MICROSCOPIC STRUCTURE.

UNIVERSITY

OF CALIFORNIA

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said to form the tissue; some which are more conspicuous are known as the cells of Deiters; they appear stellate in section and are found in the larger interstices between the nerve-fibres. Along the line of origin of the posterior roots the superficial neuroglia dips inwards towards the horn of grey matter, and expands within it to join the gelatinous substance of Rolando. This, as before mentioned, presents a transparent jelly-like appearance in the fresh condition, but in sections of the hardened cord it is finely recticular, containing numerous neuroglia-cells. Some of the bundles of nerve-fibres of the posterior root traverse this substance, and it contains many nerve-cells; these are mostly of small size, but here and there one of larger dimensions is met with.

In the constitution of the rest of the cornua the neuroglia takes part to a far less extent than in the substantia gelatinosa. It is true that it is prolonged both from this and from the white substance into the grey matter, and pervades the

fibres. In the first place are found multipolar nerve-cells, many of large size, scattered singly or occurring in groups throughout the grey substance. In connection with these and especially accumulated around the cells and cell groups, is an interlacement of the finest nerve-fibrils, which is derived partly from the ramified processes of the nerve-cells, and partly from the ramification of nerve-fibres or of collateral fibres which enter the grey matter from the nerve-fibres of the white columns. Occupying a considerable portion of the grey matter, are nerve-fibres, mostly of the medullated kind; these and the axis-cylinder processes of the nervecells traverse the grey matter in different directions, coursing for the most part in bundles which intercross with one another, and confer on the grey matter a spongy appearance. Hence it is sometimes known as the substantia spongiosa.

Some of these bundles, which are fairly constant in position in many sections of the spinal cord, have recently been described by Pal (see Bibliography).

The structure of the neuroglia is described in the part of this work which deals with General Anatomy (Vcl. I., p. 322). It is developed from the spongioblasts of His (see Embryology, Vol. I., p. 57), which at an early period of development extend from the central canal in the middle of the grey matter to the periphery of the cord (fig. 13). (The same holds good for other regions of the cerebro-spinal axis.) At a subsequent period their continuity from the central canal to the surface can no longer be recognized, although it is certain that even in the adult the cells which line

the central canal and ventricles of the brain extend a long and indefinite distance into the grey matter. It is probable however that it is by transverse division of the spongioblasts, preceded by division of their nuclei, that the neuroglia-cells are originally formed. At the same time it appears clear that although nerve-cells and neuroglia-cells are functionally very different, their origin is not so dissimilar as was at one time believed to be the case. It was taught formerly that the neuroglia is a form of connective tissue (which it resembles functionally), and it was supposed that its cells were developed from mesoblast, but of late years there has been much doubt cast upon its supposed mesoblastic origin, and the question is not yet fully decided. The researches of His have tended however to show that both the spongioblasts from which the neuroglia-cells are believed to originate and the neuroblasts which give origin to the nerve-cells are both formed from the (at first undifferentiated) cells of the neural epiblast (see Embryology, development of central nervous system). The investigations of Ramón y Cajal upon the development of these elements in the embryo chick and mammal further indicate that the distinction into spongioblasts and neuroblasts is not a fundamental one, for cells which from their shape and position would be classed amongst the spongioblasts of His may alter their character and by throwing out an axis-cylinder process become transformed into nerve-cells.

Small concentrically striated globules, termed corpora amylacea, are frequently met with in the neuroglia of the cord in man, as well as in many of the parts of the central nervous axis. They appear to be composed of proteid substance but, although long recognized, their mode of formation and their meaning are unknown.

DISTRIBUTION OF NERVE-CELLS IN THE SPINAL CORD.

In transverse sections of the cord it is seen that the nerve-cells are not equally distributed throughout the grey substance, but are arranged in definite groups, which occupy nearly the same relative position in successive sections. The groups are therefore the sections of longitudinal tracts of grey matter rich in nervecells, and these tracts are named the ganglionic or cell-columns of the grey matter. The longitudinal continuity of the groups can be seen in sections of the cord made parallel with its long axis and passing through the part of the grey matter where the groups occur. In such longitudinal sections it may also be observed that the cells tend, speaking generally, to be more extended parallel to the longitudinal axis of the cord the longer the segments of the cord, as indicated by the entering nerve-roots (Toldt). It may further be stated as a probable law, applicable at least to the cells belonging to the same group or column, that the longer the nerve-fibre which issues from a cell, the larger is the cell. Those segments of the cord from which the longest nerve-fibres issue by the anterior root have the largest anterior horn cells (Pierret).

