06 December 2012

"Integrative Action of the Nervous System": Notes to Charles Sherrington's Important Lectures


Charles Sherrington, The Integrative Action of the Nervous System, 1 ed., (London, Oxford University Press 1906).

"It is then around the cerebrum, its physiological and psychological attributes, that the main interest of biology must ultimately turn".
      
- Charles Scott Sherrington, 1906

About the author and book see here for an early precis of everything below.

Unanswered questions - that I wish someone would answer

1. Why are the prefaces from each of the editions of "The Integrative Action of the Nervous System" different and what do those differences represent?

2. Take note of the way that knowledge from medicine, zoology, histology and pathology is integrated into Sherrington'sargument. Explain the "generality" in historical terms.

3. How are metaphors used throughout this work. What do they do? What historical contexts and realities do they reveal? This may require a close reading and some creative inferences. Consider:

“The direction of the stream of liberation of energy along the pattern of the nervous web varies from minute to minute.  The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class, or of a rhythmic class, and then back to one of the previous groups again, and so on.  The conductive web changes its functional pattern within certain limits to and fro.  It changes its pattern at the entrances to common paths.  The changes in its pattern occur there in virtue of interaction between rival reflexes, "interference".  As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surface causes in the central organ a shift of functional pattern at various synapses.  The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details of connection are, at the entrance to each common path, mutable.  The gray matter may be compared with a telephone exchange, where from moment to moment, though the end-points of the system are fixed, the connections between starting points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction.” (p. 233)

4. Sherrington writes, “It is widely and wisely held that natural knowledge pursues the question "how" rather than the question "why".  The "why" involves a judgment whose data lie so beyond present human experience and comprehension that self-abnegation in regard to the desire to attempt it is not only prudent, but to the unbiased judgment a necessity” (p. 235). Does such a thoughtful remark bode well for the future direction of the neurological turn? Are these observations still true?

5. Sherrington states that “Professor James writes: "our natural way of thinking about these coarser emotions (e.g. grief, fear, rage, love) is that the mental perception of some fact excites the mental affection called the emotion, and that this latter state of mind gives rise to the bodily expression.  My theory, on the contrary, is that the bodily changes follow directly the perception of the exciting fact, and that our feeling of the same changes as they occur IS the emotion.  Every one of the bodily changes, whatsoever it be, is FELT acutely or obscurely, the moment it occurs.” How does Sherrington’s theory of emotions compare with those of other philosophers, psychologists, and physiologists?


6. Is this text "bio-medicine?"



Lecture I: The Simple Reflex

(p. 1)
The cell theory at its inception depended for exemplification largely on merely morphological observations; just as these formed originally the almost exclusive texts for Darwinian doctrine of evolution.  But with the progress of natural knowledge, biology has passed beyond the confines of the study of merely visible form, and is turning more and more to the subtler and deeper sciences that are branches of energetics.

(p.2)
The physiology of the nervous system can be studied from three main points of view.

In the first place, nerve-cells, like all other cells lead individual lives, -- they breath, they assimilate, they dispense their own stores of energy, they repair their own substantial waste; each is, in short, a living unit, with its nutrition more or less centred in itself.  Here, then, problems of nutrition, regarding each nerve-cell and regarding the nervous system as a whole, arise comparable with those presented by all other living cells.  Although no doubt partly special this specially differentiated form of cell-life, these problems are in general accessible to the same methods as apply to the study of nutrition in other cells and tissues and in the body as a whole.

Secondly, nervous cells present a feature so characteristically developed in them as to be specially theirs.  They have in exceptional measure the power to spatially transmit (conduct) states of excitement (nerve-impulses) generated within them.  Since this seems the eminent functional feature of nerve-cells wherever they exist, its intimate nature is a problem co-extensive with the existence of nerve cells, and enters into every question regarding the specific reactions of the nervous system.  This field of study may be termed that of nerve-cell conduction.

But a third aspect which nervous reactions offer to physiologists is the integrative.  In the multicellular animal, especially for those higher reactions which constitute its behaviour as a social unit in the natural economy, it is nervous reaction which par excellence integrates it, welds it together from its components and constitutes from a mere collection of organs an animal individual.  This integrative action in virtue of which the nervous system unifies from separate organs an animal possessing solidarity, an individual, is the problem before us in these lectures.

(p. 3)
But the integrative action of the nervous system is different from these, in that its agent is not mere intercellular material, as in connective tissue, nor the transference of material in mass, as by the circulation; it works through living lines of stationary cells along which dispatches waves of physico-chemical disturbance, and these act as releasing forces in distant organs (p. 4) where they finally impinge.

(p.5)
It is in view of this interconnecting function of the nervous system that the field of study of nervous reactions which was called at the outset the third or integrative, assumes its due importance.  The due activity of the interconnection resolves itself into the co-ordination of the parts of the animal mechanism by reflex action.

It is necessary to be clear as to what we understand by the expression "reflex" action.

In plants and animals occur a number of actions the initiation of which is traceable to events in their environment.  The event in the environment is some change which acts on the organism as an exciting stimulus.  The energy which is imparted to the organism by the stimulus is often far less in quantity than the energy which the organism itself sets free in the movement or other effect which it exhibits in consequence of the application of the stimulus....  p. 6 In such cases there exist, three separable structures for the three processes -- initiation, conduction, and end-effect.  Yet to cases where neither histologically nor physiologically a specific conductor can be detected, it seems better not to apply the term "reflex".  It seems better to reserve that expression for reactions employing specifically recognizable nerve-process and morphologically differentiated nerve-cells; the more so because the process of conduction in nerve is probably a specialized one, in which the qualities of speed and freedom from inertia of reaction have been attained to a degree not reached elsewhere since not elsewhere demanded.

(p. 7)
The conception of a reflex therefore embraces that of at least three separable structures, -- an effector organ, e.g. gland cells or muscles cells' a conducting nervous path or conductor leading to that organ; and an initiating organ or receptor whence the reaction starts.

For our purpose the receptor is best included as a part of the nervous system, and so it is convenient to speak of the whole chain of structures -- receptor, conductor and effector -- as a reflex-arc.

The reflex-arc is the unit mechanism of the nervous system when that system is regarded in its integrative function.  The unit reaction in nervous integrative reaction and no nervous action short of a reflex is a complete act of integration.

Co-ordination, therefore, is in part the compounding of reflexes.  In this co-ordination there are therefore obviously two grades.

The simple reflex.  There is the co-ordination which a reflex action introduces when it makes an effector organ responsive to excitement of a receptor, all other parts of the organism being supposed indifferent to and indifferent for that reaction.

