These same neuroglian filaments thus envelop the nerve-cells with their inextricable web, just as the cellular tissue, for instance, surrounds the lymphatic ganglions; and thus is constituted that immense network of connective-tissue everywhere continuous through out the nervous system, from the spinal-cord to the brain, serving to support all the individual anatomic elements, and by its softness, delicacy, and extreme divisibility, forming for them a veritable cement which solders them together, uniting them in a perfect unity, while at the same time it serves them as a means of nutrition.
This network of the neuroglia presents, moreover, a very remarkable arrangement in the cortical substance. Not only does it incorporate itself with each particular cellular element, and with the nerve-fibres, serving them in a manner as a mechanical protection, but besides this it plays an analogous part as regards the nervous elements of the cortex as a whole.
Thus, if we examine the superficial layers of each convolution in the sub-meningeal regions, we perceive that the neuroglia forms, immediately above the last zones of nerve-cells, a thin areolar layer of an appre ciable thickness, constituting a sort of spongy cushion everywhere continuous. It is, in fact, a means of protection and isolation which, as it were, filters the nutrient juices flowing from the meninges, and prevents the plexus of nerve-cells, thus protected by this variety of natural epithelium, from coming nakedly into direct contact with the capillaries of the meningeal mem branes. (See Fig. 1, A.) The capillaries similarly play a very important part in the structure of the cortical layer. They represent the most important of the nutritive elements that bring to the nerve-cell the pabulum vita' necessary for the maintenance of its daily activity.
Radiating in the form of little canals from the deep surface of the meninges, they plunge like very delicate rootlets into the midst of the nervous elements, dividing themselves into a network of greater and greater tenuity, and their meshes, becoming closer, pass around the circumference of each group of cells to form areolx extremely rich in blood-vessels. It is a very remarkable fact that the 3e same capillaries, which directly penetrate the texture of other organs and come in contact with the active elements which it is their task to nourish have a special arrangement as regards the nervous ele ments. A peculiar adventitious sheath, in fact, surrounds their walls, like a muff, for a part of their circumference, isolating them from the nervous elements themselves; so that it is but mediately that these obtain their share in the processes of nutritive life.
To sum up, the structure of the cerebral cortex may be reduced to the following propositions : The cortical substance is composed of fixed ana tomical elements, distributed in an infinite number throughout its mass—the cerebral nerve-cells.
These lie in juxtaposition and enter into close relation ship one with another. They are further arranged in regularly stratified zones one above another ; and they form by their prolongations a tissue which is everywhere continuous, and thus produces unity of action between this multitude of isolated elements.
As physiological deductions, the following conse quences spring from the considerations previously stated.
The cortical substance represents an immense instru ment constituted of nervous elements, each gifted, it is true, with its proper individuality, and yet intimately connected one with another.
The series of cells arranged in stratified zones, and the connections of the different strata communicating one with another, imply the idea that the nervous activities of each zone may be isolatedly evoked ; that they may be associated one with another ; that they may be modified in passing from one region to another, according to the nature of the intermediary cons brought into play ; that, in a word, nervous actions, like vibratory undulations, must propagate themselves through one point of contact after another, following the direction of the organic substance that underlies them, either transversely or vertically, from the super ficial to the deep regions, and vice versa.
On the other hand, as regards the physiological signi ficance of certain zones, and the relation of each to the phenomena of sensation and motion, we may, by the laws of analogy, suppose that the sub-meningeal regions, principally occupied by the small cells, may be specially connected with the phenomena of sensation, while the deeper regions occupied by groups of large cells may be considered as the most important regions that give rise to motor phenomena.
In fact, in applying to this question the data which are acquired from the study of the spinal cord, and which show us, for example, that where there are small cells (posterior horns) the phenomena connected with sensi bility take place, and where, on the contrary, there are large cells (anterior horns), motor impulses are deve loped—it is rational, I assert, to see physiological where there are morphological analogies, and to consider the sub-meningeal regions of the small cells of the cortical substance as being the natural sphere of the diffusion of general and special sensation, and therefore the great common reservoir of all the united sensibilities of the organism. And on the other hand, we may consider the deep zones as being the centres for preparing and emitting motor stimuli.
This mode of considering the cerebral cortex, in its totality, as an instrument essentially sensori-motor, conceived on the same plan as that of the sensori-motor instruments of the spinal cord, will permit of the formu lation of certain new propositions on the subject of the evolution and intra-cerebral transformation of the pheno mena of sensibility into motor reaction.