The Integrative Action of the Nervous System

The synaptic nervous system has developed as its distinctive feature a central organ, a so-called central nervous system ; it is through this that it brings into rapport one with another widely distant organs of the body, including the various portions of the diffuse nervous system itself. That portion of the synaptic system which is termed " cen- tral " is the portion where the nervous paths from the various peripheral organs meet and establish paths in common, i. e. '•' common paths y It is therefore in accord with expectation that we find the organ in which this meeting occurs situated fairly midway among them all, /. e. centrally. In bilaterally symmetri- cal animals this organ would be expected to lie where it does, namely, equidistant from the two lateral surfaces of the animal, and to exhibit as it does, laterally symmetrical halves united by a number of nervous cross ties bridging the median line. This central nervous organ contains almost all the junctions existent between the multitudinous conducting arcs. In it the afferent

paths from receptor-organs become connected with the efferent paths of effector-organs, not only those adjacent to their own receptors but, through '' intermmciaV (J. Hunter, 1778) paths, with efferent paths to effector-organs remote. This central " exchange " organ is therefore well called the central nervous system. In the higher Invertebrata it is known as the longitu- dinal nerve-cord with ganglia, supraoesophageal, suboesopha- geal, etc. ; in Vertebrata it is known as the spinal cord and brain. Under these different anatomical names the same physiological organ is designated. It would be more convenient for the biol- ogist were one general term for it in use. We have seen that it is not merely a meeting place where afferent paths conjoin with efferent, but is, in virtue of its physiological properties, an organ of reflex reinforcements and interferences, and of refractory phases, and shifts of connective pattern ; that it is, in short, an organ of co-ordination in which from a con- course of multitudinous excitations there result orderly acts, reactions adapted to the needs of the organism, and that these reactions occur in arrangements {patterns) marked by absence of confusion, and proceed in sequences likewise free from confusion. "^ — 1

By the development of these powers the synaptic system » with its central organ is adapted to more speedy, wide, and delicate co-ordinations than the diffuse nervous system allows. Out of this potentiality for organizing complex integration there is evolved in the synaptic nervous system a functional grading of its reflex arcs and centres. Thus, with allied reflexes, the mechanism of the common path knits together by plurireceptive summation not only the separate individual stimuli of similar kind, e,g. tangoreceptive or photo receptive received from some agent as this latter becomes prepotent in the environment ; but it knits together separate stimuli of even wholly different recep- tive species. C. J. Herrick has shown that in Ameiurus nebulo- sus (cat-fish) the reaction of the animal to stimulation of the barblets by meat is a reaction to a twofold stimulus, a chemical and a mechanical, and he finds that these two reactions mutually reinforce. Nagel reports a similar case with the tentacle of the

Actinian, Aiptasis saxicola. v. Uexkiill finds that the Gift- zangen of Echinus acutiis react only when a chemical and a mechanical stimulus are combined. The several qualitatively different properties of an object which is acting as stimulus are thus combined and reinforce each other in eliciting appropriate reaction. By this summation reflex complication in Herbart's sense is made possible. A touchstone for rank of a centre in this neural hierarchy is the degree to which paths from separate loci and of different receptive modality are confluent thither. Indicative of high rank is such functional position as relieves from " local work " and involves general responsibility, e. g. for a series of segments or for the whole body. The " three levels," of Hughlings Jackson is an expressive figure of this grading of rank in nerve-centres.

Integrative action of the nervous system in the segment and in the segmental series. In animal organisms of any consider- able complexity a division of the body into segments, metameres, is widely found. By the occurrence of separating constrictions or sepiments, or through the regular repetition of appendicular structures, subdivisions of the body are established which sev- erally possess analogues of functions possessed more or less similarly by the other subdivisions but also severally possess functional unity. Such is this functional unity and completeness that in some instances a metamere comes to be independent of the total organism, and able to lead a separate existence. The nervous system it is which largely gives functional solidarity to the composite collection of unit lives and organs composing the individual metamere. Further, the linkage of the several meta- meres into one functional whole is largely of nervous nature. The integrative function of the nervous system is seen to perfec- tion in the welding together of metameres into the unity of an animal individual. The kind of nervous system employed for this is the synaptic system. Although the nerve-net system is retained even in the highest vertebrates, it is then confined to unsegmentally arranged musculature, e.g. visceral and vascular. In the skeletal musculature, where segmental arrangement holds, the nervous system is synaptic. It is not surprising therefore