(p. 8)
A simple reflex is probably a purely abstract conception, because all parts of the nervous system are connected together and not part of it is probably even capable of reaction without affecting and being affected by various other parts, and it is a system certainly never absolutely at rest.  But the simple reflex is a convenient, if not a probable, fiction.  Reflexes are of various degrees of complexity, and it is helpful in analyzing complex reflexes to separate from them reflex components which we may consider apart and therefore treat as thought they were simple reflexes.

(p. 9)
Co-ordination in the simple reflex.

From the point of view of its office as integrator of the animal mechanism, the whole function of the nervous system can be summed up in the one word, conduction.

But we have first to remember that in dealing with reflexes even experimentally we very usually deal with them as reactions for which the reflex-arc as whole and without any separation into constituent parts is laid under contribution.  The reflex-arc thus taken includes the receptor.  It is assuredly as truly a functional part of the arc as any other.  But, for analysis of the arc's conduction, it is obvious that by including the receptor we are including a structure which, as its name implies, adaptation has specialized for excitation of a kind different from that obtaining for all the rest of the arc.  It is therefore advantageous, as we have to include the receptor in the reflex-arc, to consider what characters its inclusion probably grafts upon the functioning of the arc.

(p. 12)
The main function of the receptor is therefore to lower the threshold of excitability of the arc for one kind of stimulus, and to heighten it for all others.

(p. 14-15)
Indeed there is no go evidence that neuroglia is concerned directly in nervous conduction at all.

(p. 16)
We know of no reflex -arc composed of one single neurone only.  In other words, every reflex-arc must contain a nexus between one neurone and another.  The reflex-arc must, therefore, on the cell-theory, be expected to include not only intracellular conduction, but intercellular conduction.

…if there is not actual continuity of physicial phase between them, there must be a surface of separation.  Even should a membrane visible to the microscope not appear, the mere fact of non-confluence of the one with the other implies the existence of a surface of separation.  Such a surface might restrain diffusion, bank up osmotic pressure, restrict the movement of ions, accumulate electric changes, support a double electric layer, alter in shape and surface tension with changes in surface-tension or in shape, or intervene as a membrane between dilute solutions of electrolytes of difference concentration or colloidal suspensions with different signs of charge.

(p. 18)
In view, therefore, of the probably importance physiologically of this mode of nexus between neurone and neurone it is convenient to have a term for it.  The term introduced has been synapse.

(p. 19)
It would seem, therefore, that the more intense the stimulation the more the conduction along the reflex-arc comes to resemble in speed the conduction along simple nerve-trunks.

[The reflex latency may occur because:] The neurone itself is visibly a continuum from end to end, but continuity, as said above, fails to be demonstrable where neurone meets neurone -- at the synapse.  There a different kind of transmission may occur.  The delay in the grey matter may be referable, therefore, to the transmission at the synapse.

(p. 35)
Conduction along reflex-arcs presents in contrast to that along nerve-trunks characters that may be figuratively described as indicating inertia and momentum.  It is as though in the case of a weight to be pulled from a position of rest the attractive force were applied through a rigid rod in nerve-trunk conduction, but through a relatively yielding elastic band in reflex-arc conduction.  But there are other differences between the two forms of conduction which this simple simile does not figure.

Lecture II: Co-ordination in the Simple Reflex

(p. 36)
Summation of subliminal stimuli so that by repetition they become effective is practically unknown in nerve-trunk conduction.  But it is a marked feature of reflex-arc conduction.  Nor is it attributable to muscles whose contraction may serve as index of the reflex-response, since summation of this extent is not known for vertebrate skeletal muscle, though found by Richet in the claw-muscle of the crayfish.

A striking dissimilarity, therefore, between reflex-arc conduction and nerve trunk conduction is that in reflex-arc conduction and nerve-trunk conduction is considerable resistance is offered to the passage of a single nerve-impulse, but the resistance is easily forced by a succession of impulses; in other words, sub-liminal stimuli are summed.

[In contrast to the Medusa] In the chains of nerve-cells of higher animals such as Arthoropods and Vertebrates, although the conduction is reversible in each nerve-cell -- at least along that piece of it which forms a nerve-fibre -- the pluricellular chain in toto constitutes a polarized conductor, conductive in one direction only.  In such cell-chains the individual nerve-cells are characterized morphologically by possessing two kinds of cell-branches, which differ one from another in microscopic form, the one kind dendrites, the other axons.

(p. 42)
Since in many well-known instances the dendrites conduct impulses away from their free ends, while axone conducts towards its free end, it is possible on mere microscopic inspection of nerve-cells of this type to infer by analogy the normal direction of the conduction through the nerve-cell.  But in the nerve-cells forming the nerve-network of Medusa there seems no such distinct differentiation of their branches into two types.

(p. 42)
…if the synaptic membrane be permeable only in one direction to certain ions, that may explain the irreversibility of conduction.  The polarised conduction of nerve-arcs would be related to the one-sided permeability of the intestinal wall, eg to Na Cl.

(p. 45)
Refractory phase was first called attention to by Kronecker and Stirling in 1874, in the heart, and recognized by them as a fact of central importance for cardiac rhythm. In 1876 Marey met the phenomenon and gave it the name by which it is now known.  A year later Romanes' fundamental work on Medusa demonstrated the existence of the same phenomenon there.  The inconspicuous duration of the phase in nerve-trunk conduction and the progress of the view that regards the heart beat as of myogenic origin have contributed to delay recognition of refractory phase as a character of reflex-arc reactions.

By refractory period was originally meant by Marey the time during which the heart was inexcitable to a stimulus however intense.  But to-day by refractory phase is understood a state during which, apart from fatigue, the mechanism shows less than its full excitability.

(p. 55)
The reflex-arc consists, therefore, of at least three neurones.  It is convenient to have a term distinguishing the ultimate neurone FC from the rest of the arc.  For reasons to be given later it may be spoken of as the final common path.  The rest of the arc leading up to the final common path is conveniently termed the afferent arc.

(p. 69)
It is clear that an essential part of many reflexes is a more or less prolonged refractory phase succeeding nervous discharge.

Refractory phase appears therefore at the one end and the other of the animal scale as a factor of fundamental importance in the co-ordination of certain motile actions.  In the lowly animal form (Medusa) it attaches locally to the neuro-muscular organ, and so also in the visceral and blood-vascular tubes of Vertebrates.  But in higher forms (dogs) refractory phase occurs as regards the taxis of the skeletal musculature, not in the peripheral neuro-muscular organ, but in the centres of the nervous system itself.

