally from the sides of the spinal medulla and are attached by a series of pointed or tooth-like processes to the inner surface of the theca of dura mater. Between the wall of the vertebral canal and the dura mater there is a narrow interval, which is filled up by soft areolo - fatty tissue and numerous thin-walled veins arranged in a plexiform manner.
Thirty-one pairs of spinal nerves arise from the sides of the spinal medulla.