bodily actions, entailing rapid expenditure, it is the centre in which the demand for materials is indicated; and hence it becomes the regulator of the circulation, of the aeration of the blood, and of the visceral actions generally. Clearly, then, its co-ordinations are compound in comparison with those of the spinal cord — compound, because the impressions which afferent and centripetal nerves bring to it, are not only more numerous but also more heterogeneous; compound, because the impulses which it sends out are also more numerous and more heterogeneous; and compound, because it brings more involved acts into correspondence with more involved stimuli. The functions of the two still higher centres, the cere~ helium and the cerebrum, have now to be defined in "terms of the same nature. How shall we express them? Both of these great bi-lobed masses arise as buds out of tko 60 THE DATA OF PSYCHOLOGY. f originally almost-uniform cranio-spinal axis; and as tlicy enlarge, their distal portions grow more massive than their proximal portions, so that they end in being pedunculated. Each of them thus bears to the medulla oblong at a 3 a relation like that which the superior ganglion H, in the diagrammatic Fig. 4, bears to the inferior ganglion F; and we may not unreasonably infer that their functions are analogous to those hypothetically assigned to the ganglion H. That is to say, we may regard them as organs of doubly- compound co- ordination— organs which have for their common function, the re-combining into larger groups, and into countless different orders, the already-complex impressions received by the medulla oblongata; and which have the further function of so arranging the already-complex motor im- pulses issuing from, the medulla oblongata, as to form, those far more involved aggregate actions, simultaneous and successive, which, being adjusted to these involved im- pressions, achieve remote ends. The general truth of this definition may, I think, be safely assumed; since it is simply a statement in other terms, of what, in ordinary language, is called intelligent action; which habitually characterizes vertebrate animals in proportion as these centres are largely developed. Thus much being granted, there arises the further question — what are the respective parts played by these two great organs in this doubly-com- pound co-ordination? Much difference of opinion has long existed, and still exists, respecting the particular offices of these supreme ganglia; and especially respecting the office of the cerebellum. Without committing myself to it as anything more than a hypothesis, I will here venture to suggest a not improbable interpretation. The common function of the two being that of co-ordinating in larger groups and in various orders, the impressions and acts co-ordinated in the lower centres, we may fitly ask — are there any fundamentally distinct kinds of order in which impressions and acts may be co-ordinated? The obvious TEE* FUNCTIONS OP THE NERVOUS SYb'TEM. Gl answer is, tliat there are the two fundamentally distinct orders of Co-existence and Sequence. All phenomena arc presented to us either as existing simultaneously or as existing successively. If, then, these two highest nervous centres, which together perform the general function of doubly-compound co-ordination, take separate parts of this function, as, from their separateness, we must conclude that they do; we can scarcely make a more reasonable assump- tion than that the respective orders in which they co- ordinate compound impressions and acts, answer to the re- spective orders in which phenomena are conditioned. In.'1 brief, the hypothesis thus reached a, priori, is that thoj cerebellum is an organ of doubly-compound co-ordination in space; while the cerebrum is an organ of doubly-com- pound co-ordination in time. The a posteriori evidence, so far as I have examined it, appears congruous, both with this view of the general function of these centres, and with this view of their respective special functions. There is complete harmony between the hypothesis and the seemingly-strange facts that these centres may be partially destroyed without causing obvious incapacity, and that they may be wholly removed without destroying the ability to co-ordinate,. the. less. complex impressions and acts. As-^ suming, as we may fairly do, that the cells and fibres which subserve the more involved co-ordinations, are successively added at the surfaces of these centres as they develop, it is inferable that the superficial parts may be sliced off with the least-appreciable effects on the actions; and that the effects on the actions will become conspicuous in proportion as the slices destiny the parts nearer to the lower centres: and these are the results established by experiment. Besides finding, as the hypothesis leads us to expect, that these nervous masses are relatively large in all creatures capable of adjusting their involved and continuous actions to complex and distant environing- agencies; we also trace tiorne relation between the development of each and the C2 THE DATA OP PSYCHOLOGY. * peculiar capacities of tlie species. Tliere is, for instance, the fact that tlie cerebellum is unusually developed in birds of prey, which have to co-ordinate with great accuracy tlie relations of distance, direction, and complex form, as well as very precisely to co-ordinate the involved movements ap- propriate to these involved impressions. And there is, ou the other hand, the fact that the cerebrum predominates in creatures showing, like ourselves, the power of adapting, throughout long periods, concatenated compound actions to concatenated compound impressions.* Of course this classification of the functions of the nervous centres, as co-ordinations that are simple, compound,, and doubly compound, must be taken as merely approximate. No definite divisions can be made. The functions arise through increasing complications; and these general con- trasts become conspicuous only when we look at the facts in their main outlines. Here, however, where the * Let me here draw attention to papers in the Medical Times and Gazette, for "December ii and December 21, 1S67, in which Dr. Hugh! ings Jackson lias published some facts and inferences that quite harmonize with these interpre- tatioiiSj in so far as the common function of the great nervous centres is concerned.
