To aid our conceptions we will as before (§ 19) take the rude analogy furnished by a row of bricks on end, which overthrow one another in succession. If such bricks on end have been adjusted so that their faces are all at right angles to the line of the series, the change will be propagated along them with the least hindrance; or, under certain conditions, with the greatest multiplication of the original impulse, For when so placed, the impact each brick gives to the next, being exactly in the line of the series, will be wholly effec- tive y but when they are otherwise placed it- will not. If the bricks stand with their faces variously askew, each in falling will have a motion more or less diverging from, the line of the series; and hence only a part of its momentum will impel the next in the required direction. Now though in the case of a series of molecules the action can be by no means so simple, yet the same principle holds. The isomeric change of a molecule must diffuse a wave which is greater 618 PHYSICAL SYNTHESIS.
In some one direction than in all others. If so, there are certain relative positions of molecules such that each will receive the greatest amount of this wave from its predecessor, and will so receive it as most readily to produce a like change in itself. A series of molecules thus placed must stand in symmetrical relations to one another — polar relations. And it is not difficult to see that, as in the case of the bricks,, any deviation from symmetrical or polar relations will involve a proportionate deduction from the efficiency of the shock, and a diminution in the quantity of molecular motion given out at the far end. But now, what is the indirect result when a wave of change passes along a line of mole- cules thus unsymmetrically placed? The indirect result is that the motion which is not passed on by the unsymmetri- cally-placed molecules,, goes towards placing them symmetri- cally. Let us again consider what happens with our row of bricks. When one of these in falling comes against tho next, standing askew, its impact is given to the nearest angle of this next,, and so tends to give this next a motion round its axis. Further, when the next thus moved delivers its motion to its successor, it does this not through the angle on the side that was struck, but through the diagonally- opposite angle; and, consequently, the reaction, of its impact on its successor adds to the rotatory motion already re- ceived. Hence the amount of force which it does not pass on, is the amount of force absorbed in turning it towards parallelism with its neighbours. Similarly with the mole- cules. Each in falling into its new isomeric attitude, and passing on the shock to its successor, gives to its successor. a motion which is all passed on if the successor stands in polar relation towards it, but which, if the relation is not polar,is only partially passed on — some of it being taken up in moving the successor towards a polar relation. One more consequence is to be observed. Every approach of the molecules towards symmetrical arrangement, increases the amount of molecular motion transferred from one end of the THE GENESIS OF NERVES. 5] 9 series to the other. Suppose that the row of bricks, which were at first very much out of parallelism, have fallen, and that part of the motion given by each to tho next has gone towards bringing their faces nearer to parallelism; and sup- pose that, without further changing the positions of their bases, the bricks are severally restored to their vertical atti- tudes; then it will happen that if the serial overthrow of them is repeated, the actions, though the same as before in their kinds, will not be the same as before in their degrees. Each brick, falling as it now does more in the line of the series, will deliver more of its momentum to the next; and less momentum will be taken up in moving the next towards parallelism with its neighbours. If, then, the analogy holds, it must happen that in the series of isomerically-changing molecules, each transmitted wave of molecular motion is expended partly in so altering the molecular attitudes as to render the series more permeable to future waves, and partly in setting up changes at the end of the series; that in proportion as less of it is absorbed in working this struc- tural change, more of it is delivered at the far end and greater effect produced there; and that the final state i& one in which the initial wave of molecular motion is trans- mitted without deduction — or rather, with the addition of the molecular motion given out by the successive molecules of the series in their isomeric falls.
§ 225. From, beginning to end, therefore, the develop- ment of nerve results from the passage of motion along the line of least resistance, and the reduction of it to a line of less and less resistance continually. The first opening of a route along which equilibrium is restored between a place where molecular motion is in excess and a place where it is in defect, comes within this formula. The production^of a more continuous line of that peculiar colloid best fitted to transmit the molecular motion, also comes within this for- mula; as does likewise the making of this line thicker and 520 PHYSICAL SYNTHESIS.
more even. And the formula also covers that final process by which the line, having been formed, lias its molecules brought Into the polar order which least resists, and indeed facilitates, the transmission of the wave.
§ 225a. S-mie qualifications of the foregoing exposition must now be made. Instead of changing it throughout to meet criticisms,, I have thought it best to repeat the exposi- tion as given in the second, third, and fourth editions of this work and then to indicate the needful modifications.