Cell-column of anterior horn. Of these groups or columnar tracts of nervecells, the one which is most constant and contains the largest cells is found along the whole of the ventral part of the anterior horn where the nerve-cells lie among the issuing fibres of the anterior roots. There seems to be no doubt that many of these anterior or motor nerve-fibres are directly continuous with the axis-cylinder processes of nerve-cells of this group. Hence it is sometimes named the motor cell-column, but it is more generally known as the cell-column of the anterior horn. Its cells are in most parts collected into two groups, a lateral or ventro-lateral (fig. 14, b) nearer the lateral column of white matter, and a mesial (a) nearer the anterior column; in the cervical and lumbar enlargements there is a third, more deeply seated, dorsal or dorso-lateral group (b').

Of these several groups of cells into which the anterior horn cell-column is divisible, the one which is most constant along the whole length of the cord is the

Fig. 14.-SECTIONS OF SPINAL CORD IN LOWER CERVICAL, MID-DORSAL, AND MID-LUMBAR REGIONS. (E. A. S.) On the right side of each section the conducting tracts are indicated.

mesial group on this account it is thought to give origin to the nerves which supply the dorsal muscles of the spinal column. It is sometimes described as forming two sub-groups, viz.: a dorso-mesial and a ventro-mesial.

The ventro-lateral group probably gives origin to the nerves which supply the lateral and ventral muscles of the trunk, including the muscles of respiration. In the cervical region it is described as consisting of two parts, one more laterally situated, which gives origin to fibres of the spinal accessory, and another, more mesially placed, which is believed to give off the fibres of the phrenic nerve. In the lumbar enlargement it also tends to be sub-divided.

The dorso-lateral group is mainly developed in the cervical enlargement and lumbar enlargement, and has an evident relationship to the nerve-roots of the brachial and sacral plexuses. It also shows subordinate groupings, which probably have special connections with particular groups of the limb-muscles. For various conjectures regarding both these and other cell-groupings in the grey matter, the reader may consult the paper by Kaiser, which is given in the Bibliography.

Some of the cells which are nearer the anterior commissure send their axiscylinder processes through this commissure to the anterior white column of the other half of the cord: it is believed that these processes may be connected with nerve-fibres of the corresponding anterior root of the other side.

In some of the lower vertebrates it may be made out that there are variations in the size of the column of cells of the anterior horn in successive portions of the cord, the cells being more numerous opposite the points of entrance of the nerve-roots, the attachment of which to the cord is in them more localized than in man and higher vertebrates. In this way somewhat of a segmental formation of the column is indicated, and in some fishes and reptiles the enlargement of the group of cells and its enclosing grey matter is sufficiently marked to produce an external swelling opposite each nerve-pair. A similar segmentation is indicated in an early stage of development in all vertebrates, including man, by the fact that the cord is somewhat enlarged opposite each pair of nerve-roots.

Clarke's column.--A second very well marked group or column of large cells, which occupies in transverse section an area at the inner or mesial angle of the base of the posterior horn (fig. 14,ƒ), and appears cut off from the rest of the grey matter by a curved bundle of fibres derived from the posterior root, extends along the middle region of the cord from about the third lumbar to the seventh cervical nerve. This was termed by Lockhart Clarke the posterior vesicular column; it is usually known as Clarke's column, although the cells were first noticed by Stilling. It is best developed in the lower part of the dorsal (thoracic) region. From the fact that it is almost entirely confined to the thoracic region of the cord it was termed by Stilling the " dorsal nucleus." But although ceasing above and below the points mentioned, it is not altogether unrepresented in other parts, for groups of cells are found in a similar situation opposite the origin of the second and third sacral nerves ("sacral nucleus" of Stilling) and opposite the origin of the third and fourth cervical nerves (" cervical nucleus "); and elsewhere there are scattered cells of the same character in the same part of the section of the cord. The cells of this column, like most, if not all, the cells of the spinal cord, are multipolar, and their axis-cylinder processes tend towards the lateral column, where they are believed to form the direct cerebellar tract (Flechsig). The cells themselves are surrounded by a finc plexus of nerve-fibrils, probably derived from collaterals of the posterior root-fibres. They are of large size, measuring, according to Mott, in their longest diameter, which is directed longitudinally, from 40μ to 90μ, the largest being found in the lowermost part of the column. In the foetus and even in the new-born child they are much smaller (25μ to 30μ), but by the second or third year after birth they have nearly attained the same size as in the adult.