Lecture III: Co-Ordination in the Simple Reflex (concluded)

(p. 70)
A further difference between the reaction of a reflex-arc and that of a nerve-trunk lies in the greater ease with which in the latter the intensity of effect can be graded by grading the intensity of the stimulus.  

(p. 72)
The scratch-reflex, though it resembles the heart beat in relative immutability of rhythm under change of intensity of stimulation, differs from it is the change of intensity of its beat, which follows change in intensity of stimulus.  It does not observe the  "all-or-none" principle.  It is obvious that in the heart beat the object is to put a pressure on the contents of the ventricle higher than that obtaining in the aorta, and that aim reached, any further excess of pressure is useless or harmful, for it subjects the heart and arterial wall to an unnecessary strain.  Clifford Allbutt remarks: "It is the function of a healthy heart and arteries to promote the maximum of blood displacement with the minimal alteration of pressures."

(p. 80)
Again, a number of agents, e.g., strychnine, tetanus toxin, etc., that do not appreciably affect nerve-trunk conduction enormously alter reflex-arc conduction.  All these seem to exert their influence on some part of the reflex conductor which lies in gray matter.  It is interesting to ask whether they, e.g. (p. 82) strychnine, have an effect similar to their spinal effect when exhibited Bethe's preparation of the second antenna ganglion of Carcinus, whence the motor perikary have been removed.  If these agents have their locus of incidence at the synapse, it must be conceded that they act with very different intensities at different synapses.

Lecture IV: Interaction between Reflexes

(p. 114)
We have hitherto dealt with reflex reactions under the guise of a convenient but artificial abstraction, -- the simple reflex.  That is to say, we have fixed our attention on the reaction of a reflex-arc as if it were that of an isolable and isolated mechanism, for whose function the presence of other parts of the nervous system and of other arcs might be negligible and wholly indifferent.  This is improbable.  The nervous system functions as a whole.  Physiological and histological analysis finds it connected throughout its whole extent.

(p. 115)
A reflex reaction, even in a "spinal animal" where the solidarity of the nervous system has been so trenchantly mutilated, is always in fact a reaction conditioned not by one reflex-arc but by many.  A reflex detached from the general nervous condition is hardly realizable.

The compounding together of reflexes is therefore a main problem in nervous co-ordination.  For this problem it is important to recognize a feature in the architecture of gray-centred (synaptic) nervous system which may be termed "the principal of the common path". If we regard the nervous system of any higher organism from the broad point of view a salient feature in its scheme of construction is the following.

At the commencement of ever reflex-arc is a receptive neurone extending from the receptive surface to the central nervous organ.  This neurone forms the sole avenue which impulses generated at its receptive point can use whithersoever be their destination.  This neurone  is therefore a path exclusive to the impulses generated at its own receptive point, and other receptive points that its own cannot employ it.... Yet all its reflex-arcs spring from the one single shank or stem, i.e. from one afferent neurone which conducts from the receptive point at the periphery into the central nervous organ.

But at the termination of every reflex-arc we find a final neurone, the ultimate conductive link to the effector organ (muscle or gland).  This last link in the chain, e.g. the motor neurone, differs obviously in one important respect from the first link in the chain.  It does not subserve exclusively impulses generated at one single receptive source, but receives impulses from many receptive sources situated in many and various regions of the body.  It is the sole path which all impulses, no matter (p. 115) whence they come, must travel if they are to act on the muscle fibres to which it leads.

(p. 116)
Reflex-arcs show, therefore, the general features that the initial neurone of each is a private path exclusively belonging to a single receptive point (or small group of points); and that finally the arcs embouch into a path leading to an effector organ; and that their final path is common to all receptive points wheresoever they may lie in the body, so long as they have connection with the effector organ in question.  Before finally converging upon the motor neurone the arcs converge to some degree.  Their private paths embouch upon intenuncial paths common to various degree to groups of private paths.  The terminal path may, to distinguish it from internuncial common paths, be called the final common path.  The motor nerve to a muscle is a collection of final common paths.

[2 consequences here for p. 116-117:] "one of these seems the preclusion of essential qualitative difference between nerve-impulses arising in different afferent nerves."  "A second consequence is that each receptor being dependent for final communication with its effect organ upon a path not exclusive its own but common to it with certain other receptors, such a nexus necessitates successive and not simultaneous use of the common path by various receptors using it to different or opposed effect.

(p. 118)
We note an orderly sequence of actions in the movement of animals, even in cases where every observer admits that the co-ordination is merely reflex.  We see one act succeed another without confusion.  Yet, tracing this sequence to external causes, we recognize that the usual thing in nature is not for one exciting stimulus to begin immediately after another ceases, but for an array of environmental agents acting concurrently on the animal at any moment to exhibit correlative change in regard to it, so that one or other group of them becomes -- generally by increase in intensity -- temporarily prepotent.

[RESPONSE TO THIS PROBLEM TWO WAYS:] p. 118.  In the simultaneous correlation of reflexes some reflexes combine harmoniously, being reactions that mutually reinforce.  These may be termed allied reflexes, and p. 119 the neural arc which they employ allied arcs.  On the other hand some reflexes, as mentioned above, are antagonistic one to another and incompatible.  These do not mutually reinforce, but stand to each other in inhibitory relation.  One of them inhibits the other, or a whole group of others.  These reflexes may in regard to one another be termed antagonistic; and the reflex or group of reflexes which succeeds in inhibits its opponents may be termed "prepotent" for the time being.

(p. 129-130)
"There exists a further important class of cases in which reflexes have "allied" relation.  Throughout a vast range of animal types the bulk formed by the organism presents to the environment a surface sheet of cells, and, beneath that, a mass of cells more or les screened from the environment by the surface sheet.  Many of the agencies by which the environment acts on the organism do not penetrate it far enough to reach the cells of the deep mass inside.  Bedded in the surface layer of the organism are numbers of receptor cells constituted in adaptation to the stimuli delivered by environmental agencies.  But the organism itself, like the world surrounding it, is a field of ceaseless change, where internal energy is continually being liberated, whence chemical, thermal, mechanical, and electrical effects appear. It is a microcosm in which forces which can act as stimuli are at work as in the macrocosm around.  The deep tissues underlying the surface sheet are not provided with receptors of the same kinds as those of the surface, yet they are not devoid of receptors.  They have receptors specific to themselves.  The receptors which lie in the depth of the organism are adapted for excitation consonantly with changes going on in the organism itself, particularly in its muscles and their assessory organs (tendons, joints, blood-vessels).