that in metameric animals the nervous system, especially its synaptic part, should strikingly exhibit that metamerism. Various schemes of metamerism have been evolved. Where it is radiate so that each segment bears exactly similar relations to the common axis and to the other segments, the opportunity for dominance of one segment over the rest is slight. The con- ditions of life for each segment are practically those which are the average for all. The mouth, for instance, lies equidistant from them all. Evolution toward higher differentiation of the whole metameric individual and toward more intricate welding of its parts into one, is at a disadvantage in these radiate forms as compared with its opportunity in the great groups of Arthropoda and Vertebrata where the metameres are ranged serially along a single axis, the longitudinal axis of the organism. With fore and aft arrangement of its segments the animal body has its first opportunity for really high differentiation. Certain of the seg- ments of necessity lie nearer to the mouth than do others; moreover certain segments come to habitually lead, that is to say go foremost, during the animal's active locomotion.

In the integrating function of the nervous system a segmental arrangement of its functions is frequently apparent. It makes itself felt in two ways. Firstly, the various separate and differ- ent elements of the segment are knit together by nervous ties. Secondly, where kindred functions are exercised in successive segments, so that throughout a series of segments one set of organs forms a more or less functionally homogeneous system, these organs are combined by interrelated nervous arcs. But particular systems of organs common to all or many metameres of an individual present special differentiation of their function in particular metameres. In this manner the organism is built up of component segments possessing resemblance one to an- other, but presenting also specializations peculiar to certain seg- ments. Hence the segmental arrangement forms a convenient basis not merely for anatomical but for physiological description. And in dealing with the special problems of integration by the nervous system, especially those of the synaptic nervous system, analysis can employ two co-ordinate sets of descriptive factors

— one the segment, the other the line of organs of analogous i function scattered along the series of segments. In the two I great animal groups just mentioned the latter ordinate is longi- ^ tudinally extended, while the individual segment is extended j transversely. The analysis thus proceeds formally somewhat in ^ the same way as the analysis of a plane figure by rectangular] co-ordinates. , The receptive fields. The central nervous system, though | divisible into separate mechanisms, is yet one single harmoni-l ously acting although complex whole. To analyze its action we ^ turn to the receptor-organs, for to them is traceable the initia- ' tion of the reactions of the centres. These organs fall naturally , into three main groups, distributed in three main fields, each \ field being differently circumstanced.

Multicellular animals regarded broadly throughout a vast \ range of animal types are cellular masses presenting to the en- i vironment a surface sheet of cells, and under that a cellular , bulk more or less screened from the environment by the surface j sheet. Many of the agencies by which the environment acts on | the organism do not penetrate to the deep cells inside. Bedded \ in the surface sheet are numbers of receptor cells constituted in 1 adaptation to the stimuli delivered by environmental agencies. 1 The underlying tissues devoid of these receptors are not devoid \ of all receptor-organs ; they have other kinds apparently specific | to them. Some agencies act not only at the surface of the ; organism but penetratively through its mass. Of these there \ are for some apparently no receptors adapted, for instance, none j for the Rontgen rays. For others of more usual occurrence re- ■ ceptors are adapted. The most important of these deep ade- ■ quate agents seems to be mass acting in the mode of weight and \ mechanical inertia involving mechanical stresses and mechanical strain. Moreover, the organism, like the world surrounding it, : is a field of ceaseless change, where internal energy is continu- \ ally being liberated, whence chemical, thermal, mechanical, and \ electrical effects appear. It is a microcosm in which forces are \ at work as in the macrocosm around. In its depths lie receptor- \ organs adapted consonantly with the changes going on in the ;

microcosm itself, particularly in its muscles and their accessory apparatus (tendons, joints, walls of blood-vessels, and the like). There exist, therefore, two primary distributions of the recep- tor-organs, and each constitutes a field in certain respects funda- mentally different from the other. The deep field we have called ihQ proprio-ceptive ^oXd, because its stimuli are, properly speaking, events in the microcosm itself, and because that circumstance has important bearing upon the service of its receptors to the organism.

