Internal Structure of the Spinal Cord
By reflected light masses of medullated axons appear white in the fresh, and such masses are known as white substance. The spinaal cord consists of a continuous, centrally placed column of grey substance surrounded by a variously thickened tunic of white substance. The closely investing pia mater sends numerous ingrowths into the cord, bearing blood-vessels and contributing to its internal supporting tissue. The volume of white and of grey substance varies both absolutely and relatively at different levels of the cord. The absolute amount of grey substance increases with the enlargements. The absolute amount of white substance also increases with the enlargements coincident with the greater amount of grey substance in those regions. The relative amount of white substance increases in passing from the conus medullaris to the medulla oblongata, due to the fact that the ascending and descending axons associating the cord with the encephalon are the one contributed to the cord and the other gradually terminating in it at different levels along its entire descent.
The grey substance
In the embryo all the nerve-cells of the grey substance are derived from the cells lining the neural tube, and in the adult the column of grey substance, though greatly modified in shape, still retains its position about the central canal. In transverse section the column appears as a grey figure of two laterally developed halves, connected across the mid-line by a more attenuated portion, the whole roughly resembling the letter H. The cross-bar of the H is known as the grey commissure. Naturally, it contains the central canal, which is quite small and is either rounded or laterally or ventrally oval in section, according to the level of the cord in which it is examined. The canal continues upward, and in the medulla oblongata opens out into the fourth ventricle. Downward, in the extremity of the conus medullaris, it widens slightly and forms the rhomboidal sinus or terminal ventricle, then is suddenly constricted into an extremely small canal extending a short distance into the filum terminale, and there ends blindly. The grey commissure always lies somewhat nearer the ventral than the dorsal surface of the cord, and itself contains a few medullated axons which vary in amount in the different regions of the cord. The medullated axons crossing the mid-line on the ventral side of the central canal form the ventral or anterior white commissure ; those, usually much fewer in number, crossing on the dorsal side of the central canal, form the dorsal or posterior white commissure. These two commissures comprise fibers crossing in the grey substance as distinguished from others which cross in the white substance dorsal and ventral to them. The axons of these commissures serve in functionally associating the two lateral halves of the grey, column.
Each lateral half of the grey column presents a somewhat crescentic or comma- shaped appearance in transverse section, which also varies at the different levels of the cord. At all levels each half presents two vertical, well-defined horns, themselves spoken of as columns of grey substance. The dorsal horn [columna posterior] extends posteriorly and somewhat laterally toward the surface of the cord along the line of the postero-lateral sulcus. It is composed of an apex and a neck [cervix columnse posterioris].
In structure the apex is peculiar. The greater portion of it consists of a mass of small nerve-cells and neuroglia tissue, among which a gelatinous substance of questionable origin predominates, giving the horn a semi-translucent appearance. This is termed the gelatinous substance of Rolando, to distinguish it from a similar appearance immediate about the central canal, the central gelatinous substance. The apex of the dorsal horn is widest in the regions of the enlargements, especially the lumbar, and the gelatinous substance of Rolando is most marked in the cervical region. In these regions the cervix consists of a slight constriction of the dorsal horn between the apex and the line of the grey commissure. In the thoracic region, however, the base of the cervix is the thickest part of the dorsal horn. This thickness is due to the presence there of the nucleus dorsalis, or Clarke's column - a column of grey substance containing numerous nerve-cells of larger size than elsewhere in the dorsal horn, and extending between the seventh cervical and third lumbar segments of the cord. Tapering finelj' at its ends, this nucleus attains its height in the lower thoracic or first lumbar segment. About the ventro-lateral periphery of the nucleus dorsalis are scattered nerve-cells of the same type as contained in it. These cells are sometimes distinguished as Stilling's nucleus, though Clarke's column was also described by Stilling. They are more numerous about the lower extremity of the nucleus dorsalis, and they continue to appear below its termination in the lumbar region.
The ventral horn [columna anterior] of each lateral half of the grey figure is directed ventrally toward the surface of the spinal cord, pointing toward the antero-lateral sulcus. It contains the cell-bodies which give origin to the efferent or ventral root axons, and these axons make their emergence from the spinal cord along the antero-lateral sulcus. The ventral horns vary markedly in shape in the different regions. In certain segments each ventral horn is thickened later- ally and thus presents its two component columns of grey substance : the lateral horn [columna lateralis], a triangular projection of grey substance into the surrounding white substance, in line with or a little ventral to the line of the grey commissure; and the ventral horn proper [columna anterior], projecting ventrally. In the mid-thoracic region the lateral horn is relatively insignificant, and the anterior horn is quite slender; in the cervical and lumbar enlargements both horns are considerably enlarged.
The grey substance is not sharply demarcated from the white. In the blending of the two there are often small fasciculi of white substance embedded in the grey, and likewise the grey substance sends fine processes among the axons composing the white substance. Such processes or grey trabeculse are most marked along the lateral aspects of the grey figure and present there the appearance known as the reticular formation. The reticular formation of the spinal cord is most evident in the cervical region.
