_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. [Illustration: FIG. 12.--NEURON FROM THE OPTIC LOBE OF THE EMBRYO CHICK (after Koelliker). 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 {119} 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. [Illustration: 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.)]