The Integrative Action of the Nervous System

An animal organism is not a machine which merely trans- >rms a quantum of energy given it in potential form at the out- ;t of its career. It has to replenish its potential energy by continued acquisition of suitable energy-containing material from the environment, and this material it has to incorporate in itself. Moreover, since death cuts short the career of the individual organism, the species has to be maintained, and for that in most higher organisms there is required accession of material (ga- metic) from another organism (of like species) to rejuvenesce a portion of the adult, which portion then cast off leads a new individual existence. To satisfy, therefore, the primary vital re- quirements of an animal species, actual material contact with certain objects is necessary; thus, for feeding, and in many cases for sexual reproduction.

In these processes of feeding and conjugation the non- distance-receptors play an important and essential part. But ability on the part of an organism to react to an object when still distant from it allows an interval for preparatory reactive steps which can go far to influence the success of attempt either to obtain actual contact or to avoid actual contact with the object. Thus, we may take in illustration the two sets of selec- tive chemo-receptors, the gustatory and the olfactory. Both are responsive to certain chemical stimuli which reach them through solution in the moist mucous membranes of the mouth and nose. No odorous substance appears to be tasteless, and if the thresh- old value for olfaction and for taste be measured respectively, the threshold for the former as determined in weight of dissolved material is lower than for the latter. The former is the distance- receptor. Animal behavior shows clearly that in regard to these two groups of receptors the one subserves differentiation of reaction, i. e. swallowing or rejection, of material already found and acquired, e.g. within the mouth. The other, the distance-receptor, smell, initiates and subserves far-reaching complex reactions of the animal anticipatory to swallowing, namely, all that train of reaction which may be comprehensively termed the quest for food. The latter foreruns and leads up to the former. This precurrent relation of the reaction of the distance-receptor to the non-distance receptor is typical.

The "distance-receptors" initiate anticipatory, i. e. precurrent, reactions. I ventured above to use the word " attempt." Just as salient character of most of the reactions of the non-projicient eceptors taken as sense-organs is " affective tone" i, e. physical ain or physical pleasure, so " conative feeling " is salient as a sychical character of the reactions which the projicient or istance-receptors, taken as sense-organs, guide. As initiators f reflex movements the action of these latter is characterized y tendency to work or control the musculature of the animal a whole y — as a single machine, — to impel locomotion or to ut it short by the assumption of some total posture, some atti- de which involves steady posture not of one limb or one ppendage alone, but of all, so as to maintain an attitude of the ody as a whole. Take, for instance, the flight of a moth toward candle, the dash of a pike toward a minnow, and the tense iteadiness of a frog about to seize an insect. These reactions re all of them excited by distance-receptors. Though in the ne case the musculature is impelled to locomotion toward the stimulus (positive phototropism), in the other restrained (in- hibited) from locomotion. Whether the reaction be move- ment toward or movement away from (positive or negative) or Khether it be motion or its restraint (excito-motor or inhibito- lotor) does not matter here. The point here is that in both re- :tions the skeletal musculature is treated practically as a whole and in a manner suitably anticipatory of a later event. That is far less the case with the non-projicient receptors. The^decere;^ ^Jh:^ frog changes the whole direction of its path of locomotion when a visual obstacle is set in its way, but a skin impact ex- cites a movement in a small field of musculature only, e, g. the eyelidblinks oncorneal contact, the foot flexes at a digital noxa; where the part itself cannot well move itself musculature acces- sory to it but distant from it is moved.

Thus the hind limb is swept over the flank on irritation there, or the fore limb over the snout on irritation there. But in these cases the movement induced is merely local and does not affect the body as a whole. Sufficient intensity (we may include summation under intensity) I\{ a stimulus can of course impel the whole creature to move- nent even through a non-projicient receptor. A decerebrate frog touch, and again more so at a second ; at a third will, besides lowering the head, draw the front half of its trunk slightly backward; at a fourth the same movement with stronger re- traction ; at a fifth give an ineffectual sweep with its hind or fore foot; at a sixth a stronger sweep; at a seventh a feeble jump; at an eighth a free jump, and so forth. Considerable intensity or summation is required to evoke a reflex reaction of the skeletal musculature as a whole from these cutaneous receptors. The projicient receptors and their reflexes once gone, even intense stimuli do not readily move or arrest the creature as a whole. It is relatively difficult to get the " spinal " frog to spring or swim. Co-ordinate movement of the crea- ture as a whole is then obtained by general stimulation {i. e. plurireceptive summation), or if by localized stimulation the stimulus must be intense. Thus the spinal frog will swim when placed in water at 36° C. The warm water forms a noci- ceptive stimulus to the receptors of the immersed body-surface generally.