Lateral cell-column.—A third column of cells lies in the intermedio-lateral

tract (lateral cell-column, fig. 14, d). Like that tract, the ganglionic column it contains chiefly appears as a distinct formation in the thoracic region; in other parts the cells do not form so distinct a group, but they appear nevertheless to be represented to some extent along the whole cord. Its cells are bipolar or multipolar, and measure on an average 30μ in diameter, being considerably smaller than those of Clarke's column or those of the anterior horn. In the upper part of the cervical region a group of cells becomes distinct in a similar situation (lateral nucleus), and is traversed by the roots of the spinal accessory nerve.

Middle cell-column (Waldeyer).—This is formed of what in sections of the cord forms an ill-defined group of moderately-sized cells placed in the middle of the grey matter of the crescent. The cells are most distinct as a group in the cervical region, and also more numerous there, becoming fewer in number as Clarke's column becomes more evident. In the dorsal region they lie mostly at the side of Clarke's column, but further down they again move nearer the middle of the crescent. They are distinguishable right down into the sacral region. It is not known with what fibres they are connected, but fibres from the postero-lateral columns course amongst them, and it may be that, on the other hand, they give off nerve-processes to one of the conducting tracts (? to the antero-lateral ascending tract). They stain less deeply with carmine than those of the motor column or of Clarke's column.

Cells of posterior horn; solitary cells.-The cells of the posterior horn are not grouped very definitely, but for purposes of description they may be roughly sub-divided into-1, those at the base of the horn; 2, those near the middle of the norn; 3, those at the margins; and 4, those of the gelatinous substance of Rolando. They vary in size, some of the largest being found near the mesial margin of the horn; these often have a characteristic long curved process (comet-cell, Waldeyer); and even within the same group both large and small cells may be found intermixed. But in addition to the groups, a number of scattered cells are met with, distributed through the posterior horn. These cells vary much in form and size, but are for the most part spindle-shaped. They are usually spoken of as the solitary cells. The axis-cylinder processes both from some of these cells and from cells of the lateral cell-column pass towards the anterior horn and also towards the anterior commissure, and they are believed to give origin to the smaller fibres which issue with the anterior roots. Some of the axis-cylinder processes of these cells do not however leave the grey matter but are branched and their ramifications lose thenselves in the interlacement of fibrils which invests other cells. In the lamprey it has Deen shown by Freud that cells which appear to correspond with the solitary cells send their axis-cylinders into the posterior roots, and more than one observer has described a direct passage of the axis-cylinder process of one of the more deeply lying cells of the anterior horn through the grey matter into the posterior root, and thence past the ganglion into the mixed nerve without coming into connexion with any of the ganglion-cells.

The cells of the substance of Rolando were first noticed by Gierke. They are small round granule-like cells, closely packed and staining with difficulty. They appear to have been often taken for neuroglia cells, but, according to Gierke and H. Virchow, there is little doubt about their nervous nature.

Cells are occasionally found separated from the general mass of grey matter and lying out amongst the fibres of the white columns. These "outlying cells" have been described by Stilling and others, recently in detail by Sherrington.

It has been shown by Golgi that the nerve-cells of the central nervous system may be primarily classified under two heads, viz., 1, those with a long axis-cylinder process which becomes a medullated nerve-fibre; and 2, those with a short axis-cylinder process which may or may not become medullated, and soon ramifies and loses itself in neighbouring parts of the grey matter. Golgi is of opinion that the former are to be looked upon as "motor or

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