It should be remarked that the above -proposed definitions, are, to a con- siderable extent, coincident with current conceptions. The cerebrum is generally recognized as the chief organ of mind; and mind, in its ordinary acceptation, means more especially a comparatively intricate co-ordination in time — the consciousness of a creature " looking before and after," arid using past experiences to regulate future conduct. In like manner the function, ascribed to the cerebellum in the foregoing paragraph, partially agrees with that which M. Floiirens inferred from his experiments. It differs, however, in two respects. It implies that the cerebellum is not an organ for the co-ordi- nation of motions only, or of synchronous motions only; but that it is also an organ for the co-ordination of simultaneous impressions, and for the co-ordi- nation of the synchronous motions in adaptation to the simultaneous impres- sions. And it further implies that not all simultaneous impressions and adapted synchronous motions are co-ordinated by the cerebellum; but only the doubly-con: pound ones, which have for their external correlatives the intricate combinations of attributes that distinguish objects from one another, and the more multiplied and varied localizations of objects in the space that extends tey oud the immediate limits and reach of the organism.
Tflfc FUNCTIONS OF THE NERVOUS SYSTEM. f)3 object is to give an idea of the principles of nervous func- tion in its successive stages of evolution, detailed qualifica- tions do not concern us, § 23. A few words are due to the functions of that sub- ordinate nervous apparatus, the structure of which we glanced at in the last chapter — the nervous apparatus pre- siding over the vital processes. It will suffice if we take the functions of the vaso-motor division of it as exemplifying the whole.
Each vaso-motor nerve,, having roots in both the cerebro- spinal system and the sympathetic system, conveys to all branches of the artery it accompanies, the impulses arising from the activities of the great nervous centres and muscles, as well as from the activities of the viscera. Probably the ordinary amount of disturbance propagated along each vaso- motor nerve, simply excites the muscular coats of the adja- cent artery sufficiently to maintain its due elasticity. But stronger disturbances produce marked alterations of its calibre: those brought by the sympathetic fibres being said to cause contraction; and those brought by fibres from the cerebro-spinal system being said to cause dilatation. Some of these changes have relation to actions going on in the part itself; and others to actions going on in the chief vital organs, or in the body as a whole. But all of them show us that by means of the vaso-motor nervous system, the blood-vessels are so regulated as to subserve general and local needs. One further fact belonging to this class may be added; partly because of its in- trinsic interest, and partly because it illustrates certain supplementary nervous functions not hitherto named. We f have already seen that, among its many duties, the medulla ollowgata, controls, through the medium of tli3' pneumogastric nerve, the action of the heart. So long \ as the disturbance conveyed toTiKe medulla oblongata, either from the periphery of the nervous system or from its great 6i THE DATA OF PSYCHOLOGY.