Extensive internuncial paths of ** distance-receptors." Con- formably with the power of the " distance-receptors " to induce movements or postures of the individual as a whole we find the neural arcs from these receptors particularly wide and far-reach- ing. The nerve-fibre that starts from the receptor does not in many of these cases itself extend to, or send processes to, the mouths of the ^'' final common paths'' Instead of doing so it ends often far short of them, and forms connection with other nerve-fibres (internuncial paths), which in their turn reach dis- tant " final common paths." This arrangement involves an inter- calation of gray matter between the ** private receptor " path and the " final common path " not only at the mouth of the latter, but also where the internuncial path itself commences. The signifi- ca?tce of this seems that the internuncial path is itself a ** common path, and therefore a mechanism of accommodation'' Its com- munity of function is not so extensive as that of a " final com- mon path," not co-extensive for instance with all the receptors of the body, as would appear the case with a motor-nerve to a skeletal muscle. Yet it furnishes a path for use by certain sets

^■f receptors in common. In Mustelus the nerve paths from the ^Betinal and from the olfactory receptors converge toward the roof- ^Bucleus of the mid-brain, whence passes the long mesencephalo- ^^pinal path to the spinal motor nuclei. The inference is that conjoint stimulation of eye and nose exert a combined influence I^Kid impinge together on the spinal motor machinery. Similarly ^^le Reissner fibre ^^^ may serve as an internuncial path between ^paths coming in from olfactory and visual receptors on the one ^Hand and the spinal motor common paths from the spinal cord ^H> the muscles on the other. Another instance of an internun- ^Kal path is the so-called " pyramidal tract " characteristic of the ^Hiammalian nervous system. It furnishes a path of internuncial character common to certain arcs that have arisen indirectly from various receptors of various species and are knitted to- gether in the cerebral hemisphere. Another instance is the path from the thalamus to the post-central convolution (Mott, Tschermak, and others).

Precurrent reactions. Consummatory reactions. It might seem at first that all motor reflexes may be grouped into those that tend to prolong the stimulus and those that tend to cut it short. Consideration shows that such a grouping expresses I^Wie truth but partially. We argued above that the ** distance- ^^i"eceptors " induce anticipatory or precurrent reactions, that is, precurrent to final or consummatory reactions. The reflexes of certain non-projicient receptors stand in very close relation to " consummatory" events. Thus the tango-receptors of the lips and mouth initiate reflex movements that immediately precede the act which for the individual creature viewed as a conative afid a sentient agent is the final consummatory one in respect to nutriment as a stimulus, namely, swallowing. Similarly with the gustato-receptors and their reactions. The sequence of action initiated by these non-projicient receptors is a short one : their reflex leads immediately to another which is consummatory. Those receptors of the chelae of Astacus, Homarus, etc., which initiate the carrying of objects to the mouth, or again the tango-

Ieceptors of the hand of the monkey when it plucks fruit and arries it to the lips, give reactions a step further from the con- summatory than those just instanced. These reactions are all steps toward final adjustments^ and are not themselves end-points. The series of actions of which the distance-receptors initiate the earlier steps form series much longer than those initiated by the non-projicient. Their stages, moreover, continue to be guided by the projicient organs for a longer period between initiation and consummation. Thus in a positive phototropic reaction the eye continues to be the starting place of the excitation, and in many cases guides change in the direction not only of the eyeball but of the whole animal in locomotion as the reflex proceeds. The mere length of their series of steps and the vicis- situdes of relation between bodies in motion reacting on one another at a distance conspire to give to these precurrent re- flexes a multiformity and complexity unparalleled by the reflexes from the non-projicient receptors. The reaction started by ** distance-receptors " where positive not only leads up to the consummatory reactions of the non-projicient, but on the way thither associates with it stimulation of other projicient recep- tors, as when, for instance, a phototropic reaction on the part of a Selachian brings the olfactory organs into range of an odorous prey, or, conversely, when the beagle sees the hare after run- ning it by scent. In such a case the visual and olfactory recep- tor arcs would be related as " allied " arcs (Lecture IV), and reinforce each other in regard to the mesencephalo-spinal path, or in higher mammals the ** pyramidal " or other pallio-spinal path. It is easy to see what copious opportunity for adjust- ment and of side connection such a reaction demands, consist- ing as it does of a number of events in serial chain, each link a modification of its predecessor.

