Unconsciousness .—The unconsciousness due to apoplexy, or to a blow on the head, would be comparatively easy of explanation on the same theory. The hemorrhage would actually push certain neurons apart within the skull, or the intracranial pressure produced by it would keep them from making proper connections. A blow on the head may readily be supposed to jar neuronic terminal filaments so severely that it would be some time before connections could be made, and the injury might be serious enough to prevent certain cells from ever again coming in contact in such a way as to allow the passage of nerve impulses from one to the other. Concussion of the brain would, on this theory, mean that neurons were so shaken apart as to produce some confusion in their terminal filaments and consequent serious disturbances of consciousness, if not its complete loss, and corresponding disturbance of the power to move. In a word, this theory would seem to afford a reasonably satisfactory explanation for most of the extraordinary phenomena of mental life and, therefore, might also be expected to be applicable to the ordinary phenomena, though these are so elusive that it is difficult to satisfactorily apply theories to them.

FIG. 12.—NEURON FROM THE OPTIC LOBE OF THE EMBRYO CHICK (after Kölliker). The axon n runs toward the center, giving off in its course several collaterals. One of these, c, is much branched. (Barker.)

Tired States .—When fatigued, it becomes extremely difficult for us to follow a train of thought, especially if it is somewhat intricate. It becomes easy to forget things, even such as under ordinary circumstances would be readily remembered. Names are much more likely to be forgotten. Facts and, above all, dates, refuse to come as they do under normal conditions. Efforts in the direction of recalling details are eminently unsatisfactory. The command goes forth, but there is evidently hesitation about obedience. Other thoughts intrude themselves. Ideas come unbidden. The connection of thought is readily broken, and is hard to get at again. There may have been very little mental work, but somehow the fatigue of the general physical system is reflected through our central nervous system on the mind as well as the body. The early morning hours are the best for mental work, not, it seems, because the mind is fresher after its rest, but rather because the physical factors that are important for mental action are in good condition. Later they become disturbed by the fatigues of the day. The delicate cells of the brain become fatigued by sympathy with the somatic cells and it is harder to secure those nervous connections necessary for thought.

FIG. 13.—DEEP LAYER OF GIANT PYRAMIDAL CELLS OF THE POSTERIOR CENTRAL OR ASCENDING PARIETAL, CONVOLUTION OF A CHILD THIRTY DAYS OLD. a, axis-cylinder; c, collateral branch; d, long basilar dendrites; e, end tuft. (Ramon y Cajal.)

[Illustration: FIG. 14—PYRAMIDAL CELL OF CEREBRAL CORTEX OF MOUSE (after Ramon y Cajal).]

Voluntary Neuron Motion .—This theory of Duval's supposes that to some extent the neurons or nerve cells are possessed of voluntary movement. At least during certain states of the mind, they are moved and seem to have an inherent, if not quite voluntary, power of motion. There are many objections urged against the theory because of this neuronic motion. It has been said that the movement of neurons has been observed in certain of the Medusae. The observation has been doubted and it lacks confirmation. In higher animals, of course, the observation is impossible because an investigation of the nervous system for this purpose would necessarily bring about the death of the animal and the cessation of spontaneous mobility. Whether it occurs or not, therefore, is a theoretic problem. So many objections tell against Duval's theory that it is now only discussed because of its subjective value.

Neuroglia Theory.—Ramon y Cajal elaborated a second theory of explanation for the mechanism of the nervous system that has seemed to many authorities in brain physiology much more satisfactory than Duval's theory of the actual motion of the neurons themselves. The Spanish nervous histologist had made a special study of the neuroglia or connective tissue cells in the central nervous system. These are very small in size but very numerous. Ramon y Cajal suggested that it was because the terminal filaments of these neuroglia cells inserted themselves between the neuronic filaments, thus insulating one from another, somewhat as if an insulating plug were inserted between two portions of an electric circuit, that the interruption of nervous currents took place. This explanation is free from many of the objections urged against Duval's theory.

The small size of the neuroglia cells makes it easy to understand how movement may take place in them sufficient to bring about separation of neurons. It would not be surprising if they should be more or less actively contractile. Whenever they contract, neuronic filaments which they have been holding apart, come together so as to permit the passage of nervous impulses, if any are flowing at the time. When the neuroglia cells become fatigued or seriously disturbed, they refuse any longer to obey the will in any way, or at least gradually get beyond control, and in their relaxation becoming prolonged, push neurons apart. When a man is very tired it gradually becomes impossible for him to keep awake. This is partly because poisons, produced in the course of fatigue, exhaust the vitality of the neuroglia cells and also of the neurons, so that less energy is required to push these latter apart.

It is easy to understand that the neuroglia cells might well become affected by the various narcotics and intoxicants in such a way as to produce the phenomena of anesthesia and drunkenness. The rapid recovery from anesthetics seems to indicate that it is not neurons, or essential nerve cells, that are so deeply affected, but some extraneous, and less important, mechanism within the brain. The neuroglia theory explains this very well and does away with the difficulty. Certain curious phenomena of hysteria are easily explained on this theory. When there is anesthesia in a member because of hysteria, this anesthesia does not follow the distribution of certain nerves, but is limited by a line in the shape of a cuff drawn round the limb. This indicates that the trouble is not peripheral but central, and that owing to psychic disturbance, all the neurons that receive sensory impulses from a particular portion of the body are so affected by a psychic condition that they are no longer capable of receiving impulses from the periphery. The neuroglia cells in a particular area have passed from the control of the will and, relaxing themselves, have inserted their processes between the terminal filaments of neurons, thus preventing conduction.

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 till they cease to pass. This occurs in the state we usually experience as tiredness and in analogous states such as sleep, unconsciousness, narcosis and the like. Obviously this theory can be elaborated and applied parallel with the neuroglia theory except that here we are substituting synapse resistance for the hypothetical, undemonstrated action of the glial cells. But, as the latter seems a simpler process upon which to explain the various phenomena, especially to those not familiar with very recent developments in nervous histology and studies in nervous mechanism, and as it merely involves a question of the nature of the resistance and not of its site, I have used it for explanatory purposes without advocating either theory in the present state of our knowledge.