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of nerve energy, and the production of sensory and motor activity, can be further considered capable of performing the function of thought, covering the whole variety of mental occupations. Attention has been directed to the recognized diversities of nerve cells, which are unipolar, bipolar, and multipolar, on the hypothesis that these diversities may point to differences of function so great as to provide what is required. But there is a total failure of evidence to substantiate this hypothesis. The differences among the nerve cells of the brain are differences in size, and in the number of the lines of communication taking rise from them. In accordance with the plan of arrangement everywhere recognized, the number of protoplasmic lines originating from a cell gives an index to the points of contact it has in the surrounding tissue, and thus to the part it may perform in the work of coordination or interaction. A small cell with only a single line or fibre proceeding from it, must be regarded as a cell conveying nerve stimulus in only a single direction, and to only a single destination. A bipolar cell in accordance with the same rule of interpretation, is a cell having communication in two opposite directions, and thus may be capable of transmitting stimulus by the one channel or by the other, besides which it is possible, so far as structure is concerned, that such a cell may receive stimulus from one direction and send it forth in an opposite, thus proving a centre of intercommunication. On the same plan, a multipolar cell, being of greater size, and having from five to ten fibres proceeding from it, holds a more important place in the manifold ramifications of cellular tissue, sending out stimulus in an increased variety of courses according to the number of the lines pertaining to it, and proving thus an intermediate station in communication with a variety of distinct centres. No observation yet directed upon the nerve cells has proved sufficient to establish all this, but the supposition is in strict harmony with what has been ascertained aa to the laws governing the action of the nerve system.
When, however, an attempt is made to proceed farther, selecting the largest cells as "mind cells,"* or cells generating thought and volition, there is a complete break away
• Hackers Evolution of Man, vol. i p. 129.
from evidence, and from the clear lines of interpretation already established. We are dealing with conjecture, not with science. There is no reason in the interests of truth to object to hypothesis in this region, any more than in another, for conjecture has often proved the handmaid of discovery, and it is likely to be so in a still larger degree. But an essential condition of this acknowledgment is, that conjecture do not claim any respect beyond what its nature warrants, and specially do not take to itself the name of science,—knowledge, or certainty. Beyond this, it must be recognized in every intelligent circle, that conjectures, like men of different character, are entitled to different degrees of respect, some to only a moderate and guarded measure, others to a very high degree, and some to very little indeed. In a case like the present, we can have no sure test for a provisional judgment entitled to regulate provisional procedure, other than the harmony of the conjecture with scientific knowledge already acquired as to the same region of existence. Judged by this test, the conjecture that the intellectual life of man is to be accounted for by the presence in the brain of myriads of thought cells, volitional cells, memory cells, imagination cells, and emotional cells, has little on which to claim a high degree of consideration. Its most obvious scientific difficulties are these two, that it implies a departure from the scheme of brain action scientifically established, and that it passes away from the scientific appliances employed to obtain knowledge of brain action. The real discoveries which have been made are the existence of sensory and motor apparatus, and the interaction of these two branches or divisions of the system. Beyond this, science has made no advance. The scientific appliances by which these discoveries have been reached are those available in post-mortem dissection, and in experiment under such exposure of the brain tissue as has been found compatible with functional activity of the organ. This conjecture of "mind-cells" does not either experimentally or logically connect itself with the recent advances in knowledge of the brain. The system of sensory and motor apparatus spread over the body for which the brain is the great central and governing organ does not under the scientific explanation of it already obtained, lead on by natural sequence to the conjecture of additional and greatly higher functions being assigned to the brain. Besides, the suggestion that place should be found in the brain for something more and higher than sensori-motor activity, does not come from any necessity which has arisen in the course of scientific observation. It is only because we know, in a manner quite different from that in which scientific knowledge of nerve and brain has been acquired, that man does observe, and reason, construct hypotheses and cherish expectations, contemplate rules of conduct and regulate his actions in accordance with them, that scientific inquirers, attempting to include the whole range of human powers, have felt themselves urged forward to seek an explanation of the characteristics of mental life which are the familiar facts of man's experience. The course of experiment has not brought them up to these facts; common acquaintance with them has pressed on scientific inquirers the need for dealing with them in order to make good the claim that science contains the explanation of all ex