(p. 135)
But not all reflexes connected to one and the same final common path stand to one another in the relation of "allied reflexes".  Suppose during the scratch-reflex a stimulus be applied to the foot not of the scratching side, but of the opposite side.  The left leg, which is executing the scratch reflex in response to stimulation of the left shoulder skin is cut short in its movement by stimulation of the right foot, although the stimulus at the shoulder to provoke the scratch movement is maintained unaltered all the time.  The stimulus to the right foot will temporarily interrupt a scratch reflex, or will cut it short or will delay its onset; which it does of these depends on the time-relationships of the stimuli.

(p. 141)
It was argued above (Lecture III), from the morphology of the perikaryon, that it must form, in numerous cases, a nodal point in the conductive lines provided by the neurone.  The work of Ramon-y-Cajal, van Guhchten, v. Lenhossek, and others with the methods of Golgi and Ehrlich, establishes as a concept of the neurone in general that it is a conductive unit wherein a number of branches (dendrites) converge toward, meet at, and coalesce in a single outgoing stem (axone).  Through this tree-shaped structure the nervous impulses flow, like the water in a tree, from roots to stem. The conduction does not normally run in the reverse direction.  The place of junction of the dendrites with one another and with the axone is commonly the perikaryon.  This last is therefore a nodal point in the conduction system.  But it is a nodal point of particular quality.  It is not a nodal point where lines meet to cross one another, nor one where one line splits into many.  It is a nodal point where conductive lines run together into one which is the continuation of them all.  It is a reduction point in the system of lines.  The perikaryon with its convergent dendrites is therefore just such a structure as spatial summation and immediate induction would demand.

(p. 144)
It is the transference of the final common path from the group of one set of reflexes to another which constitutes the change which occurs at each step of the orderly sequence of reaction that we see normally succeeds each other in animal behavior -- leaving aside all question of consciousness in relation to the sequence.  This transference is most obvious when the sets of reflexes between which the final common path is exchange are antagonistic reflexes.  Two classes of this kind of case of specially common occurrence are "alternating reflexes" and "compensatory reflexes".

Lecture V: Compound Reflexes: Simultaneous Combination

(p. 150)
A large part of co-ordination consists in the orderly combining of reflexes.  In studying this co-ordination we have to deal with and discriminate between simultaneous combinations and successive combinations of reflexes.  We may proceed to attempt the former problem.

Irradiation
If by appropriate stimulation of the skin of the foot, say by unipolar faradization of a spot of the plantar skin of a digit, the ordinary flexion-reflex of the hind limb of the dog be evoked, the extend of the reflex increases with increase in the intensity of the stimulus.  The reflex effect spreads over a larger and larger field, irradiating as it were in various directions from a focus of reflex-discharge, which takes effect on the limb itself.

(p. 151)
The more intense the spinal reflex -- apart from strychnine and similar convulsant poisoning -- the wider, as a general rule, the extent to which the motor discharge spreads around its focal area.


(p. 152)
That the reaction should spread in its spatial extent is not surprising.  The afferent neurone on entering the central organ, the spinal cord enters a vast network of conduction of paths interlacing in all directions.  A glance at any Weiget preparation of the spinal cord shows a tangle of branching nerve fibres, the richness and intricacy of which seems practically infinite.  Into this forest the receptive neurone conducts impulses, and can itself be traced breaking up into divisions that pass in many directions and various distances.  And this web of conductive channels into which the centripetal impulses of the reflex are thus launched is known to be practically a continuum in the sense that no part of the nervous system is isolated from the rest.  [Sherrington quotes A Donaldson’s American Textbook of Physiology, 1900) "A group of nerve-cells disconnected from the other nerve-tissues of the body, as the muscles or glands are disconnected from each other, would be without physiological significance.  To understand the physiology of the nervous system it is important to keep in mind the fact that by histology it is found to be continuous throughout its entire extent.

Short and Long Reflexes

(p. 157)
The cord may, in its relation to the receptive surface and skeletal musculature, be considered divisible into right and left lateral halves, each subdivisible into regions of neck (cervical, including pinna), fore limb (brachial), trunk (thoracic), hind limb (curaral), and tail (caudal).  A reflex action in which the stimulus applied to a receptive area in one of the above regions evokes a reaction in the musculature of another of the regions is conveniently called a long spinal reflex.  A reflex reaction in which the muscular reply occurs in the same region as the application of the stimulus is conveniently called a short spinal reflex.  Short spinal reflexes are, as a rule, more easily and regularly elicitable than are long spinal reflexes.

(p. 178)
Thus at any single phase of the creature's reactions, a simultaneous combination of reflexes is in existence.  In this combination the positive element, namely, the final common paths (motor neurone groups) in active discharge, exhibits a harmonious discharge directed by the dominant reflex-arc, and reinforced by a number of arcs in alliance with it.

(p. 179)
Therefore it is that the reflex initiated by one group of receptors while in progress excludes in various directions the reflexes of other receptors, although these latter may be being stimulated.  In this way the motor paths at any moment accord in a united pattern for harmonious synergy, co-operating for one effect.

The notion, therefore, that we arrive at of such a motor reflex reaction is that it is referable to a constellation of congruous stimuli of which one is prepotent, and that the reaction taken in its totality gives the nervous intercommunications of the central organ a certain pattern, which pattern may ramify through a great extent of the central organ.... Even in extensive reflexes of the bulbo-spinal animal it is probably that though great fields of the nervous centres are involved in the reaction at any one time, large parts are still left outside the reaction.  This part of the neural network would therefore be indifferent to that particular reaction.  That amounts to saying that it is open during the reaction to be thrown into activity by some concurrent and distinctive other reaction.  But this possible neutrality and discreteness of reflex reaction and its fields is probably far less in the intact higher vertebrate than in the lower or in the mutilated higher vertebrate.  In the presence of the brain the knitting together of the whole nervous network is probably much greater than in its absence.

Lecture VI: Compound Reflexes: Successive Combination

(p. 181)
We consider last the co-ordination of reflexes in simultaneous combination.  We now turn to sequences of reflexes.  Reflexes are seen to follow one another in consecutive combination.  And in this chaining together of successive reflexes in different instances different kinds of processes seem traceable.  Of these, one consists in the reaction to one external stimulus bringing about an application of an external stimulus for a second reflex.

(p. 182)
Orderly sequence of movement characterizes the outward behaviour of animals.  Not least so where, as in the earthworm crawling, or the insect in flight, or the fish swimming, every observer admits the coadjustment is essentially reflex.  One act succeeds another without confusion.  Yet tracing this sequence to its external causes, we recognize that the usual thing in nature is not for one exciting stimulus to begin immediately after another ceases, but for an array of environmental agents acting concurrently on the animal at any moment to exhibit correlative change in regard to it, so that one or other group of them becomes -- generally by increase in intensity -- temporarily prepotent.

