terior horn. Their neuraxes, which originate either from the cellbody or from a dendrite, pass over into the posterior column. The stellate cells are supplied with dendrites, which either branch in the substance of Rolando or extend into the column of Burdach.

The gray matter contains, further, numerous medullated nervefibers, in part the neuraxes of the nerve-cells previously mentioned, and in part collateral and terminal branches of the nerve-fibers of the white matter with their telodendria; also supporting cells, known as neurogliar cells (to be discussed later), and blood-vessels.

The white matter of the spinal cord consists of medullated fibers, which are devoid of a neurilemma, of neurogliar tissue, and of fibrous connective tissue.

In each half of the cord the white substance, which surrounds the gray, is separated by the gray matter and its nerve-roots into

Fig. 306.-Schematic diagram of the spinal cord in cross-section after von Lenhossék, showing in the left half the cells of the gray matter, in the right half the collateral branches ending in the gray matter.

three main divisions or columns: The first division, lying between the anterior median fissure and the anterior horn, is the anterior column; the second, lying between the anterior and posterior horns, is the lateral column (since the anterior and lateral columns belong genetically to each other, the term anterolateral column is often used); and the third, lying between the posterior nerve-root and the posterior median septum, is the posterior column.

By means of certain methods it has been possible to separate the white substance into still smaller divisions, the most important of which may here be described.

In each anterior column is found a narrow median zone extending along the entire length of the anterior median fissure and con

taining nerve-fibers which come from the pyramids of the medulla. The majority of the pyramidal fibers cross from one side of the cord to the other in the lower portion of the medulla, at the crossing of the pyramids, and form a large bundle of nerve-fibers found in each lateral column, which will receive attention later. Some of the pyramidal fibers descend into the cord on the same side, to cross to the opposite side at different levels in the cord. These latter fibers constitute the narrow median zone, on each side of the anterior median fissure previously mentioned, forming the anterior or direct pyramidal tract, or the column of Türck. Between the direct pyramidal tract and the anterior horn lies the anterior ground bundle.

In the lateral columns are found a number of secondary columns, which may now be mentioned. In front of and by the side of the posterior horn in each lateral column lies a large group of nerve-fibers, forming a bundle which varies somewhat in size and shape in the several regions of the spinal cord, but which has in general an irregularly oval outline. These nerve-fibers are the pyramidal fibers, previously mentioned, which in the lower part of the medulla cross from one side to the other, and for this reason are known as the crossed pyramidal fibers, forming the crossed pyramidal columns. External to these columns and to the posterior horns, and extending from the posterior horns half-way around the periphery of the lateral columns, lie the direct cerebellar columns, consisting of the neuraxes of the cells of the columns of Clark, which have an ascending course. Lying just external to and between the anterior and posterior horns is a somewhat irregular zone, the mixed lateral column, containing several short bundles of fibers, the anterior of which are probably motor; the posterior, sensory. In the ventrolateral portions of the lateral columns, between the mixed lateral and the direct cerebellar columns and extending as far backward as the crossed pyramidal columns, lie two not well-defined columns, known as the ascending anterolateral or Gowers's columns and the descending anterolateral columns; the former are nearer the outer portion of the cord.

In the posterior column we distinguish a median and a lateral column. The former lies along the posterior median septum, and may even be distinguished externally by an indentation; its upper portion tapers into the fasciculus gracilis. This is the column of Goll, and it contains ascending or centripetal fibers. The lateral tract lies between the column of Goll and the posterior horn, and is known as the column of Burdach, posterior ground-bundle, or posterolateral column. It contains principally the shorter tracts, or bundles of longitudinal fibers connecting the adjacent parts of the spinal cord with one another.

Many of the nerve-fibers of the posterior column are the neuraxes of spinal ganglion cells which enter the spinal cord through the posterior roots. The cell-bodies of the spinal ganglion or sen

sory neurones are situated in the spinal ganglia found on the posterior roots of the spinal nerves. In the embryo they are distinctly bipolar, but during further development their two processes approach each other, and then fuse for a certain distance, forming finally single processes which branch like the letter T. In reality, then, there are two processes which are fused for a certain distance from the cell-body of each neurone. The peripherally directed process is regarded as the dendrite of the cell, and the proximal as the neuraxis passing to the spinal cord. The neuraxes enter the spinal cord through the posterior roots and pass to the posterior columns, where they divide, Y-shaped, into ascending and much shorter descending branches, from each of which numerous collateral branches are given off.

From the preceding account of the white matter of the spinal cord, it may be seen that it consists of longitudinally directed neuraxes arranged in so-called short and long tracts or columns. The neuraxes constituting the former, after a short course through the gray matter, emerge from it, and after giving off various collaterals, again penetrate into the gray matter, where their telodendria enter into contact with the ganglion cells. The long columns consist of

the neuraxes of neurones the cell-bodies of which are situated in the cerebrum or cerebellum, and of neurones the cell-bodies of which are in the spinal cord or spinal ganglia and the neuraxes of which terminate in the medulla or cerebellum. The nerve-fibers of the various columns give off numerous collaterals which enter the gray matter to end in telodendria. The collaterals of the posterior columns end: (1) between the cells of the gelatinous substance of the posterior horns; (2) in the columns of Clark; (3) in the anterior horns, these constituting the principal portion of the so-called reflex bundles; (4) in the posterior horn of the opposite side. The collaterals of the lateral columns pass horizontally toward the central canal, some ending in the anterior horn, others closely arranged near the columns of Clark, and some arching around the central canal, forming with the corresponding fibers of the other side the anterior bundles of the posterior commissure. The collaterals of the anterior columns form well-marked plexuses in the anterior horns of the same and opposite sides.

We have still to describe the two commissures. The anterior consists of: first, neuraxes from the commissural cells; second, dendrites from the lateral group of the anterior horn cells; and, third, the collaterals of the anterolateral column, which end in the gray substance of the other side of the cord. The posterior commissure is probably composed of the collaterals from all the remaining columns. The posterior bundle of this commissure comes from the posterior column; the middle, from the posterior portion of the lateral column; and the anterior, or least developed, from the anterior portion of the lateral column, possibly also from the anterior column. ·

In the gray commissure, nearer its anterior border, is situated the central canal of the spinal cord, continuous above with the ventricular cavity of the medulla and terminating caudally in the filum terminale. This canal is not patent in the majority of adults, being occluded from place to place. The canal is lined by a layer of columnar cells, developed from columnar cells, known as spongioblasts, lining the relatively larger canal of the embryonic spinal cord. In young individuals these cells are ciliated and their basal portions terminate in long, slender processes.

B. THE CEREBELLAR CORTEX.

In the cerebellar cortex we distinguish three general layersthe outer molecular, the middle granular (rust-colored layer), and the inner medullary tract.

Fig. 308.-Section through the human cerebellar cortex vertical to the surface of the convolution. Treatment with Müller's fluid; X 115.

The molecular layer contains three varieties of nerve-cells, those of Purkinje, which border upon the granular layer, the stel