THE WHITE SUBSTANCE OF THE BRAIN. 27 The neuroglia, with its thousands of meshes infinitely divided, similarly forms around the nerve-tubules a closely-woven network, which sustains them and con 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 abun dant between the cylinder and the sheath, these grey fibrils assume the condition of white fibres, and, having at tained 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 sympa thetic, 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.
i. The first group of the commissural fibres runs in a perceptibly tranverse direction.
Originating in the midst of the plexus of cells Qf 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 egions 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, therefore, intimate connections between homologous regions of the cortical substance. The fibres them selves are thus, by reason of their relations with the grey elements, true commissures distributed every where 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 com issural 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.