Strong affective tone an accompaniment of consummatory re- actions. We may venture to turn briefly to the psychical aspect of such sequences. To consummatory reactions affective tone seems adjunct much more than to the anticipatory, especially the remotely anticipatory of the projicient sense-organs. Thus the affective tone of *' tastes " is strong. The reaction initiated by a noci-ceptor (pp. 226-23 1) is to be regarded as consummatory. The application of an irritant to the flank of a frog evokes a movement

f the leg adapted to at once remove that stimulus from the skin f the flank. Or again, an irritant applied to the skin of the ot evokes a movement of the foot away from that stimulus, both cases the reaction is a consummatory one, because it is Iculated of itself to be final. To judge by our own introspec- on the affective tone adjunct to these reactions is strong. They stance strong affective tone pertaining to consummatory re- tions. The affective tone of the reactions of the projicient ceptors is less marked : physical pleasure or pain can hardly said to accompany them. Not of course that they are wholly nrelated to affective tone. The relative haste with which an imal when hungry approaches food offered to the visual field suggests that conation attaches to the visual reaction by asso- ciation through memory with affective tone. By associative memory a tinge of the affective tone of the consummatory reac- tion may suffuse the anticipatory. The latter becomes indirectly a pleasure-pain reaction. The neutral tango-receptive reactions of the feet of the tortoise hastening stumblingly towards its food may in this way be imbued with a tinge of affective tone derived

irom the affective tint of the leading reflex, namely the visual, i^hich itself has thus memorial association with a consummatory eflex of strong affective tone. Examples of this type of reaction urnished by new-born animals are given by Lloyd Morgan.^®* When " after a few days the new-born chick leaves ladybirds mmolested while he seizes wasp-larvae with increased energy '* ne affords evidence that reactions of his projicient receptors have acquired a new value, and that value is made up mediately f affective tone. How they have acquired it or what exact ature their new attribute has is not our question. It is enough ere that in regard to certain stimuli the new value — the mean- g — which the projicient sensation has obtained has reinforced eatly the conative intensity of the reaction to the stimulus. It as given the stimulus increased force as a spring of precurrent ctions aimed at a final consummatory one. It has given this not y altering the external stimulus, nor the receptor-organ, but by, mong other alterations, altering internal connections of the re- ptor arc. Thus it is that, be it by associative memory or other

processes, the reactions of the " distance-receptors " come in higher animals to reveal a conative driving force which is per- haps the end for which these psychoses exist. Nor are the series of reactions, short though they be, which the non-projicient receptors initiate wholly devoid of conative appearance. They show adaptation as executive of steps toward an end. Food, sexual consummation, suitable posture, preser- vation from injury, are ends to which their direction leads, as with the longer series of actions due to projicient receptors reacting to objects at a wider horizon. It is rather that the lat- ter afford a freer field for the winning more subtle adjustments with wider application of associative memory. In the latter there is more scope for the play of mind, — mind it may be of such elementary grade as to be difficult for us to picture in its operations.

We may suppose that in the time run through by a course of action focussed upon a final consummatory event, oppor- tunity is given for instinct, with its germ of memory however rudimentary and its germ of anticipation however slight, to evolve under selection that mental extension of the present backward into the past and forward into the future which in the highest animals forms the prerogative of more developed mind. Nothing, it would seem, could better ensure the course of action taken in that interval being the right one than memory and anticipatory forecast : and nothing, it would seem, could tend to select more potently the individuals taking the right course than the success which crowns that course, since the consummatory acts led up to are such — e.g. the seizure of prey, escape from enemies, attainment of sexual conjugation, etc. — as involve the very existence of the individual and the species. The problem before the lowlier organism is in some slight measure shadowed to us by the difficulties of adjustment of reaction shown by the human child. The child, although his reactions are perfect within a certain sphere of his surroundings, shows himself at the confines of that sphere a little blunderer in a world of over- whelming meaning. Hence indeed half the pathos and humour derivable from childhood.