The large cell-bodies of the ventral horn as a whole are divisible into four groups, only three of which are to be distinguished in the mid-thoracic region of the spinal cord: - (1) A ventral group of cells, sometimes separated into a ventro-lateral and a ventro- medial portion, occupies the ventral horn proper, is constant throughout the entire length of the cord, and contributes axons to the ventral root, most of which probably supply the muscles adjacent to the vertebral column; (2) a dorso-medial group of cells, situated in the medial part of the ventral horn, just below the level of the central canal, gives origin to axons some of which go to the ventral root of the same side, but most of which cross the mid- line vi& the anterior white commissure, either to pass out in the ventral root of the opposite side or to enter the white substance of that side and course upward or downward, associating with other levels of the cord. Some of its axons terminate among the cells of the ventral horn in the same level of the opposite side; (3) a lateral group of cells, sometimes separated into a dorso- lateral and a ventro-lateral portion, occupies the lateral column or horn, and is best differentiated in the cervical and lumbar enlargements. Most of the axons arising from its larger cells are contributed to the ventral root of the same side, and such axons probably supply the muscles of the extremities. Some of those from its ventral portion are distributed to the muscles of the body-wall; the dorso-lateral portion is that part of the lateral column which persists throughout the cord, and is considered as supplying the visceral efferent fibers in the ventral roots. (4) an intermediate group, occupying the mid-dorsal portion of the ventral horn. Axons arising from its cells are probably seldom contributed to the ventral root, but instead course wholly within the central nervous system. Some pass to the opposite side of the cord, chiefly via the anterior and possibly the posterior white commissure, to terminate either in the same or different levels of the grey column. Others of longer course pass to the periphery of the cord, join one of the spino-cerebellar fasoicuh, and pass upward to the cerebellum.
Furthermore, there are scattered throughout the grey substance many smaller cell-bodies of neurons. These give rise to axons of shorter course, either commissural or associational proper. Of such axons many are quite short, coursing practically in the same level as that in which their cells of origin are located, and serve to associate the different parts of the grey sub- stance of that level. Others course varying distances upward and downward for the association of different levels of the grey column.
It is evident from the above that in addition to the various nerve-cells it contains, there is also to be found a felt-work of axons in the grey substance. Many of these axons are medullated, though not in sufficient abundance to destroy the grey character of the substance. The felt-work is composed of three general varieties of fibers: - (1) The terminal branches of axons entering from the fasciculi of the white substance and forming end-brushes about the various cell-bodies in the grey substance (partly medullated) ; (2) axons given off from the cells of the grey substance and which pass into the surrounding white substance either to enter the ventral-roots or to join the ascending and descending fasciculi within the spinal cord (partly medullated); (3) axons of Golgi neurons of type 11, which do not pass outside the confines of the grey substance (non-medullated). Some axons of any of these varieties may cross the mid-line and thus become commissural. In general all fibers of long course acquire medullary sheaths a short distance from then- cells of origin, and lose them again just before termination.
The white substance of the spinal cord. - The great mass of the axons of the spinal cord course longitudinally and form the thick mantle surrounding the column of grey substance. This mantle is divided into right and left homo- lateral halves by the anterior median fissure along its ventral aspect, and along its dorsal aspect by the posterior median septum, which is for the most part a connective-tissue partition derived from the pia mater along the line of the posterior median sulcus. The mantle is supported internally by interwoven neuroglia and white fibrous connective tissue, the latter, derived chiefly from the pia mater, closely investing it without.
The axons of the white substance belong to three general neuron systems:
- (1) The spino-cerebral and cerebrospinal system, which consists of axons of long course, one set ascending and another descending, forming links in the neuron chains between the cerebrum and the peripheral organs. The ascending axons of this system collect the general bodily sensations which are conve.yed to the cerebrum, the cells of which in response contribute axons which descend the cord, conveying efferent or motor impulses.
- (2) The spino-cerebellar and cerebellospinal system consists of conduction paths, one set ascending and another descending, which are connections between cerebellar structures and the grey substance of the spinal cord.
- (3) The spinal association and commissural system of axons which serve to associate the different levels and the two sides of the spinal cord and which are proper to the spinal cord, i. e., they do not pass outside its confines.
Both the first and second systems increase in bulk as the cord is ascended. The ascending axons of each system are contributed to the white substance of the cord along its length, and therefore accumulate upward; the axons descending from the encephalon are distributed to the different levels of the cord along its length, and therefore diminish downward.
The mass of the third system of axons varies according to locality. Wherever there is a greater mass of neurons to be associated, as there is in the enlargements of the cord, a greater number of these axons is required. Their cells of origin, being in the grey substance of the cord, contribute to its bulk and thus both the cells and the axons of this system serve to make the enlargements more marked. In the lumbar and sacral regions the greater mass of the entire white substance consists of axons belonging to this system. It forms a dense felt-work about the grey column throughout the cord. Necessarily this system contains axons of various lengths. Some merely associate different levels within a single segment of the cord; others associate the different segments with each other. Axons which associate the structures of the spinal cord with those of the medulla oblongata may be included in this system. Many of these axons cross the mid- line both in the grey and in the white substance to associate the neurons of the two sides of the grey column. For purposes of distinction, such as cross the mid- line are called commissural fibers, while those which course upward and down-ward on the same side are association fibers. Coursing in longitudinal bundles about the grey figure, the latter compose the fasciculi proprii or ' ground bundles ' of the spinal cord.