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These are classified into eight cervical, twelve thoracic, five lumbar, five sacral, and one coccygeal; and according to the attachments of these groups of nerves the spinal medulla is arbitrarily subdivided into cervical, thoracic, lumbar, and sacral regions. In employing these terms, therefore, for different districts of the spinal medulla, it must be understood that the regions are determined by the nerve attachments and not by any direct relationship between these parts of the spinal medulla and the sections of the vertebral column which bear the same names.
Each spinal nerve is attached to the spinal medulla by an anterior or ventraland a posterior or dorsal root, and as these are traced to their central attachments they are seen to break up into a number of separate nerve fascicles or bundles, which spread out, in some cases very widely from each other, as they approach the side of the spinal medulla (Fig. 461). Each pair of nerves is therefore attached to a portion of spinal medulla of some length, and such a portion, with its pair of nerves, receives the name of a "segment of the spinal medulla." It must be clearly understood, however, that, in so far as the surface of the spinal medulla is concerned, there is no means of marking off one segment from another except by the nerve attachments.
In the cervical and lumbar regions of the spinal medulla the nerve-roots are somewhat crowded together, so that little or no interval is left between the adjoining root fila or fascicles of neighbouring nerves. In the thoracic region, however, distinct intervals may be observed, and the root fila are more loosely arranged. From this, it will be evident that the segments in different parts of the spinal medulla are not of equal length. In the cervical region the segments measure about 12 mm. in length, in the thoracic region from 20 to 24 mm., and in the lumbar region about 10 mm. The number of fila which attach the different nerve-roots to the spinal medulla is very different in different nerves, and is not necessarily the same in the same nerve-root in different individuals.
Roots of first lunibar nerve. -Cauda equina
Owing to the great difference which exists between the length of the spinal medulla and the length of the vertebral column, the farther we pass down the greater the distance becomes between the attachment of the various nerve-roots to the spinal medulla and the intervertebral foramina through which the corresponding nerves leave the vertebral canal. The lower nerve-roots, therefore, have to traverse the vertebral canal for a considerable distance before they reach their apertures of emergence. It thus happens that the nerve-roots which spring from the lumbar and sacral regions of the spinal medulla attain a very great length and descend vertically in the lower part of the vertebral canal in a bunch or leash, in the midst of which lie the conus medullaris and the filum terminale. This great bundle of nerve-roots receives the appropriate name of the cauda equina.
Enlargements of the Spinal Medulla.-Throughout the greater part of the thoracic region, the spinal medulla presents a uniform girth and a very nearly circular outline when seen in transverse section. In the cervical and lumbar regions, however, it shows marked swellings. The intumescentia cervicalis or cervical enlargement is the more evident of the two. It begins very gradually at the upper end of the spinal medulla, attains its greatest breadth (12 to 14 mm.) opposite the fifth or sixth cervical vertebra, and finally subsides opposite the second thoracic vertebra. To this portion of the spinal medulla are attached the great nerves which supply the upper limbs. The intumescentia lumbalis or lumbar enlargement begins at the level of the tenth thoracic vertebra, and acquires its maximum transverse diameter (11 to 13 mm.) opposite the last thoracic vertebra. Below, it rapidly tapers away into the conus medullaris. To the lumbar enlargement are attached the great nerves of the lower limbs.
FIG. 462.-SECTION THROUGH THE CONUS MEDULLARIS AND THE CAUDA EQUINA AS THEY LIE IN THE VERTEBRAL CANAL.
These enlargements of the spinal medulla are associated with the outgrowth of the limbs. In the earlier developmental stages of the spinal medulla they are not present, and they take form only as the limbs become developed. In different animals their size corresponds with the degree of development of the limbs. Thus, in the long-armed orang and gibbon the cervical swelling stands out with a remarkable degree of prominence.
Development of the Spinal Medulla.-The early stages of the process by which the originally simple epithelial neural tube becomes converted into the central nervous system have already been considered. It remains to be explained how the features specially distinctive of the spinal medulla are produced.