Richness of the extero-ceptive field in receptors ; comparative poverty of the intero-ceptive. The surface receptive field is again subdivisible. It presents two divisions. Of these one lies freely open to the numberless vicissitudes and agencies of the environment. That is to say, it is co-extensive with the so- called exterjtal surface of the animal. This subdivision may be termed the extero-ceptive field. But the animal has another surface, its so-called internal, usually alimentary in function. This, though in contact with the environment, lies however less freely open to it. It is partly screened by the organism itself. For purposes of retain- ing food, digesting and absorbing it, an arrangement of common occurrence in animal forms is that a part of the free surface is deeply recessed. In this recess a fraction of the environment is more or less surrounded by the organism itself. Into that sequestered nook the organism by appropriate reactions gathers morsels of environmental material whence by chemical action and by absorption it draws nutriment. This surface of the animal may be termed the intero-ceptive. At its ingress several species of receptors are met with whose ** adequate " stimuli are chemical {e. g. taste organs). Lining this digestive chamber, this kitchen, the intero-ceptive surface is adapted to chemical agencies to a degree such as it exhibits nowhere else. Compara- tively little is yet known of the receptor-organs of this surface, though we may suppose that they exhibit refined adaptations. But the body-surface in this recess, though possessed of certain receptors specific to it, is sparsely endowed as contrasted with that remainder of the surface (the extero-ceptive surface) lying

open fully to the influences of the great outer environment. The afferent nerve-fibres in the sympathetic system as judged by their number in the white rami are comparatively few; Warring- ton's 2^^ recent observations show this conclusively. The poverty of afferent paths from the intero-ceptive field is broadly indicated by the fact that we know no wholly afferent nerve-trunk in the sympathetic (Langley's " autonomic ") system, though such are common enough in the nervous system subserving the extero- ceptive arcs ; and that in the latter system we know no wholly efferent nerve-trunk, whereas in the sympathetic such exist, e.g. the cervical sympathetic trunk.

The extero-ceptive field far exceeds the intero-ceptive in its wealth of receptor-organs. This seems inevitable, for it is the extero-ceptive surface, facing outward on the general environ- ment, that feels and has felt for countless ages the full stream of the varied agencies forever pouring upon it from the outside world. Mere enumeration of the different species of receptor-organs recognizable in it suffices to illustrate the importance of this great field. It contains specific receptors adapted to mechanical contact, cold and warmth, light, sound, and agencies inflicting injury {noxd). Almost all these species of receptors are distrib- uted to the extero-ceptive field exclusively ; they are not known to exist in the intero-ceptive or in the proprio-ceptive fields.

It is an instructive exercise to try to classify the stimuli ade- quate for the receptors of the extero-ceptive field. Each animal has experience only of those qualities of the environment which as stimuli excite its receptors ; it analyzes its environment in terms of them exclusively. Doubtless certain stimuli causing reactions in other animals are imperceptible to man ; and in a large number of cases his reactions are different from theirs. Hence it is impossible for man to conceive the world in terms more than partially equivalent to those of other animals. Hu- manly, the classification of adequate stimuli can be made with various departments of natural knowledge as its basis. Physics and chemistry can be taken as basis usefully in a number of cases where the sources of stimulation are known to those more exact branches of experimental science. But in several ways a