At the meeting of the British Association held in Belfast in 1874, Prof: Clerk-Maxwell objected to the hypothesis that the nerve-current consists of successive waves of isomeric change, on the ground that the implied conception was that of a "heat-machine," and that a heat-machine is impossible in the absence of difference of temperature. In reply,! con- tended that my hypothesis is not at variance with this law of thermo-dynamics, since it supposes that the falling; of eac'i molecule from one isomeric form to another is accompanied by absorption of heat, and that the nerve-fibre, thereupon rendered of lower temperature than the surrounding matters, instantly takes up from them heat sufficient to cause the molecules to resume their previous form: the implication being that the nerve-current is at the cost of the heat yielded by the imbedding tissues. The discussion which ensued failed to draw from Prof. Clerk-Maxwell the admission that my reply was adequate, and failed also to make me under- stand his difficulty. This difficulty, as since explained to me by Lord Rayleigh, is that, being a lower form of molecular motion, heat cannot reproduce that higher form of molecular motion implied by the hypothesis. Here I have no alternative but to accept the dicta of these "two distinguished physicists. It is true that the heat supplied by a sitting hen apparently suffices to build up a variety of complex com- pounds, some of which, as protagon, are more complex than any of those contained in the unorganized materials of the THE GENESIS OF NERVES. 52G& egg; and it miglit seern a fair infereDce that such being the case, heat roust be capable of raising a molecule of protein from a lower isomeric form to a higher. But I suppose there must be some lack of parallelism between the two cases, and that this rhythmical isomeric change in nerve-fibre implies some further physical process which ultimate physical principles negative.
"What qualification then must the hypothesis undergo to render it tenable? Apparently we must conclude that in nerve-fibre, as in the tissues at large, performance of func- tion is accompanied by molecular disintegration, and that fitness for subsequent performance of function is to be gained only by re-integration. The implication would seem to be that the molecular change must be, to some extent, a chemical change; and that each molecule "which has under- gone modification, has to be repaired by absorption of needful matter from the nutrient liquid with which it is bathed.
This supposition appears congruous with the fact that the axis-cylinder of a nerve-fibre, small in diameter though it is, consists of a bundle of still more minute fibres. It seems not unlikely that such component fibres as have conveyed a wave of nerve-change, and become thereby temporarily in- capacitated, have their functions undertaken by another cluster of component fibres, which carry the next wave, and these, again,, leave the function to be afterwards performed by a third cluster, and so on: the fibres of the first cluster having meanwhile refitted themselves for activity. If so, there is a parallelism between the action of nerves and the action of muscles; in which last the sets t,f fibres take their turns of action and rest, while the muscle as a whole continues in a state of contraction.
CHAPTER III.
THE G-EXE3I3 OF SIMPLE ]NTEETOUS SYSTEMS/3' 5 226, Careful and extended observations have necessitated changes in the cell-doctrine as originally propounded. The statement that all organisms of sensible sizes are made up of minute nucleated bodies,, completely distinct from one another, has to be much qualified.
Among botanists a wide change of view resulted from the discovery that in the tissues of plants the protoplasm within each cell is united to that within adjacent cells by threads of protoplasm which pass through the respective cell- walls: a discovery that at once makes more comprehensible various plant-movements. Implying a kindred structure, Prof. Sedgwick writes: — "It is becoming more and more clear * In his Inaugural Address to the Section of Anatomy and Physiology, at the meeting of the British Association in 1880, Prof. Balfour, after indi- cating certain new lights thrown on the evolution of nervous systems, remarked concerning the contents of this chapter: — " These hypotheses of Herbert Spencer, which have been widely adopted in this country, are, it appears to me, not borne out by the discoveries to which I have called your attention to-day." Being, as I considered, bound to accept Prof. Balfour's representations, I was about to change essentially the first part of this chapter, when my attention was drawn to an opinion since published by Prof. Adam Seclgwiet, the successor of Prof. Balfour in the same chair at Cambridge. The opinion in question is contained in A Monograph of the Development of Peripatus Capen&is, p. 49, and is expressed as follows: — " Herbert Spencer's view of the origin of the nervous system may perhaps not be so far from the mark aS at first sight appeared." Taking advantage of the recent results of histological researches, I have been led, by the criticism and counter-criticism above quoted, to re-cast the early part of this chapter, and to give the con- tained hypothesis a more satisfactory form, I think, than previously seemed possible.
THE GENESIS OP SIMPLE NERVOUS SYSTEMS. 520cs ereiy day that the cells composing the tissues of animals are not isolated units, but that they are connected with one another. I need only refer to the connection known to exist between connective tissue cells, cartilage cells, epithelial cells, &c. And not only may the cells of one tissue be continuous with each other, but they may also be continuous with the cells of other tissues?> fpp. 47-8).
