The cortico-efferent fibers appear to proceed from all regions of the cortex, passing through the corpus striatum to converge in the cerebral peduncles. They arise as axis-cylinder processes from all the varieties of the psychic cells, large pyramids, small pyramids, and those without panicles. Most of them pass to the pyramidal tracts of the medulla and spinal cord. Collaterals from these fibers may pass either to the corpus callosum (C, Fig. 21) or to neighboring convolutions, thus becoming association fibers. They usually communicate by means of finecollaterals with the cellular elements of the corpus striatum.

The arcuate fibers (Fig. 20) may also arise from all varieties of the psychic cells. They are extremely numerous in the higher animals, sending their terminals and collaterals into every region of the cortex. The fasciculus arcuatus is almost entirely made up by their trunks. In adult forms their complication is so great as to make it beyond our power to disentangle them; it is therefore necessary to investigate them in the embryos of small mammals, where the whole may be brought within the limits of the microscopical field. The whole system offers a remarkable analogy to the fibers and collaterals found in the columns of the spinal cord. The shortest fibers. of this class are those belonging to the commissural cells of the deeper layers of the cortex, while some of the longer fibers are those of the spindle cells of the superficial layer.

The commissural fibers (Fig. 21) may pass to the opposide side either in the corpus callosum or in the anterior commissure. The fibers of the corpus callosum have long been noted for their fineness, and this is found to arise from the fact that many of them are collaterals from axis-cylinder processes. These may arise either from the arcuate fibers or from the efferent projection fibers. It is not known whether any afferent fibers send collaterals to the opposite side. Besides these, there are certain commissural fibers derived directly from the small pyramids of the cortex.

The idea so long prevalent that the fibers of the corpus callosum associated precisely identical areas of the cortex in opposite hemispheres must be abandoned. By examining the brains of small mammals it has been clearly demonstrated that a single fiber may be widely distributed throughout the opposite side. It is, in fact, doubtful whether, in any case whatever, such a union of identical areas really occurs.

The fibers of the cortex myelinate late, none receiving their medullary sheath until the ninth month of fetal life. Those of the central convolutions are the first to receive it, then those of the occipital lobe. The former appear to be connected with the motor functions, the latter with the functions of visual memory.

The Cerebellum.-If the new discoveries give promise. of leading to a rational knowledge of the elements of the cerebrum, what shall be said of the cerebellum, that organ which has caused so much conjecture and concerning which so many contradictory views have been expressed that Eckhardt said that it would be much more satisfactory if we knew nothing at all about it?

Anatomists since the time of Gall have been struck with its remarkable relations and have considered it an important organ. While they no longer assign to it the control of the procreative functions, yet its union, by means of clearly defined paths, with the spinal cord, the basal ganglia, and the cortex cerebri, make it probable that it performs some essential function; and the fact that it exists throughout the vertebrate series, increasing in size and complexity as we ascend the scale, indicates that this function is one connected with the higher activities of the nervous system. Yet, with all the elaborate researches that have been made, our knowledge of its histology has hitherto been meager.

Its cortex is usually described (Quain, Gray, Schwalbe, etc.) as composed of two layers-an outer clear gray or molecular layer, and an inner reddish-gray or granular layer. At the injunction of these two layers are found

d

e

d

A

Fig. 20.

FIG. 20.-Arcuate fibers (Ramón y Cajal). Diagram of an O-posterior section of one of the hemispheres of the rum to show that arrangement of the fibers associating ontal and occipital lobes. a, psychic cell of the frontal ending fibers to terminate in the occipital lobe; b, psychic the occipital lobe sending fiber to terminate in the l lobe; c, psychic cell of frontal lobe whose axis-cylinder Bs bifurcates; d, terminal tuft; e, collaterals to parietal J, corpus callosum cut across.

B

Fig. 21.

B

Fig. 24.

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FIG. 22.-Transverse section of a convolution of the d; Granular layer. C. White substance. a, Purkinje's cell, fron; molecular layer; d, descending terminals that surround the ce cells; f, large stellate cells of the granular layer; g, granules wi bifurcating at i; h, mossy fibers: j, neuroglia cell with rad diverging processes; n, scandent fibers; o, axis-cylinder fiber B erals to adjacent cells.'

ecular layer. B. Granular layer. C. White subcess to form a parallel fiber; d, d, d, Purkinje's cell Cocess of a cell of Purkinje.

FIG. 24. Olfactory bulb (Ramón y Cajal). Diagram sho A. Olfactory bulb. B. Mucous membrane. C. Hippocampal cell of the mucous membrane; b, glomerules; c, mitral cells; d, ugal fibers; f, amacrinal cells.

FIG. 26-Course of luminous impressions (Ramón y Cajal rigemina a, cone; b, cone bipolar; c, ganglion cell; d, e, its a filaments; g, cell of corpora quadrigemina; j, central ganglion p, q, a peripheral current terminating in an amacrinal cell, r.

Fig. 19.

FIG. 19.-Cells of first cerebral layer (Ramón y Cajal). A. Fusiform cells with two horizontal axis-cylinder processes, ac. B. Triangular cells. ante C. Polygonal cells with a single axis-cylinder process. D. Fusiform cell cere with a horizontal axis-cylinder process. E. Small cell with a bifurcated the f axis-cylinder process.

psychic cell forming with processes; d, axis-cylin4 cell of the spinal cord Ir root of the spinal cord a psychic cell with more nection of psychic cells

of cellulifugal process,

Central Canal

400 000.0000

lobel

cell

Fig. 23.

FIG. 23.-Longitudinal section of a convolution of the cerebellum. A. Mo stance. a, ascending axis-cylinder process of a granule; b, bifurcation of that pr seen in profile; e, granular terminations of the parallel fibers; f, axis-cylinder p

Fig. 25.

FIG. 25.-Cross section of the retina (Ramón y Cajal). A. Layer of rods a layer C. External plexiform layer. D. Internal granular layer. E. Internal pl lionic cells G. Layer of nerve fibers. a, rod; b, cone; c, cone nucleus; d, rod nuc g, h, i, j, k, ganglionic cells branching at different levels of the internal plexiform Muller; m, small or external horizontal cell; n, internal or large horizontal cell; 4 p, axis-cylinder process of cell m; q, terminal tuft of cell n; r, inferior ramification ification of the cone bipolars; s, t, u, v, w, amacrinal cells branching in distinct lay x, y, diffuse amacrinal cells; z, bistratal ganglion cell.

a

d Fig. 1.

ba

Fig 2.

FIG. 1.-Formation of the medullary canal. A. Section across the medullary groove. B. The groove closed to form the medullary canal. C. The formation of the neural crest. a, ectoderm; b, mesoderm; c, entoderm; d, notochord; e, neural crest forming rudiments of the spinal ganglia; f, medullary groove or canal.

FIG. 2.-The epithelium of the medullary groove of the rabbit (His). A. Before its closure. B. At the time of closure. a, a, a, columnar cells; b, b, b, germ cells.

Fig. 7.

Fig. 8.

FIG. 7.-Migration of neuroblasts (His). Young neuroblasts from the spinal cord of a hu spongioblastic network.

FIG. 8.-Bundles of neuroblasts (His). Young neuroblasts from the spinal cord of an embryo velum confine at a locality where the longitudinal and arcuate fibers are wanting.

FIG. 9.-The formation of nerve roots (Edinger). Section through spinal cord of a human em