I^P It is the long serial reactions of the " distance-receptors " that allow most scope for the selection of those brute organisms that are fittest for survival in respect to elements of mind. The '• distance-receptors " hence contribute most to the upr earing of the v^K^ednim, Swallowing was above termed a consummatory reac- ^^^on. Once through the maw, the morsel is, we know by introspection, under normal circumstances lost for conscious- ness. But it nevertheless continues to excite receptors and their nervous arcs. The significant point is that the object has passed into such a relation with the surface of the organism that " conation " is no longer of advantage. The naive notion that when we have eaten and drunken we have fed is justified practically. No effort can help us to incorporate the food further. Conation has then done its all and has no further utility in respect to that food taken. It is significant that all direct psychical accompaniment of the reactions ceases abruptly at this very point. The immediately precedent reactions that were psychically suffused with strong affective colour pass ab- ruptly over into reactions not merely affectively neutral but void — normally — of psychical existence altogether. The concomi- tance between certain nervous reactions and psychosis seems an alliance that strengthens the restless striving of the individual animal which is the passport of its species to continuance of existence.

Receptive range. The ascendency of "distance-receptors " in the organization of neural function may be partly traceable to the T^XdXxwt frequency of their use. Although it would be incorrect to assess the value of an organ by the mere frequency with which it is of service, yet caeteris paribus that seems a fair criterion. The frequency with which a receptor meets its stimuli is, other things being equal, proportionate to the size of the slice of the external world which lies within its " receptive ranged Although in a fish, for instance, the skin with its tango-receptors is much larger in area than are the retinae with their photo-receptors, the restricted " receptive-range" — the adequate stimulus requiring actual proximity — of the former gives a far smaller slice of the

imulus-containing world to the skin than pertains to the eyes. In the case of the eye not only is the slice of environment pertain- ing to it at even a short distance more wide and high than that of the skin, but it is at each moment multiplied by the third dimen- sion. There arise in it, therefore {caeteris paribus) ^ in unit of time many more stimulations, with the result that the receptor-organ of " distant" species receives many more fresh stimuli per unit of time than does the receptor-organ of restricted receptive range. The greater richness of the neural construction of the photo- receptive system than of \he tango-receptive accords with this. Thus in the photo- receptive system the so-called '* optic nerve " (which since it is the second neural link and therefore to some extent a " common path," presents numerical reduction from the first or private path in the retina itself) contains more conduc- tive channels (nerve-fibres) in man (i, 000,000, Krause) than are contained in the whole series of afferent spinal roots of one side of the body put together (634,000, Ingbert 2^*» ^^), and of these latter the cutaneous afferent fibres form only a part, and of that part the tango-receptive fibres themselves form only a fraction. The large number of the channels in the retinal path is no doubt primarily indicative of spatial differentiations of the receptive sur- face, but that spatial differentiation is itself indicative of the numbers of the stimuli frequenting that receptive field.

Locomotion and "receptive range" Locomotive progression and distance receptivity are two phenomena so fundamentally correlated that the physiology of neither can be comprehended without recognition of the correlation of the two. Evidence is forthcoming from ontogeny and phylogeny. The elaborate- ness of the photo-receptive organs of the flying Insecta corre- sponds with the great power of these forms to traverse space. When the Brachiopod passes from a motile wandering life to a fixed sedentary one its " eyes " degenerate and go. The free- swimming Ascidia with fin-like motor organs and semi-rigid axial notochord, affording elasticity and leverage, bears at its anterior end a well-formed photo-receptor organ (eye) and a well-formed otocyst (head proprio-ceptor). Connected with the nerves of these, the anterior end of its truly vertebrate central nervous system has a relatively large *' brain." Thence extends back-

