The neuroglia, with its thousands of meshes infinitely divided, similarly forms around the nerve-tubules a closely-woven network, which sustains them and con

FIG. 3.-Diagram of the commissural fibres of the anterior regions of the brain. These form a series of curves one within another, the extremities of each of which plunge into the homologous regions of each cerebral lobe, 1, 1', 2, 2',-3 and 3'. They pass through the middle line, and at 4 and 4' give rise to the various appearances which the corpus callosum presents. 5. Commissural fibres of the inferior regions. These are curved in an inverse direction as regards the former, the convexity of each set being presented towards that of the other.

stitutes a uniting frame-work, and a veritable cement in the midst of which they are imbedded.

These thousands of nervous elements, thus constituted, emerge isolatedly from the different zones of the cortex, either directly, from the essential protoplasmic structure of the nerve-cells, or indirectly, by springing from the midst of the intercellular tissue, in the form of grey transparent fibrils, covered with an exceedingly delicate sheath. By degrees, in consequence of the

interposition of the myeline, which becomes more abundant between the cylinder and the sheath, these grey fibrils assume the condition of white fibres, and, having attained the constitution of complete nervous elements, pursue their way in a given direction, to be decomposed, in the last stage of their course, in the satellite masses of grey matter with which they are particularly connected.

The white nerve fibres, like true bonds of union, serve then merely to connect two regions of associated cells, and thus to establish between them a natural channel for the propagation of nervous activity. From this standpoint they are quite comparable to the nerve-fibres interposed between each of the ganglions of the sympathetic, and serving as a bond of connection between them.

This being understood, let us see how these fibrillary elements behave, what particular direction they follow, and what relations they establish with the different central regions.

Generally speaking the white cerebral fibres take two directions.

1. The first group of the commissural fibres runs in a perceptibly tranverse direction.

Originating in the midst of the plexus of cells of the cortical substance, after having travelled with their partners for a while they separate from them one by one, abandon their primitive direction, pass across the mesial line, and are finally lost in the homologous regions of the opposite hemisphere. (Figs. 3 and 4.)

They thus constitute the transverse fibres of the vault of the corpus callosum, to which those of the anterior white commissure are attached.

They individually present themselves as curvi

linear fibres in the form of an U; and the branches of this U plunge in a similar manner into the homologous regions of both hemispheres.

This collection of transverse white fibres, which, taken as a whole, forms a little more than half the white mass of the cerebral hemispheres, establishes,

FIG. 4.-Diagram of the commissural fibres on the level of the corpus striatum—1, 1'. Groups of transverse fibres, one within another, continuous with those in the previous figure-2, 2'. Grey substance of corpus striatum-3'. Groups of inferior commissural fibres--4, 4'. These curve into the shape of an S to accommodate the corpus striatum, which they help to limit externally.

therefore, intimate connections between homologous regions of the cortical substance. The fibres themselves are thus, by reason of their relations with the grey elements, true commissures distributed everywhere in infinite numbers. We may also say that

they constitute a very distinctly defined system of fibres, which by reason of its anatomical function may be in a general manner denominated a system of commissural fibres.

From a physiological point of view, on the direction of this order of fibres we might base the induction that it is by means of them that the regions of the two cerebral hemispheres are regularly anastomosed, cell to cell; and that they are, from this very fact, the true agents in the unity of action of the two cerebral lobes.

2. The second group of white fibres (converging fibres), no less important than the preceding, follows a rectilinear and sensibly converging direction. This system of fibres is entirely developed within the same hemisphere from which it is derived. It has nothing in common with the opposite hemisphere.

The fibres of which it consists originate with their fellows, the commissural fibres, at all points of the cortical periphery, in the midst of the plexus of cells, in the form of grey fibrils, and proceed along the common track for a certain time. Arrived at the level of the wall of the superior angle of the ventricles, the commissural fibres pass to the opposite side, while these insensibly approach one another like a series of rays radiating from the periphery of a hollow sphere, group themselves in the form of great white cylindroid fascicles placed in juxtaposition, and are inserted, like pins in a pincushion, around the anterior, middle, and posterior regions of the optic thalamus of the corresponding hemisphere.

By reason of the direction and special mode of grouping of the nervous elements which thus serve as a bond

of union between the peripheral and central regions of the brain, we cannot but recognise that, anatomically, they play the part of converging elements and con

FIG. 5.-Diagram of the converging fibres and their relations to the central grey ganglions-1. Converging fibres of the posterior convolutions of the brain-2. Converging fibres of the middle convolutions of the brain-3. Converging fibres of the anterior convolutions of the brain-4, 4', 4". Cortical periphery as related to the central grey ganglions-5. Optic thalamus-6. Corpus striatum-7. Anterior (olfactory) centre8. Middle (optic) centre-9. Median (sensitive) centre-10. Posterior (acoustic) centre11. Central grey region-12. Ascending grey fibres of visceral innervation-13. Grey optic fibres-14. Ascending sensitive fibres-15. Ascending acoustic fibres-16. Series of anterolateral fibres of the axis going to be lost in the corpus striatum.

stitute a system, as well defined as the former, which we have described under the name of converging fibres.

As regards the behaviour of each group of converging