centres, does not exceed a moderate amount, the resulting1 waves of molecular change sent by it through the pneumo- gastric, do not interfere with the heart's action — perhaps en- force it. But when the medulla is excessively disturbed, the increased quantity of stimulus it sends, either diminishes the action of the heart., or stops it altogether: so causing arrest of the circulation and consequent insensibility. Noting., as we pass, that this is one of the most remarkable forms of that co-ordination which the nervous system, everywhere effects, since the arrangement is such that when the nervous system becomes abnormally active, and its chief centres sur- charged with blood, they themselves arrest the organ which propels blood to them; we have to ask how it happens that in this case the propagation of disturbance through a nerve checks action instead of causing it. The reply is that in addition to the systems of nerves which excite action., there is found to exist a system of nerves which diminish action — inhibitory nerves as they are called. Througii these it is alleged that the brain controls the spinal cord — restraining those reflex movements which, when connection with the brain is cut off, become so much more marked. And through one of these it is concluded that the medulla oblongata reins in the heart, when the cerebral irritation is excessive.
Be this as it may, the facts named illustrate the way in which the nervous system, while it co-ordinates the external actions, also co-ordinates those internal actions which make them possible. The reader has but to conceive that through other systems of nerves, other organs which absorb, secrete, excrete, &c., are similarly controlled, and he will under- stand sufficiently for present purposes, how demand_for materials and supply of materials are harmonized.
§ 24. In summing up the functions of the nervous system as thus formulated in terms of motion, it will be useful to observe the greater comprehensiveness of view we obtain, "by excluding the ordinary implications.
THE" FUNCTIONS OP THE NERVOUS SYSTEM. 65 When one part of a Zoophyte is touched, the contraction set up in that part slowly diffuses itself through the whole body. Two things are here to be noted. There is a propa- gation of disturbance through the nerveless sarcode of which the creature is composed,• for distant parts are eventually affected. There is also an increase of disturb- ance • for in successive moments the mass of tissue under- going change is greater. Thus the relatively-homogeneous substance of these simple animals, exhibits the two essential phenomena exhibited by the nervous system in all phases of its development: there is propagation of molecular motion, and there is a simultaneous augmentation of this mole- cular motion. Such essential phenomena grow more con- spicuous as the nervous system develops, partly because the changes set up become limited to narrow lines and small masses, and partly because the matter of which these are formed becomes distinguished by an increased degree of the general instability. Since, then, the functions of the nervous system as expressed in terms of molecular motion, are functions exhibited in a vague way by the undifferentiated tissue from which the nervous system insensibly arises; it is clear that by so expressing them we include alike their lowest and their highest forms, which we cannot otherwise do.
Moreover, only in these terms can there be given an adequate definition of fully -developed nervous functions. If we admit any subjective element, our definition becomes inapplicable to all those nervous actions which have no sub- jective accompaniments — which go on without feelings; and a conception of nervous functions which excludes those of organic life, cannot be a complete conception. On the other hand, the definition of nervous functions as consist- ing in the conveyance and multiplication of molecular motions, holds in all cases. It includes equally the conduc- tion of an impression made on a nerve of sense, and the excitement of chemical metamorphoses in a gland.
Go THE DATA OF PSYCHOLOGY. • The subdivision of this general function under the above- proposed heads of recipio-motor, libero-motor, and J.inijo- motor, has also the advantage of greater comprehensive* ness. jSTo word at present in use expresses the office which afferent nerves have in common, more specifically than the word afferent itself expresses it. Whether disturbance of its outer end produces in an afferent nerve a change causing a reflex contraction, or whether it produces a change causing what we call a sensation., is a circumstance of secondary import; as is proved by the fact that by use the last may become the first. The essential thing common to the two, is that molecular motion is propagated from periphery to centre. So, too, is it with the libero-motor functions. Whether, as in the ganglia of the sympathetic, the multipli- cation of communicated disturbance has no subjective con- comitant, or whether, as in the cerebrum, it has a subjective concomitant, there is in both cases a liberation of molecular motion; and this, being the common character of the changes in nerve-centres, must determine the definition of their common function. In like manner, all efferent nerves, whether conveying disturbances that set up contractions in muscles, or cause constrictions of arteries, or excite chemical transformations in glands, serve to direct the waves jof molecular motion — waves that are intrinsically alike in nature, though the results produced by them in the organs to which they are carried differ so widely, and though they are now associated with consciousness and now are not.