ward arbng the body a spinal cord. Suddenly its free-swimming habit is exchanged for a sedentary ; by adhesive projections from its head, it attaches itself permanently to some fixed object. At once there ensues a re-adaptive metamorphosis. Degeneration sets in concurrently in its locomotive musculature, its eye, its otocyst, its brain, and its cord. These vanish as by magic save that a fraction of the brain remains as a small ganglion near the mouth. The sessile creature retains, so far as can be judged from their microscopic structure, only some gustatory (?) recep- tors round the mouth, and some tango-receptors (? noci-ceptors) in the tegument, connected doubtless with an irregular diffuse subtegumental layer of unstriped muscle-tissue. Experimental observations seem wanting on the point, but we may presume that in this metamorphosis the receptive range of Ascidia dwindles from dimensions measurable by all the distance through which its free motile individual floats and swims, to a mere film of the external world, say a millimeter deep, at its own surface, espe- cially round its mouth, and unextended by succession of time, save passively by the mere flowing of the water. Such instances illustrate the fundamental connection between the function of the skeletal musculature and that of the ** distance-receptors." Did we know better the sensual aspects of these cases the more sig- nificant doubtless would be the comparison.

The " head " as physiologically conceived. As regards the objects acting on the organism at any moment through its receptors, the extension of environmental space — the animal's receptive range — is not equal in all directions as measured from the organism itself. The extension is greater in the direction about the "leading" pole.. Thus, the reactions initiated at the eye forerun reactions <^cf. Loeb's Ketten-reflexe) that will in due time come to pass through other receptor-organs. The visual receptors are usually near the leading pole, and so placed that they see into the field whither progression goes. And simi- larly with the olfactory receptors. The motor train behind, the elongated motor machinery of the rest of the body, is there- fore from this point of view a motor appendage at the behest of the distance-receptor organs in front. The segments lying at

the leading pole of the animal, armed as they are with the great ** distance " sense-organs, constitute what is termed the " head." The proprio-ceptive system and the head. We may now at- tempt to enquire whether this dominance of the leading segments which is traceable in the receptors of the extero-ceptive field ap- plies in the field of reception which we termed the proprio-ceptive. We arrived earlier at the notion that the field of reception which extends through the depth of each segment is differentiated from the surface field by two main characters. One of these was that while many agents which act on the body surface are excluded from the deep field as stimuli, an agency which does act there is mass, with all its mechanical consequences, such as weight, mechanical inertia, etc., giving rise to pressures, strains, etc., and that the receptors of this deep field are adapted for these as stimuli. The other character of the stimulations in this field we held to be that the stimuli are given in much greater measure than in the surface field of reception, by actions of the organism itself, especially by mass movements of its parts. Since these movements are themselves for the most part reac- tions to stimuli received by the animal's free surface from the environment, the proprio-ceptive reactions themselves are results in large degree habitually secondary to surface stimuli. The immediate stimulus for the reflex started at the deep receptor is thus supplied by some part of the organism itself as agent.

In many forms of animals, e.g. in Vertebrates, there lies in one of the leading segments a receptor-organ (the labyrinth) derived from the extero-ceptive field, but later recessed off from it; and this is combined in action with receptors of the proprio- ceptive field of the remaining segments. This receptive organ, like those of the proprio-ceptive field, is adapted to mechanical stimuli. It consists of two parts, both endowed with low recep- tive threshold and with refined selective differentiation. One part, the otolith organ, is adapted to react to changes in the incidence and degree of pressure exerted on its nerve-endings by a little weight of higher specific gravity than the fluid other- wise filling the organ. The other part, the semicircular canals.

reacts to minute mass movements of fluid contained within it 'hese two parts constitute the labyrinth. The incidence and legree of pressure of the otoHths upon their receptive bed :hange with changes in the position of the segment in which le labyrinth lies, relatively to the horizon line. Movements of le segment likewise stimulate the labyrinthine receptors through le inertia of the labyrinthine fluid and the otoliths. By the ibyrinth are excited reflexes which adjust the segment (and ith it the head is usually immovably conjoined) to the horizon Ine. And other parts are similarly reflexly adjusted by it. 'hus, the refined photo-receptive patches in the head — the fetinae — which conduct reflexes delicately differential in re- 'gard to space, appropriate for stimuli higher or lower or to right or to left in the photo-receptive patch, depend in their conduct of these upon a more or less constant standardization of their own normals of direction in regard to the horizon line. These photo-receptive patches are set movably in the head; by the action of muscles they can retain their bearing to the hori- zon, although the head itself shifts its relation to the horizon. |The control of these muscles lies largely with the labyrinth. The ibyrinth produces a compensatory eyeball reflex. Thus in the lead segments the labyrinth effects reflex movements analogous that which the proprio-receptive nerves from the extensor luscles of the knee excites in the leg segments, reflexes restor- ing an habitual posture that has been departed from.