physico-chemical scheme of classification of stimuli lacks sig- nificance for physiology. Thus, in the case of the noci-ceptive organs of the skin, those receptors — probably naked nerve- endings — are non-selective in the meaning that they are excit- able by physical and chemical stimuli of diverse kind, radiant, mechanical, acid, alkaline, electrical, and so on, so that a classi- fication according to mode of exciting energy on the one hand fails to differentiate them from each of a number of more specialized other groups (tango-receptors, chemo-receptors, etc.) from which biologically they are quite different, and on the other hand that classification apportions them, though physio- logically a single group, to a whole series of different classes. A physiological classification deals with them more satisfac- torily. Physiological criteria can be applied which at once separate them from other receptors and yet show their affinity one to another. Thus, physiologically the stimulus which ex- cites these end-organs must, whatever its physical or chemical nature, possess, in order to stimulate them, the quality of tending to do immediate harm to the skin. Further, the reflex they excite (i) is prepotent ; (ii) tends to protect the threatened part by escape or defence; (iii) is imperative ; and (iv) if we include psychical evidence and judge by analogy from introspection, is accompanied hy pain*

Here what we may call the physiological scheme of classifi- cation proves the more useful at present. And similarly it proves useful with a group of stimuli that may be termed " distance- stimuli," to which we must turn presently. The key to the physiological classification lies in the reaction which is produced. But the physico-chemical basis of classification also has its uses, and especially with those manifold receptors of the extero- ceptive field which possess highly developed accessory struc- tures that render them selectively receptive — and among these are some of the most highly adapted and important receptors, e.g. photo-receptors, possessed by the organism. It is to the extero-ceptive field that these belong.

Nervous integration of the segment. The edifice of the whole central nervous system is reared upon two neurones, — the affer- ent root-cell and the efferent root-cell. These form the pillars 1 these two are superposed and functionally set, mediately or im- j mediately, all the other neural arcs, even those of the cortex of I the cerebrum itself The private receptor paths and the com- i mon effector paths are in the Chordata gathered up in a single i

nerve-trunk for each segment. Close to the central nervous \ organ, however, there occurs in the segmental nerves of many | Vertebrates a cleavage of the private receptor paths from the [ common effector paths. A dorsal spinal nerve of centripetal I conduction and a ventral of centrifugal conduction results, i Among the afferent root-cells (afferent spinal root of Verte- ' (cutaneous) and from the proprio-ceptive (deep) fields. In \ many segments there is a third quotum, intero-ceptive, from the '^ visceral field. This visceral constituent of the spinal ganglion is

not present in all segments and is probably, even in those seg- ■ ments in which it is present, numerically the weakest of the three components. Cranial, caudal, and other segments exist, j therefore, in which the total afferent nerve of the segment is \ extero-ceptive and proprio-ceptive but not intero-ceptive. In the remaining segments it is intero-ceptive as well as extero- \ ceptive and proprio-ceptive, and in these its function is therefore i

The efferent segmental nerve conversely radiates outward from the central end of the afferent nerve and the central nervous organ to the various effector organs at the surface and in the : depth of the segment. Function does not, however, strictly ; respect the ancestral boundaries of segments. Among the ' efferent fibres in the ventral root are a number that extend quite beyond the boundaries of the segment in which the spinal root is placed. These pass to the viscera and muscles of the

skin. They embouch not directly into their effector organs, e.g. ^ intestinal muscle-wall, pilomotor muscle, etc., but into ganglia ' of the sympathetic system. In these ganglia, although not gray matter in the same sense as spinal cord and brain, axone-endings, perikarya, and dendrites are nevertheless found. By its distri- bution to the cells in such a ganglion and by being distributed in many cases to more than one such ganglion, a single con- stituent efferent path in the ventral spinal root obtains access to a very large number of effector organs. These ganglia seem, therefore, mechanisms for the distribution of nerve-impulses. We have seen (p. 310) how by such widening of distribution the threshold of effective reaction is lowered. But though adapted for distribution of nerve-impulses there is no evidence that these ganglia can serve for the regulation of them in the same sense as does the gray matter of the spinal cord with its synapses of variable resistance and connection. Prominent among the inte- grating connections intrinsic to each segment itself are conduct- ing paths from the extero-ceptive field to the *' final common paths " for the skeletal musculature. Thus, in the mammal we laid it down (Lect. V, p. 157) as a general rule that " for each afferent root there exists in immediate proximity to its own place of entrance into the cord, i. e, in its own segment, a reflex motor path from skin to muscle of as low resistance as any open to it anywhere."

