By reason of the direction and special mode of group ing 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 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 fibres, it is not our business in this work to give a detailed anatomic description of each of them. We shall only mention that, whether we consider them in the posterior, middle, or anterior regions of the brain, we find them everywhere disposed in a similar manner, and directed towards their proper centre of attraction.
Thus the converging fibres of the posterior convo lutions follow a common postero-anterior direction ; those of the anterior the reverse ; while those of the superior convolution run from above downwards, and those of the inferior from below upwards.
Such are the special characters of the two great sys tems of fibres which constitute the white substance of the brain. These fibres run in a fixed direction, obey definite laws, and thus become the fundamental frame work which on the one hand binds together the homo logous regions of the two hemispheres, and on the other establishes the organic union between the peripheral regions and central ganglions of the brain.
This concentration of the converging fibres around the optic thalamus once effected, what becomes of these nervous elements, and how do they become lost in its mass ? From the moment in which they are implanted in the circumference of the optic thalamus, they become dispersed by degrees, insensibly taper away, and we then see them, in the form of whitish rectilinear fibrils, continue the converging direction of the primitive fascicles, and finally lose themselves in the midst of the different agglomerations of grey matter that they meet in their passage (centres of the optic thalamus and corpus striatum).
Thus it is that each region of the cortical periphery is united, by means of these white fibres, to a symmetrical region in this common ganglion of grey matter (the optic thalamus), and that these two foci of nervous activity, the cortical periphery and the central ganglion, like two electric piles united by a common wire, are intimately united into a single instrument.