A more special view of the functions thus classed, dis- ] closes two essential facts. Considered as an agent for f generating movements, we see that the nervous system acts by liberation of successively-larger amounts of molecular i..-«-• - • * •vj...-•-• — -...'"motion in the centres successively disturbed. A very small •{ change at the outer end of an afferent nerve, sets up a frelatively-large quantity of change in some adjacent un- datable nerve-matter; whence the change, thus increased, is ? propagated to some internal ganglion; to be passed on by TOE FUNCTIONS OF THE NERVOUS SYSTEM. 67 it immensely multiplied as before; until there is unlocked an amount of disturbance capable of causing muscular con- tractions throughout the whole body.
Meanwhile these centres in which molecular motion is liberated,, are also the centres in which it is co-ordinated; and the successively higher and larger centres which evolve successively larger quantities of molecular motion, are also centres in which successively more complex co-ordinations are effected. Whence follows the general result that along with each farther development of the nervous system, enabling it to make all parts of the body work together more efficiently in simultaneous and successive actions, there goes an increased power of evolving the energy re-f quired for such larger aggregates of actions.
These principles we found to be well exemplified in the case which most nearly concerns us. It is needless to re-state the results so recently arrived at. One remark, however, may be added. In the functions of the suc- cessively-higher vertebrate centres, reaching their climax in the human being, we see well exemplified the law of development of functions in general (First Principles, Part II. §142). This progress from co-ordinations that' are small and simple to those that are larger and compound,; and to those that are still larger and doubly compound, is one of the best instances of that progressive integration of motions, simultaneously becoming more heterogeneous and more definite, which characterizes Evolution under all its forms.
CHAPTER IT,, CONDITIONS ESSENTIAL TO NERVOUS ACTION.
§25. Of these, the first in order is continuity..ofjoervgr. sujj>^,ance. Disturbance is not conveyed from end to end of a nerve that has been cut in two; and section of a nerve- centre similarly prevents the transfer of an impulse from one of the dissevered parts to the other.
The requisite continuity is not simply the continuity of unbroken contact: there must be continuity^. Q£ _moleculav cohesion.. Placing in apposition the two ends of a divided nerve., does not re-establish nervous communication. Even when,, after a cut, the surrounding flesh has been healed, it is long before the sundered nerve-threads re-unite so com- pletely that they transmit stimuli as well as before.
Further, there must be no destructipn of continiutyjby molecular disorganization. Without division of a nerve, and without injury of its sheath, there may result from disease a change which incapacitates the nerve-fibres — an atrophy, or a breaking-up by decomposition: the result being a de- rangement of those lines of peculiar nitrogenous molecules which receive and pass on the waves of disturbance.
§ 26. Xerve-structures, whether peripheral or cental, permanently disabled as they are by actual discontinuity, either molar or molecular, are temporarily disabled by dis- continuity of molecular equilibrium. Pressure is capable of THS CONDITIONS ESSENTIAL TO NERVOUS ACTION. GO producing re-arrangement of particles, even in substances that are simple and comparatively hard; as is shown by its power of altering the direction of diamagnetic polarity in metals. We may therefore expect that in substances of com- plex composition and little cohesion, pressure will readily cause the particles to change their relative positions. Hence there is no difficulty in understanding why nerve-substance, having a balanced molecular structure such that it is ever ready to pass when disturbed from one of its isomeric states to the other, may be so modified by pressure, even when small, as to be incapacitated for undergoing these alternate molecular re-arrangenients. Be this as it may, however, the fact is that one of the conditions to nervous action is.ab- In the case of nerve -trunks, demonstration of this general; truth is easy. A ligature round a nerve prevents a dis-f turbance set up at one end of it from producing any effect at the other end. Partial results of this nature are familiar. By external pressure on a limb, the conducting power of the nerves affected is much diminished.