The extero-ceptive arcs appear in most segments less closely connected with the visceral musculature than with the skeletal musculature. The intero-ceptive arcs appear in most segments less closely connected with the skeletal musculature than with the visceral. In physiological parlance a resistance to conduc- tion seems intercalated between the two. But both extero- ceptive and intero-ceptive fields easily influence through their nervous arcs the musculature of the blood vascular organs. So also do the receptors of the proprio-ceptive field itself; and these latter are in particularly close touch with the skeletal muscula- ture, exerting tonic influence on it. In certain segments these general relations are modified in special ways. Thus, in those segments where the intero-ceptive and extero-ceptive fields con- join, e.g. at the mouth and the cloaca, closer nervous connec- tions exist between the intero-ceptive arcs and the skeletal musculature, and conversely between the extero-ceptive arcs and the visceral musculature. Thus stimuli acting on the pharyngeal receptors evoke or inhibit activity of skeletal muscles subserving

respiration and deglutition ; stimuli to the cloacal mucosa evoke movements of the caudal skeletal muscles ; and so forth. It is not merely specific difference between the receptors of the extero-ceptive field and those of the intero-ceptive which brings the former into closer relationship with the skeletal musculature. Receptors of the one and the same species, if they lie in the extero-ceptive field, work skeletal musculature ; if they lie in the intero-ceptive, work visceral musculature. Thus the chemo-receptors on the outer surface of the head (gustatory of the barblets of fish) excite reflexes which move the body round, bringing the mouth to the morsel ; while the similar chemo-receptors within the mouth excite reflex swallow- ing without outward movement of the animal (C. J. Herrick).

Receptors of the same specific system, where they lie close together, mutually reinforce reaction. On the contrary, where members of two different systems lie close together, e.g. tango- receptor and noci-ceptor, in one and the same piece of skin, they, as mentioned above, often have conflicting mutual relation. One relationship between receptor arcs of the same species may be particularly noted. Receptors symmetrically placed on opposite sides of the segment, especially if distant from the median plane, excite reactions which mutually ** conflict." Thus, when a noci-ceptor is stimulated on the right side of the tail of the spinal dog or cat or lizard, the reaction moves the organ to the left. The symmetrical receptor on the left side does the converse. The two reactions thus conflict. And the like holds true for the many right and left symmetrical receptors which initiate exactly converse reactions.

But a group of special cases is formed by reactions initiated from receptors distributed at or near to the median line. Stimu- lation of such a small group of receptors at the median line in many cases evokes a bilateral movement which is symmetri- cal, e. g. a touch on the decerebrate frog's lip in the median line causes both fore limbs to sweep forward synchronously over the spot. The median overlap of the distribution of the afferent fibres of the dorsal spinal roots may be connected with this.

^B Special refinements of the receptors of the "leading " segments. As the receptors that are excitable by the various adequate agencies, e.g. mechanical impact, noxa, radiant energy, chemical solutions, etc., are traced along the series of segments, it is found that in one region of the longitudinal segmental series remark- able developments exist. In motile animals constituted of segments ranged along a single axis, e.g. Vertebrata, when locomotion of the animal goes on, it proceeds for the most part along a line continuous with the long axis of the animal itself, and more frequently in one direction of that line than in the other. The animal's loco- motor appendages and their musculature are favorably adapted for locomotion in that habitual direction. In the animal's pro^7 gression certain of its segments therefore lead. The receptors of these leading segments predominate in the motor taxis of the animal. They are specially developed. Thus, in the earth- worm, while all parts of the external surface are responsive to light, the directive influence of light is greatest at the anterior end of the animal. The leading segments are exposed to exter- nal influences more than are the rest. Not only do they receive more stimuli, meet more " objects " demanding pursuit or avoid- ance, but it is they which usually first encounter the agents beneficial or hurtful of the environment as related to the indi- vidual. Pre-eminent advantage accrues if the receptors of these leading segments react sensitively and differentially to the agen- cies of the environmentj And it is in these leading segments that remarkable developments of the receptors, especially those of the extero-ceptive field, arise. Some of them are specialized in such degree as almost obscures their fundamental affinity to others distributed in other segments. Thus, among the system of receptors for which radiation is the adequate agent, there are developed in one of the leading segments a certain group, the retinal, particularly and solely, and extraordinarily highly, amenable to radiations of a certain limited range of wave-length. These are t\iQ photo-receptors, for which light and only light, e.g. not heat, is the adequate stimulus.