(p. 192)
We do not yet understand the intimate nature of inhibition. In the cases before us now, its seat is certainly central, and in all probability is, as argued above, situated at points of synapses.  I have urged that a prominent physiological feature of the synapse is a synaptic membrane.  It seems therefore to me that inhibition in such cases as those before us is probably referable to a change in the condition of the synaptic membrane causing a block in conduction.  But what the intimate nature of the inhibitory change may be we do not know.

(p. 193)
One view [of inhibition] has been that, as the process of conduction along nerve-fibres is an undulatory one in the sense that the nerve-impulse travels as a disturbance with wave-like configurations of intensity, inhibition is due to a mutual suppression of two wave-like disturbances impinging on the same point of the conductor but in opposite phases of disturbance.

(p. 194)
Moreover, as we shall see presently, central inhibition is not a neutral process, for, at least in many cases, it leaves the reflex centre surcharge for subsequent response.

The most striking thing that we know of inhibition is that it is a phenomenon in which an agent such as in other cases excites or increases an action going on in this case stops or diminishes an action going on.  Now, the activity of a tissue can be lowered or abolished by production in it of deleterious changes such as exhaustion or, in the highest degree death.  But there is no evidence that inhibition of a tissue is ever accompanied by the slightest damage to the tissue; on the contrary, it seems to predispose, the tissue to a greater functional activity thereafter.

(p.222)
The waning of a reflex under long maintained excitation is one of the many phenomena that pass in physiology under the name of "fatigue".  It may be that in this case the so-called fatigue is really nothing but a negative induction.  Its place of incidence may lie at the synapse.  It seems a process elaborated and preserved in the selective evolution of the neural machinery.  One obvious use attaching to it is the prevention of the two prolonged continuous path by any one receptor.  It precludes one receptor from occupying for long periods an effector organ to the exclusion of all other receptors.  It prevents long continuous possession of a common path by any one reflex of considerable intensity.  It favours the receptors taking turn about.  It helps to insure serial variety of reaction.  The organism, to be successful in a million-sided environment, must in its reactions be many-sided.  Were it not for such so-called "fatigue", an organism might, in regard to its receptivity, develop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvelous congeries of all those various sense-organs which it is actually found to possess.

(p. 226) Species of reflex.  A fourth main determinant for the issue of the conflict between rival reflexes seems the functional species of the reflexes. Reflexes initiated from a species of receptor apparatus that may be termed "noci-ceptive" appear to particularly dominant the majority of the final common paths issuing from the spinal cord.

(p. 227)
With its liability to various kinds of mechanical and other damage in a world beset with dangers amid which the individual and species have to win their way in the struggle for existence we may regard nocuous stimuli as part of a normal state of affairs.  It does not seem improbably therefore that there should under selective adaptation attach to the skin a so-to-say specific sense of its own injuries.

(p. 194)
Moreover, as we shall see presently, central inhibition is not a neutral process, for, at least in many cases, it leaves the reflex centre surcharge for subsequent response.

The most striking thing that we know of inhibition is that it is a phenomenon in which an agent such as in other cases excites or increases an action going on in this case stops or diminishes an action going on.  Now, the activity of a tissue can be lowered or abolished by production in it of deleterious changes such as exhaustion or, in the highest degree death.  But there is no evidence that inhibition of a tissue is ever accompanied by the slightest damage to the tissue; on the contrary, it seems to predispose, the tissue to a greater functional activity thereafter.

(p. 222)
The waning of a reflex under long maintained excitation is one of the many phenomena that pass in physiology under the name of "fatigue".  It may be that in this case the so-called fatigue is really nothing but a negative induction. Its place of incidence may lie at the synapse.  It seems a process elaborated and preserved in the selective evolution of the neural machinery.  One obvious use attaching to it is the prevention of the too prolonged continuous use of a "common path" by any one receptor.  It precludes one receptor from occupying for long periods an effector organ to the exclusion of all other receptors.  It prevents long continuous possession of a common path by any one reflex of considerable intensity.  It favours the receptors taking turn about.  It helps to insure serial variety of reaction. The organism, to be successful in a million-sided environment, must in its reactions be many-sided.  Were it not for such so-called "fatigue" an organism might, in regard to its receptivity, develop an eye, or an ear, or a mouth, or a hand or leg, but it would hardly develop the marvelous congeries of all those various sense-organs which it is actually found to possess.

(p. 226) Species of reflex: A fourth main determinant for the issue of the conflict between rival reflexes seems the functional species of the reflex.

(p. 231)
It would seem a general rule that reflexes arising in species of receptors which considered as sense-organs provoke strongly affective sensation caetris paribus prevail over reflexes of other species when in competition with them for the use of the "final common path". Such reflexes override and set aside with peculiar facility reflexes belonging to touch organs, muscular sense-organs, etc. As the sensations evoked by these arcs, e.g. pain exclude and dominate concurrent sensations, so do the reflexes of these arcs prevail in the competition for possession of the common paths.

(p. 231-232)
Therefore, intensity of stimulation, fatigue and freshness, spinal induction, functional species of reflex, all these are physiological factors influencing the result of the interaction of reflex-arcs at a common path.  It is noticeable that they all resolve themselves ultimately into intensity of reaction.  Thus, intensity of stimulus means as a rule intensity of reaction.  Those species of reflexes which are habitually prepotent in interaction with others are those which are habitually intense; those specially impotent in competition are those habitually feeble in intensity, e.g. skeletal muscular tone.  The tone reflexes of attitude are of habitually low intensity, easily interfered with and temporarily suppressed by intercurrent reflexes, these latter have higher intensity.  But these latter suffer fatigue relatively early, whereas the tonic reflexes of posture can persist hour after hour with little or no sign of fatigue.  Fatigue; therefore, in the long run advantageously re-dresses the balance of an otherwise unequal conflict.  We can recognize in it another agency working toward that plastic alternation of activities which is characteristic of animal life and increases in it with ascent of the animal scale.  The high variability of reflex reactions from experiment to experiment, and from observation to observation, is admittedly one of the difficulties that has retarded knowledge of them.    Their variability, though often attributed to general conditions due to changes produced in the central nervous organ by its own functional conductive activity apart from fatigue.  This functional activity itself causes from moment to moment the temporary opening of some connections and the closure of others.  The chains of neurons, the conductive lines, have been, especially in recent years, by the methods of Golgi, Ehrlich, Apathy, Cajal, and others richly reveals to the microscope.  Anatomical tracing of these may be likened though more difficult to accomplish, to tracing the distribution of blood vessels after Harvey's discover had given them meaning, but before the vasomotor mechanism was discovered.  The blood-vessels of an organ may be turgid at one time, constricted almost to obliteration at another.  With the conductive network of the nervous system the temporal variations are even greater, for they extend to absolute withdrawal of nervous influence.  Under reflex inhibition a skeletal muscle may relax to its post-mortem length, ie there may then be no longer evidence of even a tonic influence on it by its moto neurone.  The direction of the stream of liberation of energy along the pattern of the nervous web varies from minute to minute.  The final common path is handed from some group of a plus class of afferent arcs to some group of a minus class or of a rhythmic class, and then back to one of the previous groups again, and so on.  The conductive web changes its functional patter within certain limits to and fro.  It changes in its pattern occur there in virtue of interaction between rival reflexes, "interference".  As a tap to a kaleidoscope, so a new stimulus that strikes the receptive surface causes  in the central organ a shift of functional pattern at various synapses. The central organ is a vast network whose lines of conduction follow a certain scheme of pattern, but within that pattern the details of connection are, at the entrance to each common path, mutable.  The gray matter may be compared with a telephone exchange, where, from moment to moment, though the end-points of the system are fixed, the connections between starting points and terminal points are changed to suit passing requirements, as the functional points are shifted at a great railway junction.

