late cells, and the small cortical cells. The cells of Purkinje possess a large flask-shaped body (about 60 μ in diameter), from which one or more well-developed dendrites pass toward the periphery. The latter branch freely and the main arborization has in each case the general shape of a pair of deer's antlers. These dendrites extend nearly to the periphery of the cerebellar cortex. In a section horizontal to the surface of the organ the dendrites of the Purkinje's cells are seen to lie in a plane very nearly vertical to the surface of the convolutions, so that a longitudinal section through the latter would show a profile view of the cells. In other words, they have an appearance much like that of a vine trained upon a trellis. The neuraxes of the cells of Purkinje arise from their basal

(inner) ends and extend through the granular layer into the medullary substance. During their course they give off a few collaterals, which pass backward to the molecular layer and end in telodendria near the bodies of the cells of Purkinje. The stellate cells lie in various planes of the molecular layer. Their peculiar interest lies in the character of their neuraxes. The latter are situated in the same plane as the dendrites of the cells of Purkinje, run parallel to the surface of the convolution, and possess two types of collaterals. Those of the first are short and branched; those of the second branch at a level with the cells of Purkinje, and form, together with their telodendria, basket-like nets around the bodies of these cells. The small cortical cells of the molecular layer are found

in all parts of this layer, but are more numerous in its peripheral portion. They are multipolar cells with neuraxes which are not readily stained and concerning the fate of which little is known.

The granular layer contains two varieties of ganglion elements, the so-called granular cells (small ganglion cells) and the large stellate cells. The dendrites of the granular cells are short, few in number (from three to six), branch but slightly, and end in short, claw-like telodendria. Their neuraxes ascend vertically to the surface and reach the molecular layer. At various points some of them are seen to undergo a T-shaped division, the two branches then running parallel to the surface of the cerebellum in a plane vertical to that of the dendrites of the cells of Purkinje. Large numbers of these T-shaped neuraxes produce the striation of the molecular layer of the cerebellum. It is very probable that during their course these parallel fibers come in contact with the dendrites of the cells of Purkinje. The large stellate cells are fewer in number and lie close to the molecular layer, some of them even within this layer. Their dendrites branch in all directions, but extend principally into the molecular layer. Their short neuraxes give off numerous collaterals which end in telodendria among the granular cells.

The medullary substance is composed of the centrifugal neuraxes of the cells of Purkinje and of two types of centripetal neuraxes, the mossy and the climbing fibers. The position of their corresponding nerve-cells is not definitely known. The mossy fibers branch in the granular layer into numerous twigs, and are not uniform in diameter, but are provided at different points with typical nodular swellings. These fibers do not extend beyond the granular layer. The climbing fibers pass horizontally through the granular layer, giving off in their course numbers of collaterals, which extend to the cells of Purkinje, up the dendrites of which they seem to climb.

In the medullary portion of the cerebellum are found a number of groups of ganglion cells known as central gray nuclei. The nerve-cells of these nuclei are multipolar, with numerous, oftbranching dendrites and a single neuraxis.

C. THE CEREBRAL CORTEX.

The cell-bodies of the neurones of the cerebrum are grouped in a thin layer of gray matter, varying in thickness from 2 to 4 mm., -which, as a continuous sheet, completely covers the white matter of the hemispheres,—and in larger and smaller masses of gray matter, known as basal nuclei. In our account of the histologic structure of the cerebral hemispheres we shall confine ourselves in the main to a consideration of the cerebral cortex, the thin layer of gray matter investing the white matter.

From without inward the following layers may be differentiated in the cerebral cortex: (1) a molecular layer; (2) a layer of small pyramidal cells; (3) a layer of large pyramidal cells; (4) a layer of polymorphous cells; and (5) medullary substance or underlying nerve-fibers.

Aside from neurogliar tissue, we find in the molecular layer a large number of nerve-fibers, which cross one another in all directions, but, as a whole, have a direction parallel with the surface of the brain. Within this layer there are found: (1) the tuft-like telodendria of the chief dendritic processes of the pyramidal cells; (2) the terminations of the ascending neuraxes, arising mostly from the polymorphous cells; and (3) autochthonous fibers-i. e., those which arise from the cells of the molecular layer and terminate in this layer. The cells of the molecular layer may be classed in three general types-polygonal cells, spindle-shaped cells, and triangular or stellate cells. The polygonal cells have from four to six dendrites, which branch out into the molecular layer and may even penetrate into the underlying layer of small pyramidal cells. Their neuraxes originate either from the bodies of the cells or from one of their dendrites, and take a horizontal or an oblique direction, giving off in their course a large number of branching collaterals, which terminate in knob-like thickenings. The spindle-shaped cells give off from their long pointed ends dendrites which extend for some distance parallel with the surface of the brain. These branch, their offshoots leaving them at nearly right angles, the majority passing upward, assuming as they go the characteristics of neuraxes having collaterals. The arborization is entirely within the molecular layer. The triangular or stellate cells are similar to those just described, but possess not two, but three, dendrites. The triangular and spindle-shaped cells, with their numerous dendritic processes resembling neuraxes, are characteristic of the cerebral cortex.

The elements which are peculiar to the second and third layers of the cerebral cortex are the small (about 10 μ in diameter) and large pyramidal cells (from 20 μ to 30 μ in diameter). They are composed of a triangular body, the base of the triangle being downward and parallel to the surface of the brain, of a chief, principal, or primordial dendrite ascending toward the brain-surface, of several basilar dendrites arising from the basal surface of the cell-body, and of a neuraxis which passes toward the medullary substance and which has its origin either from the base of the cell or from one of the basilar dendrites. The ascending or chief dendrite gives off a number of lateral offshoots which branch freely and end in terminal filaments. The main stem of the dendrite extends upward to the molecular layer, in which its final branches spread out in the form. of a tuft. The neuraxis, during its course to the white substance, gives off in the gray substance from six to twelve collaterals, which divide two or three times before terminating.

Aside from the fact that the layer of polymorphous cells contains a few large pyramidal cells, it consists principally of (1) multipolar cells with short neuraxes (Golgi's cells) and (2) of cells with only slightly branched dendrites and with neuraxes passing toward the surface of the brain (Martinotti's cells). Both these types of cells are, however, not found exclusively in the layer of polymorphous cells, but may be met with here and there in the layers of the small and large pyramidal cells. The dendrites of the cells of Golgi are

projected in all directions, those in the neighborhood of the medullary substance even penetrating into this layer. The neuraxes break up into numerous collaterals, the telodendria of which lie adjacent to the neighboring ganglion cells. The cells of Martinotti, which, as we have seen, occur also in the second and third layers, are either triangular or spindle-shaped. The neuraxis of each cell originates either from the cell-body or from one of its dendrites, and