In like manner a certain group belonging to the system receptive of mechanical impacts attains such susceptibility for these as to react to the vibrations of water and air that constitute physical sounds. The retina is thus a group of glorified " warm-spots," the cochlea a group of glorified " touch-spots." Again, a group belonging to the sys- tem adapted to chemical stimuH reach in one of the leading segments such a pitch of delicacy that particles in quantity un- weighable by the chemist, emanating from substances called odorous, excite reaction from them.

The refined receptors of the leading segments are " distance- receptors." The after-coming segments form a motor train actuated chiefly by the " distance -receptors." It is in the leading segments that we find the " distance-receptors," For so may be called the receptors which react to objects at a distance. These are the same receptors which, acting as sense-organs, initiate sensations having the psychical quality termed projicieiice. The receptor- organs adapted to odors, light, and sound, though stimulated by the external matter in direct contact with them, — as the vibrating ether, the vibrating water or air, or odorous particles, — yet generate reactions which show ** adaptation," e.g. in direction of movements, etc., to the environmental objects at a distance, the sources of those changes impinging on and acting as stimuli at the organism's surface. We know that in ourselves sensations initiated through these receptors are forthwith '* pro- jected " into the world outside the " material me." The proji- cience refers them, without elaboration by any reasoned mental process, to directions and distances in the environment fairly accurately corresponding with the " real " directions and dis- tances of their actual sources. None of the sensations initiated in the proprio-ceptive or intero-ceptive fields possess this prop- erty of projicience. And with the distance-receptors considered simply as originators of reflex actions, their reflexes are found to be appropriate to the stimuli as regards the direction and distance of the sources of these latter. Thus, the patch of light constituting a retinal image excites a reflex movement which turns the eyeball toward the source of the image and adjusts ocular accommodation to the distance of that source from the animal itself. Even a negative stimulus suffices. The

shadow of the hand put out to seize the tortoise excites, as it blots the retinal illumination, withdrawal of the animal's head to within the shelter of the shell. How this result of ** distance " has been acquired is hard to say. The net effect is reached in various ways, and with very various gain in the degree of *' distance " acquired. By long vibrissae certain tango-receptors obtain excitation from objects still at a distance from the general surface of the organism. By reduc- tion of their threshold value of stimulus, certain other receptors akin to tactual, inasmuch as their adequate stimuli are mechan- ical, become responsive to vibratory movements of water and air so as to react to physical sounds whose sources lie remote from the animal. Certain chemo-receptors acquire so low a threshold that they react not merely to food and other substances in con- tact with them in mass, but react to almost inconceivably diluted traces of such, traces which drift off from the objects and per- meate the environment through long distances, as so-called odors, before impinging upon the delicate receptors in ques- tion. The leading segments thus come to possess not only taste, but taste at a distance, namely smell. In such cases it ieems chiefly by lowering of their threshold that these receptors »f the leading segments have been brought to react to objects till remote from the organism.

The " distance-receptors " seem to have peculiar importance ►r the construction and evolution of the nervous system. In le higher grades of the animal scale one part of the nervous sys- \m has, as Gaskell insists, evolved with singular constancy a (ominant importance to the individual. That is the part which called the brain. T/ie brain is always the part of the nervous fstem which is constructed upon and evolved upon the " distance- \eceptor'' organs. Their effector reactions and sensations are idently of paramount importance in the functioning of the lervous system and of the individual. This seems explicable, ^t least partly, in the following manner.