FIG. 15.—NEUROGLIA CELLS OR THE FASCIA DENTATA; IN THE NEW-BORN RABBIT (method of Golgi). A, molecular layer; B, granular layer; C, layer of polymorphis cells; D. horn of ammon; a. neuroglia cell furnished with a descending appendage; b, another neuroglia cell; piroform; c. a cell more deeply situated; d, spider cell; e, fusiform neuroglia cell. (Ramon y Cajal.)
Varieties of Neuroglia .—The connective tissue cells are of many kinds, each probably exercising a special function. Ramon y Cajal has described and pictured a special kind of neuroglia cells for the gray and another for the white matter. In his description of these cells he has pointed out many interesting diversities of form, and probably also of function. He has also described particularly a special form of neuroglia cells which lie close to the blood vessels. These he calls perivascular cells, and they seem to have an important function in regulating the amount of blood that goes to a particular part of the brain. He has written so clearly and yet so concisely with regard to these that it seems better to cite his own words:14
Under the term neuroglia are included at least three kinds of cells,—those of the white brain substance, those of the gray substance, and the perivascular cells, which have been described by Golgi. The neuroglia cells of the white brain material are easily recognizable, being large and with rather prominent, smooth, and sharply outlined processes. As my brother seems to have shown, their object appears to be to furnish an insulating, or, at least, a badly conducting, substance to serve as an interrupter of nerve-currents. They certainly do not represent interstices of true nerve substance through which lymphatic fluid can conveniently find its way.
The neuroglia cells of the gray matter present a very special and highly characteristic appearance. They are of manifold form,—at times star-shaped, at times like a comet drawn out in length. These are the tall cells of von Retzius. They have very numerous prolongations, with a large number of short branched collaterals which give the whole cell the appearance of having feathers projecting from its periphery. These cells have been observed in two different conditions. One is that of relaxation, and the picture is that given above. The other is that of contraction, during which the cell body has more protoplasm in it, and the processes become shorter and thicker, and some of the secondary branches disappear entirely. These cells resemble, in certain ways at least, the pigment cells which occur in the skin of some animals. By means of their contractility, these pigment cells can stretch out their processes while in a state of contraction. It must be remembered that this form of neuroglia cells is most abundantly present in those parts of the brain in which it might be expected that a number of nerve currents would frequently come together. They occur, for example, with special frequency in the molecular layer of the cerebral cortex, where the bundle of pyramidal fibers, with their immense number of terminal nerve-endings, come in contact with one another.
FIG. 16.—NEUROGLIA CELL FROM THE SUBCORTICAL LAYER OF THE CEREBRUM FROM WHICH TWO PROCESSES GO TO A BLOOD VESSEL (Obersteiner).
FIG. 17.—NEUROGLIA CELLS FROM THE SPINAL CORD. Longitudinal section (Obersteiner).
The third form of neuroglia cells consists of those known as the perivascular cells. They are found only in the neighborhood of the capillaries of the gray matter and they send one or more firm prolongations to the outer surface of the endothelium of the blood vessels.
These processes are inserted in the walls of the blood vessels. Every capillary has thousands of these little pseudopod prolongations, and from the vessel the cell reaches out in a number of directions. The object of these cells undoubtedly is by contraction of the prolongations to bring about local dilatation of the blood vessels. This dilatation of the blood vessels causes greater or less intensity of the psychical processes in certain parts of the brain, because of the greater or less congestion of the circulation in a part which it produces.
With the exception of these last cells the object of the neuroglia cells is to insulate nerve fibrils and cells from one another. When the cells are relaxed, the passage of a nerve current is either entirely prevented or rendered much less easy than before. It is in this way that the true nature of intellectual rest is explained. Sleep—not only natural sleep, but also artificial narcosis, such as is produced by narcotics, hypnotics or hypnotization—is evidently the result of the same conditions.
During the state of contraction the pseudopod of the neuroglia cells are drawn in; that is to say, the protoplasm of the cells absorbs the processes, and so the true nerve cells and nerve fibrils which were separated from each other by the interposition of neuroglia come into contact. By this mechanism the brain passes from the condition of rest into one of activity. These neuroglia contractions may, particularly in certain parts of the brain, occur automatically. Often, however, they are produced by the action of the will, which, in this manner is able to influence the definite groups of neuroglia cells. As the result of this influence of the will the association of intellectual operations can be guided in various directions. The unusual course that the association of ideas sometimes takes, the flow of words and of thoughts at certain moments, the passing difficulty of speech, the recurrence of tormenting thoughts, the disappearance of expressions or ideas from the memory, even the increase of mental activity and of every kind of motor reaction as well as many other phenomena of intellection, can be satisfactorily explained on this hypothesis. It is only necessary to suppose that in certain parts of the brain the neuroglia cells are at rest, while at other parts they are in a condition of active contraction.
To put it all in a few words, the neuroglia cells of the gray substance of the brain represent an insulating and switching apparatus for nerve currents. They are an insulation apparatus when in a state of contraction, a switching and insulating apparatus when in a state of rest. It is to be remarked, then, that according to this theory the contraction of brain cells does not take place, as in Duval's theory, during intellectual rest, but, on the contrary, during the state of activity of the cerebral cortex. It is much more probable that the action of cells coincides with the active stage of intellection than that brain cellular activity—that is, contraction—should correspond with psychic rest.
The application of some of these theories enables us to understand just how short-circuiting may come about, how many of the curious phenomena of memory happen, and what are the effects, as well as the causes, of attention and distraction of attention and of diversion of mind. It is particularly the latter portion of Ramon y Cajal's theory, with regard to attention and the more or less voluntary though unconscious and usually indeliberate control of blood supply to various portions of the brain, that is of special interest. If the neuroglia cells, whose end plates are attached to blood-vessel walls, become over-contracted or lose their power of relaxation or of contraction, many of the curious phenomena of over-tiredness in neurotic conditions, and the lack of the power of concentration, and sufficient attention to things, can be readily understood. In a word, the theory enables us to translate many expressions that are vague and indefinite, from terms of mind into terms of the physical basis of mind—the anatomy and physiology of the brain.
While I have dwelt on Ramon y Cajal's theory, because for years it has been familiar, of course I must reëcho his own warning that it is, after all, only a theory. It presupposes an active interposition of the glia cells between the axon of one neuron and the dendrons of another. This cannot be demonstrated. A third theory of mental operations, then, has been suggested, and the English school, so ably led by Sherrington ("Integrative Action of the Nervous System," London, 1903) and McDougal ("Synapse Theory of Fatigue," Brain, 1910) has deservedly attracted wide attention. They contend that all the phenomena can be more simply explained without postulating the movement required for the Duval Theory or the glial activity of Ramon y Cajal's hypothesis. They consider that each nerve cell has, as it were, a certain potential energy which it sends forth in nerve impulses. These are transferred from neuron to neuron through the synapse. If what we might call, to borrow a figure from electricity, the voltage of the cell impulse be sufficient to overcome the resistance at the synapse, the impulse passes from neuron to neuron. In fatigue the potential energy of the cell is gradually dissipated. The impulses become feebler