The revised conception to which we are thus introduced, is that throughout those aggregations of Protozoa by which Metazoa have been formed, there has been an incompleteness of those spontaneous fissions which, if complete, would have multiplied the Protozoa: the units have remained connected by prolongations homologous with pseudopodia. As the members of a compound Rhizopod, say one of the Foramini- fera, are not wholly cut off from one another, but maintain some continuity of substance through perforations in the septa — as the living units which make up a Volvos or a Raphldiopkrys are held together by threads of protoplasm which traverse their respective limiting membranes; so it appears that segmentation in a fertilized ovum does not abso- lutely isolate the contained matter of each segment. In the ovum of Peripalus, at any rate, which is exceptionally adapted for displaying the early changes, there results a network of protoplasm which unites the cell-masses with one another. And Prof. Sedgwick, saying that In Peripatus "the connec- tion of cell widi cell is not a secondary feature acquired late in development, but is primary," leans to the conclusion that " the continuity in the various cells of the adult:> is " due to a primitive continuity which has never been broken" (p. 49).
Thus, then, we must conceive of animal tissue as having from the beginning consisted of a matrix of. relatively inert substances throughout which there runs a nucleated network of living and active protoplasm.
§ 226$. As shown in the actions of a Rhizopod, protoplasm displays at once the properties of nerve and muscle: it 52CM PHYSICAL SYNTHESIS.
conducts and it contracts. These united properties still characterize it when it assumes the form of an imbedded net- work. That it continues to possess them when permeating vegetal tissues is proved by such actions as those of the Sensitive Plant and the Dioncsa — actions which show us both the conveyance of a disturbance and the production of movement at a distance. And that the protoplasmic net- work of animal tissue lias the like combined traits we see in such simple types as the Hydra.
Observe, next, that these properties are most markedly displayed where the protoplasm exists in an elongated form. If the tentacle of a polype is touched it contracts with tolerable promptness — with greater promptness than the body contracts. Among the oceanic Hydrozoa which,, float- ing or swimming, have long pendant tentacles, such as Diphyes and Physalia, the threads of nucleated sarcode thus trailing behind or hanging down, are quickly drawn up when struck by small creatures serving for prey. These traits are in great measure cause and consequence. Molecular change set up at the end of a thread-shaped portion of substance is necessarily limited to the line formed by the substance. It cannot be lost by diffusion through a large mass like that of the body, but must be concentrated within the channel formed by the sides of the thread.
This much premised, let us now ask what will result in the body of a creature which is as yet but little differentiated?
what will happen to the nucleated network of protoplasm diffused through it? From the general law of the instability of the homogeneous, it is to be inferred that the originally- united properties of protoplasm will not remain the same throughout. We may expect a specialization such as will restrict the contractile power to some parts of it and leave the power of conduction to other parts. How will this differentiation be likely to arise? Suppose the incipiently- organized mass to be from time to time rudely disturbed by passing bodies— How will the effects of the shocks be TEE GENESIS OP SIMPLE NERVOUS SYSTEMS. 521 localized? May we not reasonably say that the parts of the protoplasmic network which become contractile, will be those of which the contraction is least resisted; and, con- versely, that the parts which cannot contract without greater resistance will remain conductive only. So far as ascertained; the facts harmonize with this supposition. Muscular fibres first make their appearance as processes of the epithelial cells forming the surface of the bod}7" (see Bal four's Embryology, vol. II. p. 657); and this is the part in which movement is least resisted, since the inert matter on one side only of the layer, has to be carried along with the contracting elements. Conversely, though the primitive nervous elements, being the recipients of external impressions,, first arise on the outer surface, yet the protoplasmic network connected with them, which lakes on the function of conducting impressions, soon becomes established below the surface: contraction in this pare of the network being opposed by inert tissue imbedding1 the fibres on all sides, and the assumption of conducting function solely being hence favoured.
A lurcher question now presents itself — By what physical process is the functional relation established between these muscular and nervous elements as fast as they differentiate? There is a not unsatisfactory answer. A portion of tissue which, by disturbance of one or other kind, has been made to contract,, is a portion in which molar motion has been pro- duced at the cost of molecular motion: the one implies the other. Contrariwise, a portion of tissue which, by some incident force or stimulus, has been disturbed, but does not undergo contraction, is one in \vhicli the molecular motion generated remains free. In the first place, then, there is caused a deficiency, and in the second place there is caused a surplus. Consequently if, between the two, there is any channel through \vhich an equilibrium can be established, a flow of molecular motion through it may he anticipated. Such a channel exists in the protoplasmic network. Hence, when the iucipiently-differeiitiatiug tissues are disturbed, there will 5:22 PHYSICAL SYNTHESIS.
result a draught from the portion which is beginning to act as a sensorv organ to the portion which is beginning to act as a contractile organ — a draught which, repeated on each occa- sion, will, in conformity to the principle set forth in the last chapter, tend ever to make the channel more permeable.