In the early stages of the development of the spinal medulla (Fig. 463), the neuroblasts are found to be scattered in the intermediate of the three bands of
FIG. 463.-DIAGRAM OF TRANSVERSE SECTION OF THE LEFT HALF OF EARLY NEURAL TUBE.
which the thick side wall of the neural tube is composed-the mantle layer. These primitive nerve-cells soon congregate in much larger numbers in the ventral part of the basal lamina (Fig. 464), so that the mantle layer expands there into a broad excrescence, which is the rudiment of the columna anterior or anterior cornu of gray matter. This anterior column contains the efferent or motor nervecells, the axons of which emerge as the anterior root of a spinal nerve. At this stage the rest of the mantle layer consists of a thin stratum of neuroblasts (Fig. 463), mainly intercalated cells, which receive the sensory impressions entering the spinal medulla through the radix posterior, and transmit impulses into axons passing (a) to the motor nuclei, (b) to the other side of the spinal medulla through the floor-plate (Fig. 463), or (c) into the superficial stratum (peripheral layer) of the spinal medulla where they bend upwards or downwards as constituent elements of the funiculi (or white columns). As development proceeds (Fig. 463) the substantia grisea (gray substance) formed of these intercalated cells becomes much more abundant and forms a broad blunt boss (Figs. 464, B and C), which is the rudiment of the columna posterior (O.T. posterior cornu).
The surfaces of these gray columns become coated with a layer of white sub
istance, composed at first mainly of the axons of cells in the root ganglia and intercalated cells in the spinal medulla; and as these funiculi increase in size they help to mould the form of the gray columns. This is displayed best in the case of the posterior column (O.T. posterior cornu). major portion of the white substance, funiculus posterior, which accumulates behind (and afterwards lies on the medial side of) the posterior column, does not consist of fibres springing from intercalated cells, either of the spinal medulla or any other part of the central nervous system, but of the direct continuations of the central processes of the cells in the spinal ganglion on the posterior root (Figs. 463 and 464). A large proportion of the fibres of the posterior root do not enter the gray columns immediately after their insertion into the alar lamina, but bifurcate to form two vertical nerve-fibres, one passing upwards, and the other downwards, in the funiculus posterior before they end in the gray column, some distance above or below the place where they gained admission to the medulla spinalis. As the spinal medulla grows, the originally blunt posterior column becomes drawn backwards into an increasingly attenuated process, and the funiculus posterior, which was placed originally upon its lateral surface (Fig. 464, A), and then upon its posterior surface (Fig. 464, B), gradually assumes a wedge-shaped form (Figs. 464, C, and 466), upon the medial side of the gray matter.
Development of the Anterior Median Fissure, Posterior Median Septum, and of the Central Canal.-As the anterior columns of gray matter and the anterior funiculi of white matter increase in size, the anterior surface of the spinal medulla, on each side of the median plane, bulges forwards, and the fissura mediana anterior (Fig. 464, A, B, and C) is produced as the natural result.
There has been considerable discussion as to the mode of formation of the posterior median septum; but there is now no doubt as to the essential facts. Early in the third month the walls of the posterior three-fourths (of the sagittal extent) of the central canal of the spinal medulla become approximated (Fig. 464), and later they fuse to obliterate that part of the canal. But the part of the septum thus formed is only an insignificant portion of the whole. For most of the septum is produced by the gradual elongation of the epithelial cells lining the remnant of the central canal as the fibremasses of the posterior funiculi expand and separate the posterior surface of the spinal medulla further and further from the situation of the canal (see Fig. 453, p. 511).
Furrows of the Spinal Medulla.-When cross-sections of the adult spinal
Early posterior funiculus
FIG. 464.-THREE STAGES IN THE DEVELOP-
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.
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.
Posterior median septum
Cervical swellingSulcus intermedius posterior Posterior lateral sulcus
FIG. 465.-DIAGRAM OF THE
CVI shows the level of the 1st cervical vertebra; CVv of the 5th cervical vertebra; THVп of the 2nd thoracic vertebra; THVx of the 10th thoracic vertebra; THVx11 of the 12th thoracic vertebra; LVII of the 2nd lumbar vertebra.
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. Origin of accessory.
Each crescentic mass of gray matter presents certain well-defined parts. The projecting portions which extend behind and in front of the connecting transverse gray commissure are termed respectively the posterior and the anterior columns of gray matter (columnæ grisea). 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 cervical
FIG. 466.-TRANSVERSE SECTION THROUGH THE SUPERIOR PART OF 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.)