What we have before us as clearly admitted on all sides is that human life presents the common characteristics of organic life, and is subjected to the ordinary laws of organism. The problem with which we have now to deal in view of this admission is this,—How far do the functions of organism account for the universally recognized characteristics of human life?
In facing this problem there are not a few scientific inquirers who look upon the mere raising of it as a claim to include all that belongs to human nature within the realm of physical science. They have allowed themselves to regard the two things as interchangeable, and all their researches are in their view so involved in this identification, that they resent the challenging of it, as if it implied antagonism to science. But the scientific inconsistency of this is easily shown. That science must extend its investigations to human organism, admits of no doubt; that by means of this investigation all the phenomena of human life will be traced to organism, is the very thing to be proved, and until established on clear and full evidence is not to be regarded otherwise than problematic. If we are in this matter to be influenced by regard to the slow and difficult procedure in cases of much greater simplicity, we shall be guarded in the utterance of expectations; if we make account of the enormous difficulties to be encountered in arranging the facts to be explained, we shall be still more guarded; and if we remember that the practical demands of life must all be met day by day without waiting for science as an aid, it will not appear strange that the non-scientific thinker regards the whole scientific investigation as wide of the sphere in which questions of self-government are settled, even though this view seems to affirm, without knowledge of both sides, that there is a sphere belonging to human life into which science can not enter.
Still, it must be allowed that in the pathway of science nothing is to be foreclosed, and no area, whether large or small, is to be shut off on which the appliances of science can be brought to bear. Science can not exclude man from the range of investigation; can not on any warrant supplied by the conditions of its own procedure, draw a line within the circumference of nature, even though it may be constrained to allow that there are many things within nature of which it can offer no explanation.
That science has by recent research done much to explain phases of human activity previously unexplained, may be clearly shown. The modification of previously received opinion may be indicated thus,—that many forms formerly regarded as in the true sense voluntary, and so described in the life not only of man, but also of the higher animals, can be explained by the action of brain and nerve. This involves a considerable extension of the area of the mechanical in human action, and a considerable restriction of the area of the voluntary. In seeking to indicate roughly the form of this restriction, we may find enough for our purpose in the distinction between what we may describe as muscular action, and what we would more naturally denominate personal conduct. This contrast will serve throughout, as we proceed to estimate the explanations which science has reached in dealing with the characteristics of human life.
The proved superiority of brain and nerve in man affords an adequate explanation of his generally recognized superiority in the variety of the forms of his muscular activity. In mere muscular power man can not compete with the more powerful animals. His practical superiority is seen in manipulation and the vastly greater variety of occupations to which he can turn; and in the greater wisdom he has for self-government. Leaving meanwhile out of account comparative intelligence, we have only to consider the superior use man has of the general sensibilities of the body, and of the special senses of touch and sight; the greater variety of the joints and muscles in his body; the more complicated arrangements of his nerve system; and the relation of all these in a single economy, in order to perceive a distinct phase of the superiority of man, sufficiently accounted for by clearly recognized facts, anatomical and physiological. In a multitude of well-known forms of action, of which the mechanical arts afford illustration, man can do what can not be attempted by lower forms of organism.
Another step higher is taken by the advance of physiological science, involving an explanation of acquired aptitudes. The interaction of sensibility and motor activity has been shown to be great. A message conveyed along a sensory line is readily transferred to a motor line; the sense of touch becomes a natural guide to familiar forms of action; a form of sensibility may thus be connected with a given range of motor apparatus, just as the history of the blind illustrates how much more can be accomplished by aid of touch without sight, than is ordinarily achieved. By these means, what at first requires consideration and care (neither of which is accounted for by physiological explanations), comes at last to be done without deliberation, and with so much facility, that it does not seem to engage much attention. Physiological science thus accounts for a considerable amount of superior activity characteristic of man in his daily engagements. It must, however, be noticed that the explanation is not a complete one, inasmuch as the action of the sensory and motor apparatus referred to, presupposes consideration and care, that is intellectual and voluntary guidance commensurate with the initial difficulties of attainment, in order that the nerve system may be brought to accomplish what becomes possible afterwards by mere mechanical and chemical contrivance within the living organism.