Unlike reflexes have successive but not simultaneous use of the common path; like reflexes mutually reinforce each other on their common path.  Expressed teleologically, the common path, although economically subservient for many and various purposes, is adapted to serve but one purpose at a time.  Hence it is a co-ordinating mechanism and prevents confusion by restricting the use of the organ, its minister, to but one action at a time.

Lecture VII: Reflexes as Adapter Reactions

(p. 235)
It is of course as impossible to disprove as to prove that psychical events accompany, or that they do not accompany the nervous reactions of the spinal animal.  It is significant, however, that the best-known controversy (Pfluger, Lotze) as the psychical powers of the spinal cord, occurred prior to the advent of the Darwinian theory of evolution. This latter suggests how purposive neural mechanisms may arise.  If furnishes a key to the genesis and development of adapted reactions and among these latter, reflexes.

That a reflex action should exhibit purpose is no longer considered evidence that a psychical process attaches to it; let along that it represents any dictate of choice or will.  In light of Darwinian theory every reflex must be purposive.  We here trench upon a kind of teleology.  It is widely and wisely held that natural knowledge pursues the question "how" rather than the question "why".  The "why" involves a judgment whose data lie so beyond present human experience and comprehension that self-abnegation in regard to the desire to attempt it is not only prudent, but to the unbiased judgment a necessity.

Older writings on reflex action concerned themselves boldly with the purpose of the reflexes they described.  The language in which they are couched shows that for them the interest of the phenomena centred in their being regarding as manifestations of an informing spirit resident in the organism, lowly or mutilated thought that might be.  Progress of knowledge has tended more and more to unseat this anthropomorphic image of the observer himself which he projected into the object of his observations.  The teleological speculations accompanying such observations have been proportionately discredited.

The impetus given to biology by the doctrine of adaptation under natural selection, felt so strongly by morphological studies, seems hardly as yet to have begun its course as a motive force in physiology.  But signs being to be numerous that such an era is at hand.  The infinite fertility of the organism as a field for adapted reaction has become more apparent.  The purpose of a reflex seems as legitimate and urgent an object for natural enquiry as the purpose of the colouring of an insect or a blossom.  And the importance to physiology is, that the reflex reaction cannot be really intelligible to the physiologist until he knows its aim.

In general terms we may say that the effect of any reflex is to enable the organism in some particular respect to better dominate the environment.  One often hears objections take to epithets -- common in writings on biology -- "lower" and "higher" as applied to organisms, plant and animal.   Such objection seems valid if the phrase assumes that the "lower" organism any less perfectly fulfils its "purpose" or "design" than does the higher, or in those respects in which it has commerced with the environment is any less admirably adjusted than is the higher. But "lower" and "higher" may be used without any connotation of that kind.  In the course of evolution a number of organisms have become so adapted to the environment as to dominate it more various and extensively than do other organisms.  In that sense some organisms are higher and some are lower.  In that sense man is the highest organism.  And if evolution be a process of gradual and more or less uninterrupted course it is obvious that the highest form achieved will also be among the latest of the forms achieved.  This grading of rank in the animal scale will be nowhere more apparent than in the nervous system in its office as integrator of the individual.  The more numerous and extensive the responses made by a creature to the actions of the world around upon its receptors, the more completely will the bundle of reflexes, which from this standpoint the creature is, figure the complexity of the world around, mirroring it more completely than do the bundles of reflexes composing "lower" creatures.

(p. 238)
Grainger's conclusion was that spinal cutaneous reflexes "are either of a preservative character or resemble the movements which the functions of the organ require."

(p. 239)
There are of course two modes of preservation, namely, escape and defense.

(p. 255)
Bodily Resonance of Emotions.  Some sensations are neutral or devoid of affective tone, while others are rich in affective tone.  The development of these latter is closely connected with the origin of the coarser emotions.  A physiological interest attaches to these states of emotion since certain reactions of the bodily organs are, as is well known, characteristic of them.  That marked reactions of the nervous arcs regulating the thoracic and abdominal organs and the skin contribute characteristically to the phenomena of emotion has been common knowledge from time immemorial.

To this bodily resonance of the emotion has in recent years been assigned by some authorities a prominent role in the mechanism of the production of the emotional state itself in certain of the coarser emotions.  Instead of the emotional state beginning as Ladd puts it as "a sort of nerve storm in the brain, whence there descend an excitement which causes commotion in the viscera and vascular regions -- thus secondarily inducing an organic reverberation" -- the vie has been advance that the cerebral and psychological processes of emotion are secondary to an immediate reflex reaction of vascular and visceral organs of the body suddenly excited by certain stimuli of peculiar quality.

(p. 256)
Of points where physiology and psychology touch, the place of one lies at "emotion".  Built upon a sense-feeling much as cognition is built upon sense-perception, emotion may be regarded almost as a "feeling" -- a feeling excited, not by a simple little-elaborated sensation, but by a group or train of ideas.  To such compound ideas it holds relation much as does "feeling" to certain species of simple sense-perceptions.  It has a special physiological interest in that certain visceral reactions are peculiarly colligate with it.  Heart, blood-vessels, respiratory muscles, and secretory glands take special and characteristic part in the various emotion.  These viscera, though otherwise remote from general place of psychical process are affected vividly by the emotional.  Hence many a picturesque metaphor of proverb and phrase and name -- "the heart is better than the head" and "swells within the breast", "Richard Coeur de Lion".  It was Descartes who first promoted the emotion to the brain.  Even last century, Bichat wrote, "The brain is the seat of cognition and is never affected by emotions, whose sole seat lies in the viscera".  But the brain is now thought to be a factor necessary in all higher animals to every mechanism whose working has consciousness as an adjunct.

