The number of cells in the cortical substance must he estimated at many thousands. The following data are sufficient to make this clear.
In a space of cortical substance equal to i square millimetre, and of a thickness of th of a millimetre, 100 to 12o nerve-cells of various sizes have, on an average, been counted. If now we form in imagination an esti mate of the ratio of this small portion of the cortical substance to the whole, we shall arrive at an estimate of many thousands.
The colour of the cortical cells in fresh healthy brains is an amber yellow. They are apparently provided with a bright nucleus and with a nucleolus.* The internal structure of the cerebral cell individually considered, seems to grow more complex the deeper we proceed in the minute study of its elements.
Some years ago anatomists admitted of an investing membrane, and a contained substance,with a nucleus and a nucleolus, in the constitution of the cell ; later they discovered that its investing membrane was nothing more than the external layer of an amorphous proto plasm, surrounding the nucleus of the-cell, and prolonging itself externally in the form of multiple ramifications.
At the present day things seem to complicate them selves still further ; for I have lately discovered in some researches as yet unpublished, that this substance which we call the protoplasm of the cell is formed by a true tissue organized in a special manner ; that this tissue, consisting of very delicate fibrillae interlaced like the wickerwork of an osier-basket, has a tendency to agglomerate towards the nucleus of the cell, which thus becomes a true point of concentration ; that the nucleus itself is not homogeneous ; that it is endowed with a special structure, radiated in appearance; and that lastly the nucleolus, considered as the final expression of the unity of the nerve-cell, is in its turn divisible into secondary filaments.
Imagination is confounded when we penetrate into this world of the infinitely little, where we find the same infinite divisions of matter that so vividly impress us in the study of the sidereal world ; and when we thus behold the mysterious details of the organization of an anatomical element, which only reveal them.
selves when magnified from 700 to 800 diameters, and think that this same anatomical element repeats itself a thousandfold throughout the whole thickiess cf the cerebral cortex, we cannot help being seized with admiration ; especially when we think that each of these little organs has its autonomy, its individuality, its minute organic sensibility ; that it is united with its fellows; that it participates in the common life; and that above all it is a silent and indefatigable worker, discreetly elaborating those nervous forces of the psychic activity, which are incessantly expended in all directions and in the most varied manners, according to the different calls which are made upon it, and set it vibrating.
The nerve-fibres which represent the bonds union between the cortical substance and the central regions of the brain emerge from the midst of the plexus of cells. They all at first appear as isolated filaments, as a derivation, mediate or immediate, from the tissue proper to each cell ; then by degrees, as they proceed between the ranges of cells, they enlarge, their sheath thickens, the interposed fatty substance becomes more abundant, and they are insensibly transformed from grey to white fibrils. As to their mode of central arrangement they behave in a manner which we shall explain further on.
the elements which enter into the structure of the cerebral cortex, the uniting sub stance, the nenroglia as it is called, plays a primary part as regards its anatomical connections and physiological properties.
Imagine a web of extreme delicacy, radiating from the walls of the sheaths of the capillaries of the cerebral membranes, and immediately enveloping the cortical sub stance; its prolongations, like an infinite number of root lets, everywhere plunging into this substance. Imagine this delicate web, having resolved itself into a network of greater and greater attenuation, forming meshes more and more closely woven, in the midst of which the nerve cells are, to borrow the picturesque simile of Malpighi, embedded like pomegranate seeds, in the midst of the white fibrous tissue which encloses them on all sides.