And from the above it seems clear that there is another fea- ire of resemblance between the labyrinthine receptor and the >roprio-ceptors of the limb. Stimulation of the labyrinth must In preponderant measure be given not by external agents directly but by the reaction of the organism itself. Posture and move- ment of the head are the immediate causes which stimulate the labyrinth, whether or not they be part of a total movement or posture of the whole individual. Such movement is most fre- quently an active one on the part of the animal itself. Thus, when Ascidia becomes sedentary and its locomotor muscalature atrophies its otocyst disappears. But an animal's active move- ment is in its turn usually traceable as a reaction to an environ-

mental stimulus affecting the receptors at the surface of the animal. Thus the labyrinthine receptors like the proprio-ceptors in other segments, are stimulated by the animal itself as agent, though secondarily to stimulation of the animal itself via some extero-ceptor. And there is another point of likeness between labyrinth re- flexes and those of the proprio-ceptors of the limb and other segments. The proprio-ceptors of the limbs appear productive of certain continuous, that is tonic, reflexes. Thus, in the decere- brate dog the tonic extensor rigidity of the leg appears reflexly maintained by afferent neurones reaching the cord from the deep structures of the leg itself. Similarly, if the knee-jerk be accepted as evidence in the spinal animal of a spinal tonus in the extensor muscle, this tonus seems maintained by afferent fibres from the extensor muscle itself, since the knee-jerk is extinguished by severence of those fibres. Again, the rapidity of onset of rigor mortis in a muscle is speedier when its tonus prior to death has been high. Section of the afferent roots ot the limb prior to death delays onset of rigor mortis ^^ in that limb as judged by stiffness at the knee ; but that delay is not observable when skin-nerves only have been severed. The labyrinthine receptors appear likewise to be the source of cer- tain maintained, that is tonic, reflexes. Destruction of the laby- rinth also delays the onset of rigor-mortis in the muscles to which its field of tonus can be traced. Ewald has shown that each labyrinth maintains tonus especially in the neck and trunk muscles and in the extensor-abductor limb-muscles of the homonymous side.

In regard to these tonic reflexes it is difficult to see how a steady mechanical stimulus can continue to elicit a reflex con- stantly for long periods. If we take sensation as a guide, a touch excited by constant mechanical pressure of slight intensity fades quickly below the threshold of sensation. It is said that a spinal frog may even be crushed by mechanical pressure without ex- citing from it a reflex movement provided that the pressure be applied by very slowly progressive increments. The office of a receptor would seem to be, placed across the line of a

^■tream of energy, to react under the transference of energy Btross it, as for instance from the environment to the organism, or vice versa. We have many instances in which the Hving material adapts itself to, and maintains its own equilibrium under, different grades of environmental stress, treating each fairly continuous or slowly altering grade as a normal zero. (he slow changes of barometric pressure on the body surface riginate no skin-sensation, though they are much above the threshold value for touch. There streams constantly from the body through the skin a current of thermal energy much above the threshold value of stimuli for warmth sensations ; yet this current evokes under ordinary circumstances no sensation. It is the stationary condition, the fact that the transference of energy continues at constant speed, which makes it unperceived. The receptor apparatus is not stimulated unless there is a change of rate in the transference, and that change of rate must occur in most cases with considerable quickness, otherwise there is a mere unperceived shift in the stationary equilibrium which forms the resting zero of the sensual apparatus. Over and over in the elicitation of reflexes as well as in the artificial excitation of nerve or muscle we meet this same feature. Both for sensa- tion and for reflex action a function in the threshold value of stimulus is time as well as intensity and quantity. If a weak agent is to stimulate, its application must be abrupt. But in the tonic reflexes whose source lies at the proprio-ceptors and the labyrinth a weak stimulus, although apparently unchanging, seems to continue to be an effective stimulus.