What is the meaning of the intimate linkage of visceral actions to psychical states emotional?  To the ordinary day's consciousness in the healthy individual the life of the viscera contributes little at all, except under emotion.  The perceptions of the normal consciousness are rather those of outlook upon the circumambient universe than inlook into the microcosm of the "material me".  Yet heightened beating of the heart, blanching or flushing of blood-vessels, the pallor of fear, the blush of shame, the Rapelaisian effect of fright upon the bowel, the secretion by the lachrymal gland in grief, all these are prominent characters in the pantomime of natural emotion.  Visceral disturbance is evidently a part of the corporeal expression of emotion.   The explanation is a particular case in the problem of movements of expression in general.

(p. 257)
They must have an explanation the same in kind as that of instinctive movement.  There is no real break between man and brute even in the matter of mental endowment.  The instinctive bodily expressions of emotion arose, in the opinion of those quotes above, as attitudes and movements useful to the animal for defence, escape, seizure, embrance, etc.  These as survivals have become symbolic for states of mind.  Hence, an intelligible nexus between muscular attitude, the pose of feature, etc., and the emotional state of mind.  But between action of the viscera and the psychical state the nexus is less obvious.  This latter connection adds a difficult corollary to the general problem.

The fact of the connection is one all hands admitted, but as to the manner of it opinion is at issue.  Does (I) the psychical part of the emotion arise and its correlate nervous action then excite the viscera? p. 258 Or (2) does the same stimulus which excites the mind excite concurrently and per se the nervous centres ruling the viscera?  Or (3) does the stimulus which is the exciting  cause of the emotion act first on the nervous centres ruling the viscera, and their reaction then generate visceral sensations; and do these latter, laden with affective quality as we know they will be, induce the emotion of the mind?  On the first of the three hypotheses the visceral reaction will be secondary to the psychical, on the second the two will be collateral and concurrent, on the third the psychical process will be secondary to the visceral.

Professor James writes: "our natural way of thinking about these coarser emotions (e.g. grief, fear, rage, love) is that the mental perception of some fact excites the mental affection called the emotion, and that this latter state of mind gives rise to the bodily expression.  My theory, on the contrary, is that the bodily changes follow directly the perception of the exciting fact, and that our feeling of the same changes as they occur IS the emotion."  Every one of the bodily changes, whatsoever it be, is FELT acutely or obscurely, the moment it occurs."

(p. 267)
In view of these general considerations and of the above experiments, we may with James accept visceral and organic sensations and the memories and associations of them as contributory to primitive emotions, but we must regard them as reinforcing rather than initiating the psychosis.  Organic and vascular reaction, though not the actual excitant of emotion, strengthen it.  This is the kernel of the old contention about actuality of emotion in the art of the artist.  Hamlet's description of the actor as really moved by his expression may be accepted as an answer.

(p. 270)
Progress of knowledge in regard to the nervous system has been indissolubly linked with determination of localization of function in it.  This has been so from the time of the Bell-Magendie discovery of the difference of function in the two spinal roots and Flourens delimitation of the respiratory centre in the bulb. The discovery of localization of function in parts of the cortex has given the knowledge now supplies to the student charts of the functional topography of the brain much as maps of continents are supplied in a geographical atlas.  The student looking over the political map of a continent may little realize the complexity of the populations and states so simply represents.  We looking at the brain chart of the text-book may never forget the unspeakable complexity of the reactions thus rudely symbolized and spatially indicated.

(p. 303)
Hughlings Jackson with characteristic penetration of thought argued nearly thirty years ago that rigidity ensuing in hemiplegia (hemiplegic contracture) is not owing to the cerebral lesion nor to the lateral sclerosis.  He said, "Whilst the primary cerebral lesions can account for the paralytic element it cannot (nor can the sclerosis of the lateral column) account for the tonic condition of the muscles. My speculation is that the rigidity is owing to unantagonized influence of the cerebellum. Whilst the cerebrum innervates the muscles in the order of their action from the most voluntary movements (limbs) to the most automatic (trunk), the cerebellum innervates them in the opposite order.  This is equivalent to saying that the cerebellum is the centre for continuous movements and cerebrum for changing movements.  Thus, in 'walking' the cerebellum tends to stiffen all the muscles; the changing movements of walking are the result of cerebral discharges overcoming in a particular and orderly way the otherwise continuous cerebellar influence.  When the influence of the cerebrum is permanently taken off by disease of the cerebrum, as in hemiplegia, from the parts which it most specially governs (arm and leg) the cerebellar influence (p. 304) is no longer antagonized; there is unimpeded cerebellar influx and hence rigidity of the muscles which in health the cerebrum chiefly innervates.  The spinal muscles are those which the cerebrum influences least and the cerebellum most.  In health the whole of the muscles of the body are double innervated -- innervated both by the cerebrum and cerebellum: there being a co-operation of antagonism between the two great centres."

(p. 306)
Finally, it seems to me that the number of reflex actions which are "neutral" to each other, in the sense expressed in Lecture II, is less with the cerebral cortex present than without it.  This amounts to expressing concretely an inference that the cerebral cortex augments the motor solidarity of the creature.  Since there is more solidarity as well as more diversity in those movements of an animal which are directed to its outer environment than to its inner -- meaning by this latter the fraction of environment embraced within its own pulmono-digestive cavity -- the representation of visceral movement in the cortex will be relatively slight and chiefly concern parts where alimentary canal open on outer surface.

Lecture IX: The Physiological Position and Dominance of the Brain

(p. 308)
We may now attempt to gather from the various notions, however fragmentary, that have occupied us, some general conception of the neural architecture of an animal as a whole; though of course only in its motor aspect, for its truly sensorial aspects we have hardly had before us. The problem is too difficult for me to expect much success.  Yet it will repay us if from the attempt we glean something at least of one cardinal feature of the scheme, namely, the dominance attained by one limited set of neural segments, the brain, over all the rest.

(p.350)
The head is in many ways the individual's greater part.  It is the more so the higher the individual stands in the animal scale.  It has the mouth, it takes in the food, including water and air, it has the main receptive organs providing data for the rapid and accurate adjustment of the animal to time and space.  To it the trunk, an elongated motor organ with a share of the digestive surface, and the skin, is appended as an apparatus for locomotion and nutrition.  The latter must of necessity lie at the command of the great receptor-organs of the head.  The co-ordination of the activities of the trunk with the requirements of the head is a cardinal function of the synaptic nervous system.  Conducting arcs must pass from the cephalic receptors to the contractile masses of the body as a whole.  The spinal cord contains these strands of conductors in vertebrates and is from this point of view a mere appendage of the brain.  A salient feature of these conducting arcs is that the nerve-fibres from the cephalic receptors do not run, as might perhaps a priori have been thought natural, direct from their cephalic segment backwards to react the common effector paths upon which they embouch.