That pressure on the centrally-seated tracts of fibres, hinders or arrests their actions, is shown in every case of paralysis. By a clot of blood that has escaped from a ruptured vessel, or by a quantity of lymph that has oozed through the walls of capillaries over-distended, bundles of fibres at the base of the brain, or in the spinal cord, are un- duly squeezed; and if afferent or centripetal fibres they cease to bring disturbances from the periphery, while if efferent or centrifugal fibres they cease to convey impulses to the muscles.
The like is true of nerve-centres as wholes. Indeed pres- sure appears to be a greater hindrance to their actions than to the actions of nerve-trunks. That certain forms of the abnormal arrest of nervous action called jsoma^are ^ue to excessive congestion of the blood-vessels oiflCe encephalon, seems possible; but as some question this interpretation wo 70 _ -i. There is, liowever, one cannot safely base an inference °^ ^^ of tte skull that conclusive piece of evicien, ^ areaj and leaves the causes indentation ^ ^ ^slj filled by the brain,. bse intruding on ^/P^^distnrbances sent to it .-.ops the functions of the b ^ ^.^ ^.^^ caU forth no ^^X^ means of a trephine, the - ° m0tieTortion of bone is cnt out, the brain, relieved from pre, at once ^ may be regarded as converse evidence. It esc * re allows t]ie aedon, and if the ^^.^Jt is inferable that no^l amount of nerve- ^ ^.f ^ if tlie cerebra «^J^ iy being subject to less of nerve-nbres amid ^^ y of moleCular change to £St HenC:the Jeemingly-anomalous fact that grea P of blood or ^reat local anemia caused by stoppage o7Sa cer brai arte^rv, causes convulsions. Such a result £ Tmicreated as the first result, before innutri- Z tSTtlll, though innutrition.ill afterwards Zse prostration or paralysis And this is the order in which the phenomena actually occur. ^ ^ IP; L trulof the peripheral nervous system. The afferen nerves of individuals who, though otherwise healthy, have L tissues, are often unduly impressible And ttere a« in,tinces of undue local impressibility which, I think, admit of this interpretation. It has been found that an arm rendered anasmio by unnatural constriction of its arteries, therebv reduced in temperature and beginning to atrophy, mav nevertheless have its afferent nerves affected by electric THE CONDITIONS ESSENTIAL TO NERVOUS ACTION.?1 discharges in an unusual degree.* Deficient pressure on the nerve-trunks appears a possible cause of this otherwise strange result.
§ 27. Proof that hejiJkept above a certain level is a con- dition to the maintenance of nervous action,, is difficult to disentangle from proof that the maintenance of nervous action depends on a due circulation of blood; for the one condition is usually but a concomitant of the other. Never- theless, there is reason to infer that a supply of free mole- cular motion is requisite, apart from a supply of nutritive materials.
The general fact that cold-blooded animals are com- paratively inactive, admits of the interpretation that their low temperature is due to their inactivity, as well as of the interpretation that their inactivity is a consequence of their low temperature; for the two act and react in such a way that neither can properly be called the cause of the other. Bat reptiles which remain quiescent in cold weather, and become active when they are warmed by the summer's sun, yield us good evidence. Though it may be alleged that, their greater activity arises from accelerated circulation and aeration of the blood, yet as the heart and lungs are set going by their respective nervous centres, we must infer that the warming of these nervous centres by external heat, is the initial change in these animals that have but little power of producing heat by their own actions. In support of this interpretation may be cited the converse fact. When active creatures, capable under ordinary conditions of generating enough heat within themselves, are exposed to conditions under which they lose heat faster than they make it, their nervous actions decrease, and they eventually cease to move. In hybernating mammals we see