And then,, lastly, observe that though there is implied a simultaneity in the production of the deficiency and the surplus in the case described, so that the contraction has ahva rly arisen at the time that the stimulus has been received, yet it is inferable that when the connecting channel has become easily permeable, the reception of a stimulus will cause a flow through the channel, and consequent arrival at the contracting part before this has been otherwise dis- turbed: the result being that the surplus molecular motion received will initiate a contraction. Sensation, or that which corresponds to it, will produce motion through incipient nerves, § '227. In many coelenterate creatures the contractile sii'1- stance is partly differentiated into muscular fibres; which, however,, are distributed in a diffused way. In an Actiniay the average equality of the forces to which the body is ex- posed all round, is unfavourable to the formation of distinct muscles and a distinct nervous system. There is nothing which tends to bring the contractility to one place; and therefore nothing which causes the waves of molecular dis- turbance to take special courses. Probably in a sea-anemone, the incipient lines of nervous discharge are as much diffused as the muscular fibres are diffused. Noting only the fact that the contractile tissue which, when it acts, absorbs molecular motion, becomes differentiated before there arise distinct nerve-fibres conveying molecular motion from places where it has been evolved, let us take a hypothetical case fitted to m«tke intelligible the first step in nervous development.
Suppose that the process of continuous gemmation, by which creatures of these low types very generally multiply, is so carried on that the individuals successively produced THE GENESIS OF SIMPLE NERVOUS SYSTEMS.
are interfered witli by the colony more on one side than on the other side. Being unsymnietrically conditioned they will become unsymmetrically developed. (Principles Let Fig. 5 represent a creature of this kind that grows obliquely away from its elder neighbours; and let A B stand for the surface over which the colony is spreading. Then it must happen that when moving objects in the adjacent water, larger than those minute ones serving for prey, come against the creature, first striking its expanded tentacles and then its body, the most exposed part of its body C will be most fre- quently disturbed. Each time it is disturbed there will be propagated through it that form of molecular change from which contraction results, and there will occasionally be produced more molecules of this same type. (Principles of Biology § 302.) That is to say C will become a place where the contractions ars relatively frequent and decided, and where contractile protoplasm is greater in amount than else- where. What further will happen? Mostly when a collision occurs the tentacles are touched before the body; and, for reasons above given, the propagation of molecular change along them is comparatively rapid. Now at the part C, each evolution of mechanical motion is necessarily accompanied by an absorption of molecular motion. Consequently when from the disturbed end of the tentacle D there has been sent a wave of molecular motion, part of which is absorbed in the contraction of each successive portion of the tentacle but a surplus of which passes on, setting up contractions of the portions below, the final surplus when the wave has reached d, will be drafted off to the contractile portion C; 524 PHYSICAL SYNTHESIS.
since this, being struck the instant after, and made to con - tract, becomes a place where molecular motion is absorbed. But- such an action does not constitute a true nervous action. For the stimulus applied at D is not the cause of the con- craction at G. The contraction at C is caused by a collision at 0: and the discharge from d to C cannot take place until after the contraction at C has commenced. Nevertheless, though not a nervous action proper, it may, by frequent repetition, grow into one. If restorations of equilibrium be- tween d and C recur often — if they continually ta,ke the same route through the network of protoplasm — if this becomes a line of less and less resistance that drafts off the molecular motion with rapidity; then, eventually, when an approaching body touches the end of the tentacle D, the impulse conveyed down it and along the incipient nerve from d to C will reach C before the approaching body touches it. Now the contractile colloid at G is capable of having its special molecular transformation set up by various stimuli — by communicated molecular motion as well as by a blow. Hence when a wave of disturbance reaches it before it re- ceives a blow, it will be^iu to contract in anticipation of the blow. A rude touch at the end of the tentacle D, will, by the shrinking it sets up at C, cause withdrawal of the body from the source of danger.
§ 228. To avoid complications of statement, I have pre- sented this primitive nervous action under a simpler forir. than that which actually occurs. For the wave of molecular motion has to be conveyed not to a single point but to a portion of contractile colloid having considerable extension, many parts of which, simultaneously become places where molecular motion is being absorbed. Hence the wave pass- ing to it will somewhere on its way tend to divide according to the respective tensions towards these respective parts. What will result?
Fig, 6 represents the same general distribution as "before, THE G EXE SIS OF SIMPLE NERVOUS SYSTEMS. 525 with the difference that the mass of contractile colloid G, is marked in dotted lines, and that at e the line of nervous com- munication is shown to take divergent and re-divergent courses towards differento parts of C. For this is the structure implied. The same tendency towards re- storation of equilibrium which causes the wave to go from d to C,, will also cause \ \\ it to distribute itself with tolerable even- *&& \ \ ness to all parts of C -y since to any park