(p. 353)
To say this is to say no more than that the motile and consolidated individual is driven, guided, and controlled by, above all organs, its cerebrum.


Lecture X: Sensual Fusion

(p. 354)
The animal whose nervous construction we have been attempting to follow thus far, we have supposed merely a puppet moved by the external world in which it is immersed; and we have supposed it a puppet without passions, memory, feelings, sensations, let alone ideas concrete or abstract.  From time to time we have purposely invoked appeal to sensations and feelings such as out own experience of ourselves provides in order to see better whither lead the blind  reactions of the thing that we have been imagining a fatal mechanism.  Whether such sensation or feelings accompany or do not accompany the reactions we have been studying we have left open.  We tacitly consented that our point of study of those reactions leaves that question to which the present time gives no clear answers, as one with which we are not concerned.  But we may agree that if such sensations and feelings or anything at all closely like them do accompany the reactions we have studied, the neural machinery to whose working they are adjunct lies not confined in the nervous arcs we have so far traced but in fields of nervous apparatus that, though connected with those arcs, lie beyond them, in the cerebral hemispheres.

(p. 357)
Can we at all compare with the simultaneous co-ordination of the nervous factors in a motor reflex the synthesis of the nervous elements whose combination underlies a simple sense-perception?

(p. 386)
We are thus warned against any hasty conclusion that the neural mechanisms which synthesize reflex movements illustrate in their arrangement also those concerned where sensual fusion is the phenomenon.  But that does not invalidate a broad practical interference which study of the nervous system in regard to motor reaction allows.  This inference is that toward the solution of the problems of motor taxis help is obtainable by appeal to characters evident in sensual reaction.  This practical inference need not in the least involve any doctrinal attitude whatever toward the hypothesis of pscyho-physical parallelism.  It simply insists on the likeness of nervous reactions expressed by muscular and other effector-organs to reactions whose evidence is sensual.  It insists on this likeness being close and fundamental enough to make each of the two classes of phenomena of use to the student of the other.

(p. 387)
Instances might be multiplied, but they have risen prominently in several minds now.  A practical inference from them is that physiology and psychology; instead of prosecuting their studies, as some now recommend, more strictly apart one from another than at present, will find it serviceable for each to give to the results achieved by the other even closer heed than has been customary hitherto.

Besides this similarity of time-relation and other features between the physiological and psychical signs of neural activity, another link connects the psychological and physiological for the biologist.  To the physiology of pure reflexes, that is, reflexes devoid of psychical accompaniment so far as introspection can discover, psychological interest nevertheless attaches, and on a very distinct ground.  This ground of connection is seen if inquiry is followed along the animal scale in the direction from higher forms to lower rather than by the usually more favorable reverse approach.  This is partly because we directly observe psychical phenomena by introspection only, that is, only in ourselves; and the facts discovered by introspection are applicable to other beings the more readily the more those being resemble ourselves, namely, are animals ranking near to man.

Pure reflexes are admirably adapted to certain ends.  They are reactions which have long proved advantageous in the phylum, of which the existent individual is a representative embodiment p. 388.  Perfected during the course of ages, they have during that course attained a stability, a certainty, and an ease of performance beside which the stability and facility of the most ingrained habit acquired during an individual life is presumably small.  But theirs is of itself a machine-like fatality.  Their character in this stands revealed when the neural arcs which execute them are separated, e.g. by transection of the spinal cord, from the higher centres of the nervous system.  They can be checked, it is true, as we have seen, by collision with other reflexes as ancestral and as fatally operative as themselves.  To these ancient invariable reflexes, consciousness, in the ordinary meaning of the term, is not adjunct.  The subject as active agent does not direct them, and cannot introspect them.

Yet it is clear, in higher animal especially so, that reflexes are under control.  Their intrinsic fatality lies under control by higher centres unless their nervous arcs are sundered from ties existing with those higher centres.  In other words, the reactions of reflex arcs are controllable by mechanisms to whose activity consciousness is adjunct.  By these higher centres, this or that reflex can be checked, or released, or modified in its reaction with such variety and seeming independence of external stimuli that the existence of a spontaneous internal process expressed as "will" is the naive inference drawn.  Its spring of action is now our question; its seat in the nervous system seems to correspond with that of processes of perceptual level.  It is urgently necessary for physiology to know how this control -- volitional control -- is operative upon reflexes, that is how it intrudes and makes its influence felt upon the running of the reflex machinery.

No exposition of the integrative action of the nervous system is complete, even in outline, if this control is left without consideration.  Reflexes ordinarily outside its pale can by training be brought within it.  The actor, it is asserted, can shed tears at will, or blush, or blanch.

(p. 390)
It is significant that, although the reflexes controlled are so often unconscious, consciousness is adjunct to the centres which exert control.  A biologist, Professor Lloyd Morgan, has urged that "the primary aim, object, and purpose of consciousness is control.  Consciousness in a mere automaton is a useless and unnecessary epiphenomenon."

A biological inference arises at this point.  We have admitted that the organs to which psychosis is adjunct, namely, the brain, and especially in higher vertebrates the cerebral hemispheres, supply the surest touchstone to rank in the scale of animal creation.  That is to admit, in other words, that development of these organs constitutes on the whole, the best criterion to the success of an animal form in the competition which lies at the toot of animal evolution.

(p. 392-3)
We thus, from the biological standpoint, see the cerebrum, and especially the cerebral cortex, as the latest and highest expression of a nervous mechanism which may be described as the organ of, and for, the adaptation of nervous reactions.

(p. 393)
These adjustments, though not transmitted to the offspring, yet in higher animal form the most potent internal condition for enabling the species to maintain and increase in sum its dominance over the environment in which it is immersed.  A certain measure of such dominance is its ancestral heritage; in this is based its innate right to success in the competition for existence.  But the factors and elements of that competition change in detail as the history of the earth proceeds.  The creature has to be partially readjusted if it is to hold its own in the struggle.  Only by continual modification of its ancestral powers to suit the present can it fulfill that which its destiny, if it is to succeed, requires from it as its life's purpose, namely the extension of its dominance over its environment.  For this conquest its cerebrum is its best weapon.  It is then around the cerebrum, its physiological and psychological attributes, that the main interest of biology must ultimately turn.

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