BOOK I. $49. make up the nervous organism, are the causes of changes in consciousness. It is, then, these changes Combination in the nervous organism which we must keep in view, of physiology with meta- abstracting from their subjective aspect, in which physic. they are portions of space and time filled by feelings of sight and touch; and these changes it is which we must imagine as producing the changes or movements in those states of consciousness which have been analysed and classified in the preceding chapter. It is obvious, then, that the enquiry has two distinct branches, which must be pursued separately and then brought to bear upon each other; that only in their combination, after each has been separately pursued, is the result, the knowledge of the mode in which one causes the other, attainable; we must endeavour to ascertain and analyse each series separately, in order to determine what phenomena, what states, what movements, in the one series are the causing moments, the supporters of corresponding phenomena, in the other. Metaphysical analysis of states of consciousness, and processes of consciousness from state to state, must therefore not be disregarded, but go hand in hand with physiological analysis, the one supplying hints or hypotheses to the other, according as either happens to have made the greater progress. In the first place, then, I will turn to the nervous organism, and endeavour to exhibit as well as I can what I have been able to gather from the writings of physiologists as seemingly conducive to the purpose in view. $50. The nervous organism. § 50. 1. The nervous organism contains two chief members, nerve cells and nerve tubes. A single cell in connection with a single tube, or a tube connecting two cells, would be the lowest form of the organ ism, and the highest is no more than an aggregation of this. Where there are a great number of cells gathered together, the appearance of the mass in which they are found is grey; where there is a mass of tubes, its appearance is white; hence grey substance is used sometimes to signify cellular substance, and white tubular. The tubes serve as conductors of movements to and from cells, that is, both from the cells in central organs to the nervous apparatus at the periphery of nerves of sense and muscular motion, and from the periphery to the cells in the central organs, and also between the cells in the central organs themselves. Every cell is furnished with one branch or more running out from it, which in some cases is continued so as to become a tube going towards the periphery, in others connects the cell with other cells; but in many cases the branch seems to stop short, and cannot be traced into any other cell or tube. Provision seems made here for growth and development of the nervous organism, especially of its masses of cells, by the completion of the communication between them by means of these branches from the cells. The branches running out from cells appear to be inchoate tubes, which may run in time from cell to cell or from cell to periphery. 2. The two kinds of effects produced (to describe them by words of consciousness), the two kinds of services performed, by this organism are perception and muscular motion. But for the present I will put aside all those kinds of perception and action which seem to depend upon the operation of cells upon cells in the central organs, and consider only those which require the activity of cells and tubes going to the periphery; that is to say, presentative BOOK I. $ 50. The nervous organism. BOOK I. § 50. The nervous organism. perceptions and muscular motions. In other words, I will consider first only the nerves, in connection with their central cells and peripheral apparatus ; and this is the first division of the nervous organism, which is generally described as consisting of nerves and brain. 3. In the first place, there appears to be no difference between the motor and sensor nerves in respect either of their composition or of the way in which the movements are propagated in them. When the movement begins at the periphery of any nerve and is propagated to the central cell or cells, there arises perception. When the movement begins at the central cell or cells and is propagated to the periphery, there, supposing the nerve to be distributed to a muscle, arises muscular motion. The movement in the nerve is the same in kind in both cases; the difference of the effect is due solely to the sensor nerves being exposed, by means of their peripheral apparatus, to receive certain stimuli from without, while the motor nerves are so distributed as to communicate to the muscle the stimulus which they receive from the central cells. But this very distribution of nerves to muscles appears to communicate a movement from muscle to centre, which produces perception, the perception namely of the tension of the muscle, which belongs to what we call the muscular sense. See on this point Prof. Brown-Séquard's Phys. of Nervous System, Lect. i. p. 9-10. Also on the subject of this whole paragraph Mr. G. H. Lewes' Phys. of Common Life, Chap. viii. All nerves therefore are sensor, and all would be motor also if they were distributed to muscles. Those distributed to muscles are those which go up into the spinal column BOOK I. CH. III. by the anterior roots. See on this doctrine, and the new support derived for it from Prof. Du Bois Reymond's "Negative Stromschwankung," Dr. Funke's The nervous Lehrbuch der Phys. § 162. vol. i. p. 841-4. 4th edit. of 4. The differences between the several groups perceptions, such as seeing, hearing, touch, systemic sensations, and so on, as well as the minor differences within each group, depend upon the mode of arrangement of the nervous apparatus or expansion at the peripheral extremity of the nerve, which in each case is adapted to receive a certain class of stimuli. Every nerve has in consequence its own specific group of perceptions; and the stimulus to which the peripheral apparatus of each nerve is adapted is called the adequate stimulus of that nerve; such are the ether undulations to the optic nerve, and the air undulations to the auditory nerve. Other stimuli than these applied to any nerve produce perceptions of the same specific kind as the adequate stimulus produces, but perceptions not capable of such minute modifications as those which belong to perceptions produced by the adequate stimulus. The nerve with its peripheral apparatus can act only in certain peculiar ways, and transmit only certain peculiar movements, whatever may be the stimulus applied to it; and the perceptions produced must be regarded as the result of the mode of movement proper to the nerve itself, combined with the action of the stimulus which sets that movement more or less perfectly on foot. § 51. . The questions which have the greatest interest in physiology when treated in connection with metaphysic are of two classes, those relating to the nature of the movements in nerve and brain $ 50. organism. $51. The nerve movements. BOOK I. $51. The nerve movements. which produce or support states of consciousness, and those relating to the assignment of the several portions of the nervous organism, and the several kinds of nerve movement, to the several groups of the phenomena of consciousness, groups which are distinguished by metaphysical analysis. As to the first of these classes of questions, we are able at present to speak only in the most general terms; the specific kinds of movement which take place in the nervous organism have not been determined as yet by physiologists. The most recent theory, that of Dr. Pflueger, may be read at the end of his "Untersuchungen über die Physiologie des Electrotonus." But without overstepping the modesty of the most general terms, we may distinguish, in accordance with what has been said at the end of the preceding §, two sources of movement in the processes which support perception and muscular motion; the first, of movement belonging to the nervous substance itself, the second, of that received by it from a stimulus. And the first movement, belonging to the nervous substance itself, must again be distinguished into movements of action and reaction of particles along its whole length; movements which exist prior to the reception of any stimulus, and which, on its reception, combine with it into a total movement which supports the perception or the muscular motion. The stimulus may come either from the central parts of the nervous organism, or from the forces acting upon the peripheral extremities of the nerves, In the first case the stimulus would itself be a nerve movement acting upon a motor or a sensor nerve, in consequence of some previous movements in the central organs; and this would include all cases of |