Having thus briefly indicated the advances in knowledge of the working of our own organism gained by recent research, and the explanation thus afforded of much of the superiority manifest in human life, we come upon the grand difficulty of science,—How to account for intellectual superiority. It is obvious that animals give proofs of intelligence as well as men; and that the human brain has a marked superiority in the frontal region, to which intelligence is commonly referred, as it certainly is superior also in the back part of the organ, to which intelligence is not so commonly referred. But the pressing difficulty is this, to show how nerve cells, confessedly concerned with the development 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 coördination 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 as 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,"[CX] or cells generating thought and volition, there is a complete break away 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 existence, man included. The dilemma for this conjecture that the brain thinks and wills is serious. If the brain is capable of what is commonly named mental activity, all that science has demonstrated is susceptibility and motor activity, that is nerve impulse involving molecular and muscular action, and this carries no explanation of mental action.
In this, as in previous cases, it is better to take purely scientific statements concerning the structure and functions of the nerve cells, without regard to theoretical result. The following quotations will show what account has been given of the variety of appearance and position of the nerve cells. Professor Turner says, nerve cells are "the characteristic structures in the nerve centres, are susceptible to impressions or nervous impulses, and are the texture in which the molecular changes occur that produce or disengage the special form of energy named nerve energy, the evolution of which is the distinctive mark of a nerve centre."[CY] "The central extremities of the nerve fibres lie in relation to, and are often directly connected with the nerve cells." From opposite points of the surface of the bipolar cell "a strong process is given off, which is directly continued into a nerve fibre.[CZ]" When we pass next to multipolar cells, we have the following explanations. "In many localities they present characteristic forms. In the gray matter of the spinal cord, more especially in its anterior horn,[DA] they give rise to numerous processes, and have a stellate or radiate form. In the gray matter on the surface of the convolutions of the cerebrum they are pyramidal in shape; the apex is directed to the surface of the convolution, the base towards the white matter; the processes arise from the base, apex, and sides of the pyramid."[DB] "The processes which arise from a multipolar nerve cell as a rule divide and subdivide as they pass away from the body of the cell, until at last they give rise to branches of extreme tenuity. These branching processes apparently consist exclusively of cell protoplasm, and have been called protoplasm processes. Gerlach has described the protoplasm processes of the multipolar nerve cells of the brain and spinal cord as forming an excessively minute net-work, from which minute medullated nerve fibres arise."[DC] "From the observations of Lockhart Clark, Arndt, Cleland and Meynert, there can be no doubt that the pyramidal nerve cells vary in relative size and in numbers in the different layers of the gray cortex, and that the largest sized pyramidal cells lie in the third and fourth layers."[DD] "Large pyramidal cells are found in the frontal lobe in considerable numbers," but it is added, "there is no difficulty in recognizing in the occipital lobe" (the back region of the brain) "a small proportion of cells quite equal in magnitude to the largest cells of the frontal lobe."[DE]
From these statements, it is easy to judge what value can be attached to the conjecture that multipolar or pyramidal cells are to be regarded as "mind-cells." The result may be summarized thus; (1) The larger cells are invariably distinguished by the number of fibres given off, or the lines of communication they have with other parts of the tissue; (2) As to distribution of these cell-fibres appearing in the brain, it is found that as the cells themselves are in the lower strata of the gray matter, the majority of their fibres stretch downwards, the mass of the nerve fibres thus communicating with the organism; (3) These pyramidal cells are not peculiar to any part of the brain, and they do not belong to the brain alone, but are found in the gray matter of the spinal cord, and also of the sympathetic system, which provides for the action of the heart, lungs, and other vital organs. All these characteristics are adverse to the conjecture that for the larger sized cells a claim can be made assigning to them distinctively intellectual or mental functions.