The proprio-ceptors and the labyrinthine receptors seem to have in common this, that they both originate and maintain tonic reflexes in the skeletal muscles. And they, at least in some instances, reinforce one another in this action. Thus the tonus of the extensor muscle of the knee in the cat and dog appears to have a combined source in the proprio-ceptors of that muscle itself and in the receptors of the homonymous labyrinth. The tonus of skeletal muscles is an obscure problem. Its mode of production, its distribution in the musculature, its purposive sig- nificance, are all debateable. The steadiness and slight inten-

sity of the contraction constituting the tonus render its detection difficult Part of the discrepancy between the experimental findings may be traced to the supposition that a reflex tonus if present is present in all muscles at all times. A single muscle examined for reflex tonus has been taken to represent all muscles under all conditions, although the answer has been sometimes positive and sometimes negative. It appears to me likely that reflex tonus is the expression of a neural discharge concerned with the maintenance of attitude. In many reflex reactions the eff'ect is movement and the muscles are dealt with as organs of motion. In these cases the stimuli and the reactions both of them are short-lived events. But much of the reflex reaction expressed by the skeletal muscu- late is postural. The bony and other levers of the body are maintained in certain attitudes both in regard to the horizon, to the vertical, and to one another. The frog as it rests squatting in its tank has an attitude far different from that which gravita- tion would give it were its musculature not in action. Evi- dently the greater part of the skeletal musculature is all the time steadily active, antagonizing gravity in maintaining the head raised, the trunk semi-erect, and the hind legs tautly flexed. Innervation and co-ordination are as fully demanded for the maintenance of a posture as for the execution of a movement This steady co-ordinate innervation antagonizes gravitation and other forces, e, g. as in currents of water. In these tonic as in other reflexes antagonistic muscles co-operate co-ordinately. There is nothing to show that reciprocal innervation does not obtain in the one class of reflex as in the other. If so, it be- comes easily intelligible that the slight reflex contraction termed skeletal tonus should under given conditions be found in some muscles and not in others. The slight reflex contraction will be accompanied by reflex inhibition of the antagonistic muscles. For reflex tonus to be the expression of a neural discharge which maintains attitude accords well with the ascription of its source to the proprio-ceptors, including the labyrinth. Those are exactly the receptors which, functioning as sense-organs, initiate sensations of posture and of attitude (Bonnier). And it

accords also with the share in the production and regulation of skeletal tonus which the cerebellum has (Luciani's atonia) and ,the cerebrum. Naturally, the distinction between reflexes of attitude and reflexes of movement is not in all cases sharp and abrupt. Be- /een a short lasting attitude and a slowly progressing move- lent the difference is hardly more than one of degree. Moreover, jach posture is introduced by a movement of assumption, and ifter each departure from the posture, if it is resumed, it is re- verted to by a movement of compensation. Hence the taxis of ittitude must involve not only static reactions of tonic mainte- lance of contraction, but innervations which execute reinforcing ^movements and compensatory movements. In all this kind of (function the proprio-ceptors of the body generally and of the labyrinthine receptors in the head appear to co-operate together md form functionally one receptive system.

This system as a whole may be embraced within the one term '* proprio-ceptive." Our inquiry regarding it is now, whether that part of it which is situate in the leading segments, namely Its labyrinthine part, exerts preponderance in the system as in [the extero-ceptive system do the extero-ceptors situate in the 'leading segments. It must be remembered of the extero-ceptive system that even in the segments which are not the leading segments its receptors considered as sense organs produce sensations that have some projicience ; and that in animals pro- vided with outstanding skin appendages, e.g. hair, the tango- reflexes are to a slight extent reactions to objects at a distance. This germ of distance reaction and projicience of sensation in the extero-ceptors of the ordinary body-segments is developed in the extero-ceptors of the leading segments into the vast distance-reactions of the eye and the absolute projicience of vision. But the proprio-ceptors of the limb and body segments exhibit no germ of distance-reaction nor of projicience of sensa- tion. And the specialized proprio-ceptor organ of the leading segment (the labyrinth) is similarly not a distance-receptor; although some of its sensations seem projected into the environ- ment as well as referred to the organism itself, to the " material